Core Java

Question: What invokes a thread's run() method?

Answer: After a thread is started, via its start() method or that of the Thread class, the JVM invokes the thread's run() method when the thread is initially executed.

Question: What is the difference between the Boolean & operator and the && operator?

Answer: If an expression involving the Boolean & operator is evaluated, both operands are evaluated. Then the & operator is applied to the operand. When an expression involving the && operator is evaluated, the first operand is evaluated. If the first operand returns a value of true then the second operand is evaluated. The && operator is then applied to the first and second operands. If the first operand evaluates to false, the evaluation of the second operand is skipped.

Question: Name three subclasses of the Component class.

Answer: Box.Filler, Button, Canvas, Checkbox, Choice, Container, Label, List, Scrollbar, or TextComponent

Question: What is the GregorianCalendar class?

Answer: The GregorianCalendar provides support for traditional Western calendars.

Question: Which Container method is used to cause a container to be laid out and redisplayed?

Answer: validate()

Question: What is the purpose of the Runtime class?

Answer: The purpose of the Runtime class is to provide access to the Java runtime system.

Question: How many times may an object's finalize() method be invoked by the garbage collector?

Answer: An object's finalize() method may only be invoked once by the garbage collector.

Question: What is the purpose of the finally clause of a try-catch-finally statement?

Answer: The finally clause is used to provide the capability to execute code no matter whether or not an exception is thrown or caught.

Question: What is the argument type of a program's main() method?

Answer: A program's main() method takes an argument of the String[] type.

Question: Which Java operator is right associative?

Answer: The = operator is right associative.

Question: What is the Locale class?

Answer: The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.

Question: Can a double value be cast to a byte?

Answer: Yes, a double value can be cast to a byte.

Question: What is the difference between a break statement and a continue statement?

Answer: A break statement results in the termination of the statement to which it applies (switch, for, do, or while). A continue statement is used to end the current loop iteration and return control to the loop statement.

Question: What must a class do to implement an interface?

Answer: It must provide all of the methods in the interface and identify the interface in its implements clause.

Question: What method is invoked to cause an object to begin executing as a separate thread?

Answer: The start() method of the Thread class is invoked to cause an object to begin executing as a separate thread.

Question: Name two subclasses of the TextComponent class.

Answer: TextField and TextArea

Question: What is the advantage of the event-delegation model over the earlier event-inheritance model?

Answer: The event-delegation model has two advantages over the event-inheritance model. First, it enables event handling to be handled by objects other than the ones that generate the events (or their containers). This allows a clean separation

between a component's design and its use. The other advantage of the event-delegation model is that it performs much better in applications where many events are generated. This performance improvement is due to the fact that the event-delegation model does not have to repeatedly process unhandled events, as is the case of the event-inheritance model.

Question: Which containers may have a MenuBar?

Answer: Frame

Question: How are commas used in the intialization and iterationparts of a for statement?

Answer: Commas are used to separate multiple statements within the initialization and iteration parts of a for statement.

Question: What is the purpose of the wait(), notify(), and notifyAll() methods?

Answer: The wait(),notify(), and notifyAll() methods are used to provide an efficient way for threads to wait for a shared resource. When a thread executes an object's wait() method, it enters the waiting state. It only enters the ready state after another thread invokes the object's notify() or notifyAll() methods.

Question: What is an abstract method?

Answer: An abstract method is a method whose implementation is deferred to a subclass.

Question: How are Java source code files named?

Answer: A Java source code file takes the name of a public class or interface that is defined within the file. A source code file may contain at most one public class or interface. If a public class or interface is defined within a source code file, then the source code file must take the name of the public class or interface. If no public class or interface is defined

within a source code file, then the file must take on a name that is different than its classes and interfaces. Source code files use the .java extension.

Question: What is the relationship between the Canvas class and the Graphics class?

Answer: A Canvas object provides access to a Graphics object via its paint() method.

Question: What are the high-level thread states?

Answer: The high-level thread states are ready, running, waiting, and dead.

Question: What value does read() return when it has reached the end of a file?

Answer: The read() method returns -1 when it has reached the end of a file.

Question: Can a Byte object be cast to a double value?

Answer: No, an object cannot be cast to a primitive value.

Question: What is the difference between a static and a non-static inner class?

Answer: A non-static inner class may have object instances that are associated with instances of the class's outer class. A static inner class does not have any object instances.

Question: What is the difference between the String and StringBuffer classes?

Answer: String objects are constants. StringBuffer objects are not.

Question: If a variable is declared as private, where may the variable be accessed?

Answer: A private variable may only be accessed within the class in which it is declared.

Question: What is an object's lock and which object's have locks?

Answer: An object's lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the object's lock. All objects and classes have locks. A class's lock is acquired on the class's Class object.

Question: What is the Dictionary class?

Answer: The Dictionary class provides the capability to store key-value pairs.

Question: How are the elements of a BorderLayout organized?

Answer: The elements of a BorderLayout are organized at the borders (North, South, East, and West) and the center of a container.

Question: What is the % operator?

Answer: It is referred to as the modulo or remainder operator. It returns the remainder of dividing the first operand by the second operand.

Question: When can an object reference be cast to an interface reference?

Answer: An object reference be cast to an interface reference when the object implements the referenced interface.

Question: What is the difference between a Window and a Frame?

Answer: The Frame class extends Window to define a main application window that can have a menu bar.

Question: Which class is extended by all other classes?

Answer: The Object class is extended by all other classes.

Question: Can an object be garbage collected while it is still reachable?

Answer: A reachable object cannot be garbage collected. Only unreachable objects may be garbage collected..

Question: Is the ternary operator written x : y ? z or x ? y : z ?

Answer: It is written x ? y : z.

Question: What is the difference between the Font and FontMetrics classes?

Answer: The FontMetrics class is used to define implementation-specific properties, such as ascent and descent, of a Font object.

Question: How is rounding performed under integer division?

Answer: The fractional part of the result is truncated. This is known as rounding toward zero.

Question: What happens when a thread cannot acquire a lock on an object?

Answer: If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object's lock, it enters the waiting state until the lock becomes available.

Question: What is the difference between the Reader/Writer class hierarchy and the InputStream/OutputStream class hierarchy?

Answer: The Reader/Writer class hierarchy is character-oriented, and the InputStream/OutputStream class hierarchy is byte-oriented.

Question: What classes of exceptions may be caught by a catch clause?

Answer: A catch clause can catch any exception that may be assigned to the Throwable type. This includes the Error and Exception types.

Question: If a class is declared without any access modifiers, where may the class be accessed?

Answer: A class that is declared without any access modifiers is said to have package access. This means that the class can only be accessed by other classes and interfaces that are defined within the same package.

Question: What is the SimpleTimeZone class?

Answer: The SimpleTimeZone class provides support for a Gregorian calendar.

Question: What is the Map interface?

Answer: The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values.

Question: Does a class inherit the constructors of its superclass?

Answer: A class does not inherit constructors from any of its superclasses.

Question: For which statements does it make sense to use a label?

Answer: The only statements for which it makes sense to use a label are those statements that can enclose a break or continue statement.

Question: What is the purpose of the System class?

Answer: The purpose of the System class is to provide access to system resources.

Question: Which TextComponent method is used to set a TextComponent to the read-only state?

Answer: setEditable()

Question: How are the elements of a CardLayout organized?

Answer: The elements of a CardLayout are stacked, one on top of the other, like a deck of cards.

Question: Is &&= a valid Java operator?

Answer: No, it is not.

Question: Name the eight primitive Java types.

Answer: The eight primitive types are byte, char, short, int, long, float, double, and boolean.

Question: Which class should you use to obtain design information about an object?

Answer: The Class class is used to obtain information about an object's design.

Question: What is the relationship between clipping and repainting?

Answer: When a window is repainted by the AWT painting thread, it sets the clipping regions to the area of the window that requires repainting.

Question: Is "abc" a primitive value?

Answer: The String literal "abc" is not a primitive value. It is a String object.

Question: What is the relationship between an event-listener interface and an event-adapter class?

Answer: An event-listener interface defines the methods that must be implemented by an event handler for a particular kind of event. An event adapter provides a default implementation of an event-listener interface.

Question: What restrictions are placed on the values of each case of a switch statement?

Answer: During compilation, the values of each case of a switch statement must evaluate to a value that can be promoted to an int value.

Question: What modifiers may be used with an interface declaration?

Answer: An interface may be declared as public or abstract.

Question: Is a class a subclass of itself?

Answer: A class is a subclass of itself.

Question: What is the highest-level event class of the event-delegation model?

Answer: The java.util.EventObject class is the highest-level class in the event-delegation class hierarchy.

Question: What event results from the clicking of a button?

Answer: The ActionEvent event is generated as the result of the clicking of a button.

Question: How can a GUI component handle its own events?

Answer: A component can handle its own events by implementing the required event-listener interface and adding itself as its own event listener.

Question: What is the difference between a while statement and a dostatement?

Answer: A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once.

Question: How are the elements of a GridBagLayout organized?

Answer: The elements of a GridBagLayout are organized according to a grid. However, the elements are of different sizes and may occupy more than one row or column of the grid. In addition, the rows and columns may have different sizes.

Question: What advantage do Java's layout managers provide over traditional windowing systems?

Answer: Java uses layout managers to lay out components in a consistent manner across all windowing platforms. Since Java's layout managers aren't tied to absolute sizing and positioning, they are able to accomodate platform-specific differences among windowing systems.

Question: What is the Collection interface?

Answer: The Collection interface provides support for the implementation of a mathematical bag - an unordered collection of objects that may contain duplicates.

Question: What modifiers can be used with a local inner class?
Answer: A local inner class may be final or abstract.

Question: What is the difference between static and non-static variables?

Answer: A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.

Question: What is the difference between the paint() and repaint() methods?

Answer: The paint() method supports painting via a Graphics object. The repaint() method is used to cause paint() to be invoked by the AWT painting thread.

Question: What is the purpose of the File class?

Answer: The File class is used to create objects that provide access to the files and directories of a local file system.

Question: Can an exception be rethrown?

Answer: Yes, an exception can be rethrown.

Question: Which Math method is used to calculate the absolute value of a number?

Answer: The abs() method is used to calculate absolute values.

Question: How does multithreading take place on a computer with a single CPU?

Answer: The operating system's task scheduler allocates execution time to multiple tasks. By quickly switching between executing tasks, it creates the impression that tasks execute sequentially.

Question: When does the compiler supply a default constructor for a class?

Answer: The compiler supplies a default constructor for a class if no other constructors are provided.

Question: When is the finally clause of a try-catch-finally statement executed?

Answer: The finally clause of the try-catch-finally statement is always executed unless the thread of execution terminates or an exception occurs within the execution of the finally clause.

Question: Which class is the immediate superclass of the Container class?

Answer: Component

Question: If a method is declared as protected, where may the method be accessed?

Answer: A protected method may only be accessed by classes or interfaces of the same package or by subclasses of the class in which it is declared.

Question: How can the Checkbox class be used to create a radio button?

Answer: By associating Checkbox objects with a CheckboxGroup.

Question: Which non-Unicode letter characters may be used as the first character of an identifier?

Answer: The non-Unicode letter characters $ and _ may appear as the first character of an identifier

Question: What restrictions are placed on method overloading?

Answer: Two methods may not have the same name and argument list but different return types.

Question: What happens when you invoke a thread's interrupt method while it is sleeping or waiting?

Answer: When a task's interrupt() method is executed, the task enters the ready state. The next time the task enters the running state, an InterruptedException is thrown.

Question: What is casting?

Answer: There are two types of casting, casting between primitive numeric types and casting between object references. Casting between numeric types is used to convert larger values, such as double values, to smaller values, such as byte values. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference.

Question: What is the return type of a program's main() method?
Answer: A program's main() method has a void return type.

Question: Name four Container classes.

Answer: Window, Frame, Dialog, File Dialog, Panel, Applet, or ScrollPane

Question: What is the difference between a Choice and a List?

Answer: A Choice is displayed in a compact form that requires you to pull it down to see the list of available choices. Only one item may be selected from a Choice. A List may be displayed in such a way that several List items are visible. A List supports the selection of one or more List items.

Question: What class of exceptions are generated by the Java run-time system?

Answer: The Java runtime system generates Runtime Exception and Error exceptions.

Question: What class allows you to read objects directly from a stream?

Answer: The ObjectInputStream class supports the reading of objects from input streams.

Question: What is the difference between a field variable and a local variable?

Answer: A field variable is a variable that is declared as a member of a class. A local variable is a variable that is declared local to a method.

Question: Under what conditions is an object's finalize() method invoked by the garbage collector?

Answer: The garbage collector invokes an object's finalize() method when it detects that the object has become unreachable.

Question: How are this() and super() used with constructors?

Answer: this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.

Question: What is the relationship between a method's throws clause and the exceptions that can be thrown during the method's execution?

Answer: A method's throws clause must declare any checked exceptions that are not caught within the body of the method.

Question: What is the difference between the JDK 1.02 event model and the event-delegation model introduced with JDK 1.1?

Answer: The JDK 1.02 event model uses an event inheritance or bubbling approach. In this model, components are required to handle their own events. If they do not handle a particular event, the event is inherited by (or bubbled up to) the component's container. The container then either handles the event or it is bubbled up to its container and so on, until the highest-level container has been tried.

In the event-delegation model, specific objects are designated as event handlers for GUI components. These objects implement event-listener interfaces. The event-delegation model is more efficient than the event-inheritance model because it eliminates the processing required to support the bubbling of unhand led events.

Question: How is it possible for two String objects with identical values not to be equal under the == operator?

Answer: The == operator compares two objects to determine if they are the same object in memory. It is possible for two String objects to have the same value, but located indifferent areas of memory.

Question: Why are the methods of the Math class static?

Answer: So they can be invoked as if they are a mathematical code library.

Question: What Checkbox method allows you to tell if a Checkbox is checked?

Answer: getState()

Question: What state is a thread in when it is executing?

Answer: An executing thread is in the running state.

Question: What are the legal operands of the instance of operator?
Answer: The left operand is an object reference or null value and the right operand is a class, interface, or array type.

Question: How are the elements of a GridLayout organized?

Answer: The elements of a GridBad layout are of equal size and are laid out using the squares of a grid.

Question: What an I/O filter?

Answer: An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.

Question: If an object is garbage collected, can it become reachable again?

Answer: Once an object is garbage collected, it ceases to exist.It can no longer become reachable again.

Question: What is the Set interface?

Answer: The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements.

Question: What classes of exceptions may be thrown by a throw statement?

Answer: A throw statement may throw any expression that may be assigned to the Throwable type.

Question: What are E and PI?

Answer: E is the base of the natural logarithm and PI is mathematical value pi.

Question: Are true and false keywords?

Answer: The values true and false are not keywords.

Question: What is a void return type?

Answer: A void return type indicates that a method does not return a value.

Question: What is the purpose of the enableEvents() method?

Answer: The enableEvents() method is used to enable an event for a particular object. Normally, an event is enabled when a listener is added to an object for a particular event. The enableEvents() method is used by objects that handle events by overriding their event-dispatch methods.

Question: What is the difference between the File and RandomAccessFile classes?

Answer: The File class encapsulates the files and directories of the local file system. The RandomAccessFile class provides the methods needed to directly access data contained in any part of a file.

Question: What happens when you add a double value to a String?

Answer: The result is a String object.

Question: What is your platform's default character encoding?

Answer: If you are running Java on English Windows platforms, it is probably Cp1252. If you are running Java on English Solaris platforms, it is most likely 8859_1..

Question: Which package is always imported by default?

Answer: The java.lang package is always imported by default.

Question: What interface must an object implement before it can be written to a stream as an object?

Answer: An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object.

Question: How are this and super used?

Answer: this is used to refer to the current object instance. super is used to refer to the variables and methods of the superclass of the current object instance.

Question: What is the purpose of garbage collection?

Answer: The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources may be reclaimed and reused.

Question: What is a compilation unit?

Answer: A compilation unit is a Java source code
file.

Question: What interface is extended by AWT event listeners?

Answer: All AWT event listeners extend the java.util.EventListener interface.

Question: What restrictions are placed on method overriding?

Answer: Overridden methods must have the same name, argument list, and return type. The overriding method may not limit the access of the method it overrides. The overriding method may not throw any exceptions that may not be thrown by the overridden method.

Question: How can a dead thread be restarted?

Answer: A dead thread cannot be restarted.

Question: What happens if an exception is not caught?

Answer: An uncaught exception results in the uncaughtException() method of the thread's ThreadGroup being invoked, which eventually results in the termination of the program in which it is thrown.

Question: What is a layout manager?

Answer: A layout manager is an object that is used to organize components in a container.

Question: Which arithmetic operations can result in the throwing of an ArithmeticException?

Answer: Integer / and % can result in the throwing of an ArithmeticException.

Question: What are three ways in which a thread can enter the waiting state?

Answer: A thread can enter the waiting state by invoking its sleep() method, by blocking on I/O, by unsuccessfully attempting to acquire an object's lock, or by invoking an object's wait() method. It can also enter the waiting state by invoking its (deprecated) suspend() method.

Question: Can an abstract class be final?

Answer: An abstract class may not be declared as final.

Question: What is the ResourceBundle class?

Answer: The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to tailor the program's appearance to the particular locale in which it is being run.

Question: What happens if a try-catch-finally statement does not have a catch clause to handle an exception that is thrown within the body of the try statement?

Answer: The exception propagates up to the next higher level try-catch statement (if any) or results in the program's termination.

Question: What is numeric promotion?

Answer: Numeric promotion is the conversion of a smaller numeric type to a larger numeric type, so that integer and floating-point operations may take place. In numerical promotion, byte, char, and short values are converted to int values. The int values are also converted to long values, if necessary. The long and float values are converted to double values, as required.

Question: What is the difference between a Scrollbar and a ScrollPane?

Answer: A Scrollbar is a Component, but not a Container. A ScrollPane is a Container. A ScrollPane handles its own events and performs its own scrolling.

Question: What is the difference between a public and a non-public class?
Answer: A public class may be accessed outside of its package. A non-public class may not be accessed outside of its package.

Core Java

Question: What are the uses of Serialization?

Answer: In some types of applications you have to write the code to serialize objects, but in many cases serialization is performed behind the scenes by various server-side containers.

These are some of the typical uses of serialization:

* To persist data for future use.
* To send data to a remote computer using such client/server Java technologies
as RMI or socket programming.
* To "flatten" an object into array of bytes in memory.
* To exchange data between applets and servlets.
* To store user session in Web applications .
* To activate/passivate enterprise java beans.
* To send objects between the servers in a cluster.

Question: what is a collection ?

Answer: Collection is a group of objects. java.util package provides important types of collections. There are two fundamental types of collections they are Collection and Map. Collection types hold a group of objects, Eg. Lists and Sets where as Map types hold group of objects as key, value pairs Eg. HashMap and Hashtable.

Question: For concatenation of strings, which method is good, StringBuffer or String ?

Answer: StringBuffer is faster than String for concatenation.

Question: What is Runnable interface ? Are there any other ways to make a java program as multithred java program?

Answer: There are two ways to create new kinds of threads:

- Define a new class that extends the Thread class
- Define a new class that implements the Runnable interface, and pass an object of that class to a Thread's constructor.
- An advantage of the second approach is that the new class can be a subclass of any class, not just of the Thread class.
Here is a very simple example just to illustrate how to use the second approach to creating threads: class myThread implements Runnable { public void run() { System.out.println("I'm running!"); } } public class tstRunnable { public static void main(String[] args) { myThread my1 = new myThread(); myThread my2 = new myThread(); new Thread(my1).start(); new Thread(my2).start(); }

Question: How can i tell what state a thread is in ?

Answer: Prior to Java 5, isAlive() was commonly used to test a threads state. If isAlive() returned false the thread was either new or terminated but there was simply no way to differentiate between the two.

Starting with the release of Tiger (Java 5) you can now get what state a thread is in by using the getState() method which returns an Enum of Thread.States. A thread can only be in one of the following states at a given point in time.

NEW


A Fresh thread that has not yet started to execute.

RUNNABLE


A thread that is executing in the Java virtual machine.

BLOCKED


A thread that is blocked waiting for a monitor lock.

WAITING


A thread that is wating to be notified by another thread.

TIMED_WAITING


A thread that is wating to be notified by another thread for a specific amount of time

TERMINATED


A thread whos run method has ended.


The folowing code prints out all thread states. public class ThreadStates{ public static void main(String[] args){ Thread t = new Thread(); Thread.State e = t.getState(); Thread.State[] ts = e.values(); for(int i = 0; i < ts.length; i++){ System.out.println(ts[i]); } } }

Question: What methods java providing for Thread communications ?

Answer: Java provides three methods that threads can use to communicate with each other: wait, notify, and notifyAll. These methods are defined for all Objects (not just Threads). The idea is that a method called by a thread may need to wait for some condition to be satisfied by another thread; in that case, it can call the wait method, which causes its thread to wait until another thread calls notify or notifyAll.

Question: What is the difference between notify and notify All methods ?

Answer: A call to notify causes at most one thread waiting on the same object to be notified (i.e., the object that calls notify must be the same as the object that called wait). A call to notifyAll causes all threads waiting on the same object to be notified. If more than one thread is waiting on that object, there is no way to control which of them is notified by a call to notify (so it is often better to use notifyAll than notify).

Question: What is synchronized keyword? In what situations you will Use it?

Answer: Synchronization is the act of serializing access to critical sections of code. We will use this keyword when we expect multiple threads to access/modify the same data. To understand synchronization we need to look into thread execution manner.

Threads may execute in a manner where their paths of execution are completely independent of each other. Neither thread depends upon the other for assistance. For example, one thread might execute a print job, while a second thread repaints a window. And then there are threads that require synchronization, the act of serializing access to critical sections of code, at various moments during their executions. For example, say that two threads need to send data packets over a single network connection. Each thread must be able to send its entire data packet before the other thread starts sending its data packet; otherwise, the data is scrambled. This scenario requires each thread to synchronize its access to the code that does the actual data-packet sending.

If you feel a method is very critical for business that needs to be executed by only one thread at a time (to prevent data loss or corruption), then we need to use synchronized keyword.

EXAMPLE

Some real-world tasks are better modeled by a program that uses threads than by a normal, sequential program. For example, consider a bank whose accounts can be accessed and updated by any of a number of automatic teller machines (ATMs). Each ATM could be a separate thread, responding to deposit and withdrawal requests from different users simultaneously. Of course, it would be important to make sure that two users did not access the same account simultaneously. This is done in Java using synchronization, which can be applied to individual methods, or to sequences of statements.

One or more methods of a class can be declared to be synchronized. When a thread calls an object's synchronized method, the whole object is locked. This means that if another thread tries to call any synchronized method of the same object, the call will block until the lock is released (which happens when the original call finishes). In general, if the value of a field of an object can be changed, then all methods that read or write that field should be synchronized to prevent two threads from trying to write the field at the same time, and to prevent one thread from reading the field while another thread is in the process of writing it.

Here is an example of a BankAccount class that uses synchronized methods to ensure that deposits and withdrawals cannot be performed simultaneously, and to ensure that the account balance cannot be read while either a deposit or a withdrawal is in progress. (To keep the example simple, no check is done to ensure that a withdrawal does not lead to a negative balance.)
public class BankAccount { private double balance; // constructor: set balance to given amount public BankAccount( double initialDeposit ) { balance = initialDeposit; } public synchronized double Balance( ) { return balance; } public synchronized void Deposit( double deposit ) { balance += deposit; } public synchronized void Withdraw( double withdrawal ) { balance -= withdrawal; } }

Note: that the BankAccount's constructor is not declared to be synchronized. That is because it can only be executed when the object is being created, and no other method can be called until that creation is finished.

There are cases where we need to synchronize a group of statements, we can do that using synchrozed statement.
Java Code Example synchronized ( B ) { if ( D > B.Balance() ) { ReportInsuffucientFunds(); } else { B.Withdraw( D ); } }

Question: What is serialization ?

Answer: Serialization is the process of writing complete state of java object into output stream, that stream can be file or byte array or stream associated with TCP/IP socket.

Question: What does the Serializable interface do ?

Answer: Serializable is a tagging interface; it prescribes no methods. It serves to assign the Serializable data type to the tagged class and to identify the class as one which the developer has designed for persistence. ObjectOutputStream serializes only those objects which implement this interface.

Question: How do I serialize an object to a file ?

Answer: To serialize an object into a stream perform the following actions:

- Open one of the output streams, for exxample FileOutputStream
- Chain it with the ObjectOutputStream <
- Call the method writeObject() providinng the instance of a Serializable object as an argument.
- Close the streams
Java Code --------- try{ fOut= new FileOutputStream("c:\\emp.ser"); out = new ObjectOutputStream(fOut); out.writeObject(employee); //serializing System.out.println("An employee is serialized into c:\\emp.ser"); } catch(IOException e){ e.printStackTrace(); }

Question: How do I deserilaize an Object?

Answer: To deserialize an object, perform the following steps:

- Open an input stream
- Chain it with the ObjectInputStream - Call the method readObject() and cast the returned object to the class that is being deserialized.
- Close the streams
Java Code try{ fIn= new FileInputStream("c:\\emp.ser"); in = new ObjectInputStream(fIn); //de-serializing employee Employee emp = (Employee) in.readObject(); System.out.println("Deserialized " + emp.fName + " " + emp.lName + " from emp.ser "); }catch(IOException e){ e.printStackTrace(); }catch(ClassNotFoundException e){ e.printStackTrace(); }

Question: What is Externalizable Interface ?

Answer: Externalizable interface is a subclass of Serializable. Java
provides Externalizable interface that gives you more control over what is being serialized and it can produce smaller object footprint. ( You can serialize whatever field values you want to serialize)

This interface defines 2 methods: readExternal() and writeExternal() and you have to implement these methods in the class that will be serialized. In these methods you'll have to write code that reads/writes only the values of the attributes you are interested in. Programs that perform serialization and deserialization have to write and read these attributes in the same sequence.

Question: What is a transient variable?

Answer: A transient variable is a variable that may not be serialized.

Question: Which containers use a border Layout as their default layout?

Answer: The window, Frame and Dialog classes use a border layout as their default layout.

Question: Why do threads block on I/O?

Answer: Threads block on i/o (that is enters the waiting state) so that other threads may execute while the i/o Operation is performed.

Question: How are Observer and Observable used?

Answer: Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.

Question: What is synchronization and why is it important?

Answer: With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often leads to significant errors.

Question: Can a lock be acquired on a class?

Answer: Yes, a lock can be acquired on a class. This lock is acquired on the class's Class object.

Question: What's new with the stop(), suspend() and resume() methods in JDK 1.2?

Answer: The stop(), suspend() and resume() methods have been deprecated in JDK 1.2.

Question: Is null a keyword?

Answer: The null value is not a keyword.

Question: What is the preferred size of a component?

Answer: The preferred size of a component is the minimum component size that will allow the component to display normally.

Question: What method is used to specify a container's layout?

Answer: The setLayout() method is used to specify a container's layout.

Question: Which containers use a FlowLayout as their default layout?

Answer: The Panel and Applet classes use the FlowLayout as their default layout.

Question: What state does a thread enter when it terminates its processing?

Answer: When a thread terminates its processing, it enters the dead state.

Question: What is the Collections API?

Answer: The Collections API is a set of classes and interfaces that support operations on collections of objects.

Question: Which characters may be used as the second character of an identifier, but not as the first character of an identifier?

Answer: The digits 0 through 9 may not be used as the first character of an identifier but they may be used after the first character of an identifier.

Question: What is the List interface?

Answer: The List interface provides support for ordered collections of objects.

Question: How does Java handle integer overflows and underflows?

Answer: It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.

Question: What is the Vector class?

Answer: The Vector class provides the capability to implement a growable array of objects

Question: What modifiers may be used with an inner class that is a member of an outer class?

Answer: A (non-local) inner class may be declared as public, protected, private, static, final, or abstract.

Question: What is an Iterator interface?

Answer: The Iterator interface is used to step through the elements of a Collection.

Question: What is the difference between the >> and >>> operators?

Answer: The >> operator carries the sign bit when shifting right. The >>> zero-fills bits that have been shifted out.

Question: Which method of the Component class is used to set the position and size of a component?

Answer: setBounds()

Question: How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters?

Answer: Unicode requires 16 bits and ASCII require 7 bits. Although the ASCII character set uses only 7 bits, it is usually represented as 8 bits. UTF-8 represents characters using 8, 16, and 18 bit patterns. UTF-16 uses 16-bit and larger bit patterns.

Question: What is the difference between yielding and sleeping?

Answer: When a task invokes its yield() method, it returns to the ready state. When a task invokes its sleep() method, it returns to the waiting state.

Question: Which java.util classes and interfaces support event handling?

Answer: The EventObject class and the EventListener interface support event processing.

Question: Is sizeof a keyword?

Answer: The sizeof operator is not a keyword.

Question: What are wrapped classes?

Answer: Wrapped classes are classes that allow primitive types to be accessed as objects.

Question: Does garbage collection guarantee that a program will not run out of memory?

Answer: Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection

Question: What restrictions are placed on the location of a package statement within a source code file?

Answer: A package statement must appear as the first line in a source code file (excluding blank lines and comments).

Question: Can an object's finalize() method be invoked while it is reachable?

Answer: An object's finalize() method cannot be invoked by the garbage collector while the object is still reachable. However, an object's finalize() method may be invoked by other objects.

Question: What is the immediate superclass of the Applet class?

Answer: Panel

Question: What is the difference between preemptive scheduling and time slicing?

Answer: Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.

Question: Name three Component subclasses that support painting.

Answer: The Canvas, Frame, Panel, and Applet classes support painting.

Question: What value does readLine() return when it has reached the end of a file?

Answer: The readLine() method returns null when it has reached the end of a file.

Question: What is the immediate superclass of the Dialog class?

Answer: Window

Question: What is clipping?

Answer: Clipping is the process of confining paint operations to a limited area or shape.

Question: What is a native method?

Answer: A native method is a method that is implemented in a language other than Java.

Question: Can a for statement loop indefinitely?

Answer: Yes, a for statement can loop indefinitely. For example, consider the following: for(;;) ;

Question: What are order of precedence and associativity, and how are they used?

Answer: Order of precedence determines the order in which operators are evaluated in expressions. Associatity determines whether an expression is evaluated left-to-right or right-to-left

Question: When a thread blocks on I/O, what state does it enter?

Answer: A thread enters the waiting state when it blocks on I/O.

Question: To what value is a variable of the String type automatically initialized?

Answer: The default value of an String type is null.

Question: What is the catch or declare rule for method declarations?

Answer: If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause.

Question: What is the difference between a MenuItem and a CheckboxMenuItem?

Answer: The CheckboxMenuItem class extends the MenuItem class to support a menu item that may be checked or unchecked.

Question: What is a task's priority and how is it used in scheduling?

Answer: A task's priority is an integer value that identifies the relative order in which it should be executed with respect to other tasks. The scheduler attempts to schedule higher priority tasks before lower priority tasks.

Question: What class is the top of the AWT event hierarchy?

Answer: The java.awt.AWTEvent class is the highest-level class in the AWT event-class hierarchy.

Question: When a thread is created and started, what is its initial state?

Answer: A thread is in the ready state after it has been created and started.

Question: Can an anonymous class be declared as implementing an interface and extending a class?

Answer: An anonymous class may implement an interface or extend a superclass, but may not be declared to do both.

Question: What is the range of the short type?

Answer: The range of the short type is -(2^15) to 2^15 - 1.

Question: What is the range of the char type?

Answer: The range of the char type is 0 to 2^16 - 1.

Question: In which package are most of the AWT events that support the event-delegation model defined?

Answer: Most of the AWT-related events of the event-delegation model are defined in the java.awt.event package. The AWTEvent class is defined in the java.awt package.

Question: What is the immediate superclass of Menu?

Answer: MenuItem

Question: What is the purpose of finalization?

Answer: The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.

Question: Which class is the immediate superclass of the MenuComponent class.

Answer: Object

Core Java

Question: What are different types of inner classes ?

Answer: Inner classes nest within other classes. A normal class is a direct member of a package. Inner classes, which became available with Java
1.1, are four types

* Static member classes
* Member classes
* Local classes
* Anonymous classes

Static member classes - a static member class is a static member of a class. Like any other static method, a static member class has access to all static methods of the parent, or top-level, class.

Member Classes - a member class is also defined as a member of a class. Unlike the static variety, the member class is instance specific and has access to any and all methods and members, even the parent's this reference.

Local Classes - Local Classes declared within a block of code and these classes are visible only within the block.

Anonymous Classes - These type of classes does not have any name and its like a local class
Java Anonymous Class Example public class SomeGUI extends JFrame { ... button member declarations ... protected void buildGUI() { button1 = new JButton(); button2 = new JButton(); ... button1.addActionListener( new java.awt.event.ActionListener() <------ Anonymous Class { public void actionPerformed(java.awt.event.ActionEvent e) { // do something } } );

Question: What are the uses of Serialization?

Answer: In some types of applications you have to write the code to serialize objects, but in many cases serialization is performed behind the scenes by various server-side containers.

These are some of the typical uses of serialization:

* To persist data for future use.
* To send data to a remote computer using such client/server Java technologies
as RMI or socket programming.
* To "flatten" an object into array of bytes in memory.
* To exchange data between applets and servlets.
* To store user session in Web applications.
* To activate/passivate enterprise java beans.
* To send objects between the servers in a cluster.

Question: what is a collection ?

Answer: Collection is a group of objects. java.util package provides important types of collections. There are two fundamental types of collections they are Collection and Map. Collection types hold a group of objects, Eg. Lists and Sets where as Map types hold group of objects as key, value pairs Eg. HashMap and Hashtable.

Question: For concatenation of strings, which method is good, StringBuffer or String ?

Answer: StringBuffer is faster than String for concatenation.

Question: What is Runnable interface ? Are there any other ways to make a java program as multithred java program?

Answer: There are two ways to create new kinds of threads:

- Define a new class that extends the Thread class
- Define a new class that implements the Runnable interface, and pass an object of that class to a Thread's constructor.
- An advantage of the second approach is that the new class can be a subclass of any class, not just of the Thread class.
Here is a very simple example just to illustrate how to use the second approach to creating threads: class myThread implements Runnable { public void run() { System.out.println("I'm running!"); } } public class tstRunnable { public static void main(String[] args) { myThread my1 = new myThread(); myThread my2 = new myThread(); new Thread(my1).start(); new Thread(my2).start(); }

Question: How can i tell what state a thread is in ?

Answer: Prior to Java 5, isAlive() was commonly used to test a threads state. If isAlive() returned false the thread was either new or terminated but there was simply no way to differentiate between the two.

Starting with the release of Tiger (Java 5) you can now get what state a thread is in by using the getState() method which returns an Enum of Thread.States. A thread can only be in one of the following states at a given point in time.

NEW


A Fresh thread that has not yet started to execute.

RUNNABLE


A thread that is executing in the Java virtual machine.

BLOCKED


A thread that is blocked waiting for a monitor lock.

WAITING


A thread that is wating to be notified by another thread.

TIMED_WAITING


A thread that is wating to be notified by another thread for a specific amount of time

TERMINATED


A thread whos run method has ended.
The folowing code prints out all thread states. public class ThreadStates{ public static void main(String[] args){ Thread t = new Thread(); Thread.State e = t.getState(); Thread.State[] ts = e.values(); for(int i = 0; i < ts.length; i++){ System.out.println(ts[i]); } } }

Question: What methods java providing for Thread communications ?

Answer: Java provides three methods that threads can use to communicate with each other: wait, notify, and notifyAll. These methods are defined for all Objects (not just Threads). The idea is that a method called by a thread may need to wait for some condition to be satisfied by another thread; in that case, it can call the wait method, which causes its thread to wait until another thread calls notify or notifyAll.

Question: What is the difference between notify and notify All methods ?

Answer: A call to notify causes at most one thread waiting on the same object to be notified (i.e., the object that calls notify must be the same as the object that called wait). A call to notifyAll causes all threads waiting on the same object to be notified. If more than one thread is waiting on that object, there is no way to control which of them is notified by a call to notify (so it is often better to use notifyAll than notify).

Question: What is synchronized keyword? In what situations you will Use it?

Answer: Synchronization is the act of serializing access to critical sections of code. We will use this keyword when we expect multiple threads to access/modify the same data. To understand synchronization we need to look into thread execution manner.

Threads may execute in a manner where their paths of execution are completely independent of each other. Neither thread depends upon the other for assistance. For example, one thread might execute a print job, while a second thread repaints a window. And then there are threads that require synchronization, the act of serializing access to critical sections of code, at various moments during their executions. For example, say that two threads need to send data packets over a single network connection. Each thread must be able to send its entire data packet before the other thread starts sending its data packet; otherwise, the data is scrambled. This scenario requires each thread to synchronize its access to the code that does the actual data-packet sending.

If you feel a method is very critical for business that needs to be executed by only one thread at a time (to prevent data loss or corruption), then we need to use synchronized keyword.

EXAMPLE

Some real-world tasks are better modeled by a program that uses threads than by a normal, sequential program. For example, consider a bank whose accounts can be accessed and updated by any of a number of automatic teller machines (ATMs). Each ATM could be a separate thread, responding to deposit and withdrawal requests from different users simultaneously. Of course, it would be important to make sure that two users did not access the same account simultaneously. This is done in Java using synchronization, which can be applied to individual methods, or to sequences of statements.

One or more methods of a class can be declared to be synchronized. When a thread calls an object's synchronized method, the whole object is locked. This means that if another thread tries to call any synchronized method of the same object, the call will block until the lock is released (which happens when the original call finishes). In general, if the value of a field of an object can be changed, then all methods that read or write that field should be synchronized to prevent two threads from trying to write the field at the same time, and to prevent one thread from reading the field while another thread is in the process of writing it.

Here is an example of a BankAccount class that uses synchronized methods to ensure that deposits and withdrawals cannot be performed simultaneously, and to ensure that the account balance cannot be read while either a deposit or a withdrawal is in progress. (To keep the example simple, no check is done to ensure that a withdrawal does not lead to a negative balance.)
public class BankAccount { private double balance; // constructor: set balance to given amount public BankAccount( double initialDeposit ) { balance = initialDeposit; } public synchronized double Balance( ) { return balance; } public synchronized void Deposit( double deposit ) { balance += deposit; } public synchronized void Withdraw( double withdrawal ) { balance -= withdrawal; } }

Note: that the BankAccount's constructor is not declared to be synchronized. That is because it can only be executed when the object is being created, and no other method can be called until that creation is finished.

There are cases where we need to synchronize a group of statements, we can do that using synchrozed statement.
Java Code Example synchronized ( B ) { if ( D > B.Balance() ) { ReportInsuffucientFunds(); } else { B.Withdraw( D ); } }

Question: What is serialization ?

Answer: Serialization is the process of writing complete state of java object into output stream, that stream can be file or byte array or stream associated with TCP/IP socket.

Question: What does the Serializable interface do ?

Answer: Serializable is a tagging interface; it prescribes no methods. It serves to assign the Serializable data type to the tagged class and to identify the class as one which the developer has designed for persistence. ObjectOutputStream serializes only those objects which implement this interface.

Question: How do I serialize an object to a file ?

Answer: To serialize an object into a stream perform the following actions:

- Open one of the output streams, for exaample FileOutputStream
- Chain it with the ObjectOutputStream - Call the method writeObject() providingg the instance of a Serializable object as an argument.
- Close the streams

Java Code --------- try{ fOut= new FileOutputStream("c:\\emp.ser"); out = new ObjectOutputStream(fOut); out.writeObject(employee); //serializing System.out.println("An employee is serialized into c:\\emp.ser"); } catch(IOException e){ e.printStackTrace(); }

Question: How do I deserilaize an Object?

Answer: To deserialize an object, perform the following steps:

- Open an input stream
- Chain it with the ObjectInputStream - Call the method readObject() and cast tthe returned object to the class that is being deserialized.
- Close the streams
Java Code try{ fIn= new FileInputStream("c:\\emp.ser"); in = new ObjectInputStream(fIn); //de-serializing employee Employee emp = (Employee) in.readObject(); System.out.println("Deserialized " + emp.fName + " " + emp.lName + " from emp.ser "); }catch(IOException e){ e.printStackTrace(); }catch(ClassNotFoundException e){ e.printStackTrace(); }

Question: What is Externalizable Interface ?

Answer : Externalizable interface is a subclass of Serializable. Java
provides Externalizable interface that gives you more control over what is being serialized and it can produce smaller object footprint. ( You can serialize whatever field values you want to serialize)

This interface defines 2 methods: readExternal() and writeExternal() and you have to implement these methods in the class that will be serialized. In these methods you'll have to write code that reads/writes only the values of the attributes you are interested in. Programs that perform serialization and deserialization have to write and read these attributes in the same sequence.

Question: Explain garbage collection ?

Answer: Garbage collection is an important part of Java's security
strategy. Garbage collection is also called automatic memory management as JVM automatically removes the unused variables/objects from the memory. The name "garbage collection" implies that objects that are no longer needed by the program are "garbage" and can be thrown away. A more accurate and up-to-date metaphor might be "memory recycling." When an object is no longer referenced by the program, the heap space it occupies must be recycled so that the space is available for subsequent new objects. The garbage collector must somehow determine which objects are no longer referenced by the program and make available the heap space occupied by such unreferenced objects. In the process of freeing unreferenced objects, the garbage collector must run any finalizers of objects being freed

Question : How you can force the garbage collection ?

Answer : Garbage collection automatic process and can't be forced. We can call garbage collector in Java by calling System.gc() and Runtime.gc(), JVM tries to recycle the unused objects, but there is no guarantee when all the objects will garbage collected.

Question : What are the field/method access levels (specifiers) and class access levels ?

Answer: Each field and method has an access level:

* private: accessible only in this class
* (package): accessible only in this package
* protected: accessible only in this package and in all subclasses of this class
* public: accessible everywhere this class is available

Similarly, each class has one of two possible access levels:

* (package): class objects can only be declared and manipulated by code in this package

public: class objects can be declared and manipulated by code in any package

Question: What are the static fields & static Methods ?

Answer: If a field or method defined as a static, there is only one copy for entire class, rather than one copy for each instance of class. static method cannot accecss non-static field or call non-static method

Example Java Code

static int counter = 0;

A public static field or method can be accessed from outside the class using either the usual notation:

Java-class-object.field-or-method-name

or using the class name instead of the name of the class object:

Java- class-name.field-or-method-name

Question: What are the Final fields & Final Methods ?

Answer: Fields and methods can also be declared final. A final method cannot be overridden in a subclass. A final field is like a constant: once it has been given a value, it cannot be assigned to again.

Java Code

private static final int MAXATTEMPTS = 10;

Question: Describe the wrapper classes in Java ?

Answer: Wrapper class is wrapper around a primitive data type. An instance of a wrapper class contains, or wraps, a primitive value of the corresponding type.

Following table lists the primitive types and the corresponding wrapper classes:

Primitive


Wrapper

boolean


java.lang.Boolean

byte


java.lang.Byte

char


java.lang.Character

double


java.lang.Double

float


java.lang.Float

int


java.lang.Integer

long


java.lang.Long

short


java.lang.Short

void


java.lang.Void

Question: What are different types of inner classes ?

Answer: Inner classes nest within other classes. A normal class is a direct member of a package. Inner classes, which became available with Java 1.1, are four types

* Static member classes
* Member classes
* Local classes
* Anonymous classes

Static member classes - a static member class is a static member of a class. Like any other static method, a static member class has access to all static methods of the parent, or top-level, class.

Member Classes - a member class is also defined as a member of a class. Unlike the static variety, the member class is instance specific and has access to any and all methods and members, even the parent's this reference.

Local Classes - Local Classes declared within a block of code and these classes are visible only within the block.

Anonymous Classes - These type of classes does not have any name and its like a local class
Java Anonymous Class Example public class SomeGUI extends JFrame { ... button member declarations ... protected void buildGUI() { button1 = new JButton(); button2 = new JButton(); ... button1.addActionListener( new java.awt.event.ActionListener() <------ Anonymous Class { public void actionPerformed(java.awt.event.ActionEvent e) { // do something } } );

Core Java

Question: How could Java classes direct program messages to the system console, but error messages, say to a file?

Answer: The class System has a variable out that represents the standard output, and the variable err that represents the standard error device. By default, they both point at the system console. This how the standard output could be re-directed:

Stream st = new Stream(new FileOutputStream("output.txt")); System.setErr(st); System.setOut(st);

Question: What's the difference between an interface and an abstract class?

Answer: An abstract class may contain code in method bodies, which is not allowed in an interface. With abstract classes, you have to inherit your class from it and Java does not allow multiple inheritance. On the other hand, you can implement multiple interfaces in your class.

Question: Why would you use a synchronized block vs. synchronized method?

Question: Does it matter in what order catch statements for FileNotFoundException and IOExceptipon are written?

Answer: Yes, it does. The FileNoFoundException is inherited from the IOException. Exception's subclasses have to be caught first.

Question: Can an inner class declared inside of a method access local variables of this method?

Answer: It's possible if these variables are final.

Question: What can go wrong if you replace && with & in the following code: String a=null; if (a!=null && a.length()>10) {...}

Answer: A single ampersand here would lead to a NullPointerException.

Question: What's the main difference between a Vector and an ArrayList

Answer: Java Vector class is internally synchronized and ArrayList is not.

Question: When should the method invokeLater()be used?

Answer: This method is used to ensure that Swing components are updated through the event-dispatching thread.

Question: How can a subclass call a method or a constructor defined in a superclass?

Answer: Use the following syntax: super.myMethod(); To call a constructor of the superclass, just write super(); in the first line of the subclass's constructor.

For senior-level developers:

Question: What's the difference between a queue and a stack?

Answer: Stacks works by last-in-first-out rule (LIFO), while queues use the FIFO rule

Question: You can create an abstract class that contains only abstract methods. On the other hand, you can create an interface that declares the same methods. So can you use abstract classes instead of interfaces?

Answer: Sometimes. But your class may be a descendent of another class and in this case the interface is your only option.

Question: What comes to mind when you hear about a young generation in Java?

Answer: Garbage collection.

Question: What comes to mind when someone mentions a shallow copy in Java?

Answer: Object cloning.

Question: If you're overriding the method equals() of an object, which other method you might also consider?

Answer: hashCode()

Question: You are planning to do an indexed search in a list of objects. Which of the two Java collections should you use: ArrayList or LinkedList?

Answer: ArrayList

Question: How would you make a copy of an entire Java object with its state?

Answer: Have this class implement Cloneable interface and call its method clone().

Question: How can you minimize the need of garbage collection and make the memory use more effective?

Answer: Use object pooling and weak object references.

Question: There are two classes: A and B. The class B need to inform a class A when some important event has happened. What Java technique would you use to implement it?

Answer: If these classes are threads I'd consider notify() or notifyAll(). For regular classes you can use the Observer interface.

Question: What access level do you need to specify in the class declaration to ensure that only classes from the same directory can access it?
Answer: You do not need to specify any access level, and Java will use a default package access level

Question: When you declare a method as abstract method ?

Answer: When i want child class to implement the behavior of the method.

Question: Can I call a abstract method from a non abstract method ?

Answer: Yes, We can call a abstract method from a Non abstract method in a Java abstract class

Question: What is the difference between an Abstract class and Interface in Java ? or can you explain when you use Abstract classes ?

Answer: Abstract classes let you define some behaviors; they force your subclasses to provide others. These abstract classes will provide the basic funcationality of your applicatoin, child class which inherited this class will provide the funtionality of the abstract methods in abstract class. When base class calls this method, Java calls the method defined by the child class.

* An Interface can only declare constants and instance methods, but cannot implement default behavior.
* Interfaces provide a form of multiple inheritance. A class can extend only one other class.
* Interfaces are limited to public methods and constants with no implementation. Abstract classes can have a partial implementation, protected parts, static methods, etc.
* A Class may implement several interfaces. But in case of abstract class, a class may extend only one abstract class.
* Interfaces are slow as it requires extra indirection to find corresponding method in the actual class. Abstract classes are fast.

Question: What is user-defined exception in java ?

Answer: User-defined expections are the exceptions defined by the application developer which are errors related to specific application. Application Developer can define the user defined exception by inherite the Exception class as shown below. Using this class we can throw new exceptions.
Java Example : public class noFundException extends Exception { } Throw an exception using a throw statement: public class Fund { ... public Object getFunds() throws noFundException { if (Empty()) throw new noFundException(); ... } } User-defined exceptions should usually be checked.


Answer: Synchronized blocks place locks for shorter periods than synchronized methods.

Question: Explain the usage of the keyword transient?

Answer: This keyword indicates that the value of this member variable does not have to be serialized with the object. When the class will be de-serialized, this variable will be initialized with a default value of its data type (i.e. zero for integers).

Question: How can you force garbage collection?

Answer: You can't force GC, but could request it by calling System.gc(). JVM does not guarantee that GC will be started immediately.


Question: How do you know if an explicit object casting is needed?

Answer: If you assign a superclass object to a variable of a subclass's data type, you need to do explicit casting. For example:

Object a; Customer b; b = (Customer) a;

When you assign a subclass to a variable having a supeclass type, the casting is performed automatically.

Question: What's the difference between the methods sleep() and wait()

Answer: The code sleep(1000); puts thread aside for exactly one second. The code wait(1000), causes a wait of up to one second. A thread could stop waiting earlier if it receives the notify() or notifyAll() call. The method wait() is defined in the class Object and the method sleep() is defined in the class Thread.

Question: Can you write a Java class that could be used both as an applet as well as an application?

Answer: Yes. Add a main() method to the applet.

Question: What's the difference between constructors and other methods?

Answer: Constructors must have the same name as the class and can not return a value. They are only called once while regular methods could be called many times.

Question: Can you call one constructor from another if a class has multiple constructors

Answer: Yes. Use this() syntax.

Question: Explain the usage of Java packages.

Answer: This is a way to organize files when a project consists of multiple modules. It also helps resolve naming conflicts when different packages have classes with the same names. Packages access level also allows you to protect data from being used by the non-authorized classes.

Question: If a class is located in a package, what do you need to change in the OS environment to be able to use it?

Answer: You need to add a directory or a jar file that contains the package directories to the CLASSPATH environment variable. Let's say a class Employee belongs to a package com.xyz.hr; and is located in the file c:\dev\com\xyz\hr\Employee.java. In this case, you'd need to add c:\dev to the variable CLASSPATH. If this class contains the method main(), you could test it from a command prompt window as follows:

c:\>java com.xyz.hr.Employee

Question: What's the difference between J2SDK 1.5 and J2SDK 5.0?

Answer: There's no difference, Sun Microsystems just re-branded this version.

Question: What would you use to compare two String variables - the operator == or the method equals()?
Answer: I'd use the method equals() to compare the values of the Strings and the == to check if two variables point at the same instance of a String object.

Question: What is the difference between checked and Unchecked Exceptions in Java ?

Answer: All predefined exceptions in Java are either a checked exception or an unchecked exception. Checked exceptions must be caught using try .. catch() block or we should throw the exception using throws clause. If you dont, compilation of program will fail.
Java Exception Hierarchy +--------+ | Object | +--------+ | | +-----------+ | Throwable | +-----------+ / \ / \ +-------+ +-----------+ | Error | | Exception | +-------+ +-----------+ / | \ / | \ \________/ \______/ \ +------------------+ unchecked checked | RuntimeException | +------------------+ / | | \ \_________________/ unchecked

Question: Explain garbage collection ?

Answer: Garbage collection is an important part of Java's security
strategy. Garbage collection is also called automatic memory management as JVM automatically removes the unused variables/objects from the memory. The name "garbage collection" implies that objects that are no longer needed by the program are "garbage" and can be thrown away. A more accurate and up-to-date metaphor might be "memory recycling." When an object is no longer referenced by the program, the heap space it occupies must be recycled so that the space is available for subsequent new objects. The garbage collector must somehow determine which objects are no longer referenced by the program and make available the heap space occupied by such unreferenced objects. In the process of freeing unreferenced objects, the garbage collector must run any finalizers of objects being freed.

Question: How you can force the garbage collection ?

Answer: Garbage collection automatic process and can't be forced. We can call garbage collector in Java by calling System.gc() and Runtime.gc(), JVM tries to recycle the unused objects, but there is no guarantee when all the objects will garbage collected.

Question: What are the field/method access levels (specifiers) and class access levels ?

Answer: Each field and method has an access level:

* private: accessible only in this class
* (package): accessible only in this package
* protected: accessible only in this package and in all subclasses of this class
* public: accessible everywhere this class is available

Similarly, each class has one of two possible access levels:

* (package): class objects can only be declared and manipulated by code in this package
* public: class objects can be declared and manipulated by code in any package

Question: What are the static fields & static Methods ?

Answer: If a field or method defined as a static, there is only one copy for entire class, rather than one copy for each instance of class. static method cannot accecss non-static field or call non-static method

Example Java Code

static int counter = 0;

A public static field or method can be accessed from outside the class using either the usual notation:

Java-class-object.field-or-method-name

or using the class name instead of the name of the class object:

Java- class-name.field-or-method-name

Question: What are the Final fields & Final Methods ?

Answer: Fields and methods can also be declared final. A final method cannot be overridden in a subclass. A final field is like a constant: once it has been given a value, it cannot be assigned to again.

Java Code

private static final int MAXATTEMPTS = 10;

Question: Describe the wrapper classes in Java ?

Answer: Wrapper class is wrapper around a primitive data type. An instance of a wrapper class contains, or wraps, a primitive value of the corresponding type.

Following table lists the primitive types and the corresponding wrapper classes:

Primitive


Wrapper

boolean


java.lang.Boolean

byte


java.lang.Byte

char


java.lang.Character

double


java.lang.Double

float


java.lang.Float

int


java.lang.Integer

long


java.lang.Long

short


java.lang.Short

void


java.lang.Void

SCJP-WRAPPER

Wrapper Class Summary - Methods of interest for SCJP
Class Name parseXXX getXXX toString toHexString
Boolean static boolean getBoolean(String) String toString()
static String toString(boolean)*
Character String toString()
static String toString(char)*
Byte static byte parseByte(String)
static byte parseByte(String, int) String toString()
static String toString(byte)
Short static short parseShort(String)
static short parseShort(String, int) toString()
static String toString(short)
Integer static int parseInt(String)
static int parseInt(String, int) static Integer getInteger(String)
static Integer getInteger(String, int)
static Integer getInteger(String, Integer) String toString()
static String toString(int)
static String toString(int, int) static String toHexString(int)
Long static long parseLong(String)
static long parseLong(String, int) static Long getLong(String)
static Long getLong(String, long)
static Long getLong(String, Long) String toString()
static String toString(long)
static String toString(long, int) static String toHexString(long)
Float static float parseFloat(String) String toString()
static String toString(float)
Double static double parseDouble(String) String toString()
static String toString(double)
* indicates that the method is new to JDK 1.4

.equals() for Wrapper classes

So how about Wrapper classes - when are they "equal"? In particular, we want to focus on the .equals method. The Sun Certified Programmer for the Java 2 Platform 1.4 Exam state that you must know the following methods
doubleValue, floatValue, intValue, longValue, parseXxx, getXxx, toString, and toHexString.
Ah, but where is the equals() method you might ask? It's not there, so I don't need to worry about it! That would be wrong! There are several items of confluence for the SCP for Java 2 Platform 1.4 - we previously had .equals() required knowledge based on other objectives. Second, the new hashCode method objective has the hashCode() and equals() methods of utmost importance. Even if the equals() method is not covered, knowing how to test the equality of two Wrapper classes is an important interview question or on the job scenario.
Let us use the Float and Double classes as examples. A value of 5.0 wrapped by a Double and a value 5.0 wrapped by a Float are the same, no? After all, is not 5.0 the same as 5.0? The answer is no when defined by the equals() method.
Let's take a look at the following code snippet:
Double aDouble = new Double (5.0);
Float aFloat = new Float(5.0);
if ( aFloat.equals(aDouble) )
System.out.println("equals");
The result is true if and only if the argument is not null and is a Float object that represents a float that has the identical bit pattern to the bit pattern of the float represented by this object. For this purpose, two float values are considered to be the same if and only if the method floatToIntBits(float) returns the same int value when applied to each. Note that in most cases, for two instances of class Float, f1 and f2, the value of f1.equals(f2) is true if and only if
f1.floatValue() == f2.floatValue()
also has the value true.
Thus, in the above code snippet, the aDouble object needs to be of type Float, not Double, to be considered "equal" by the equals() method.
There are two exceptions for the Float equals() method:
 If f1 and f2 both represent Float.NaN, then the equals method returns true, even though Float.NaN==Float.NaN has the value false.
 If f1 represents +0.0f while f2 represents -0.0f, or vice versa, the equal test has the value false, even though 0.0f==-0.0f has the value true. This definition allows hashtables to operate properly.

toHexString
The toHexString method does approximately what you expect in that it returns a string which is a hex string version of the number. It has a natural partner in the toBinaryString method which returns a string that represents the number in its binary version. The following example code will output the strings 100 followed by 10.
public class NumberFormats{
public static void main(String argv[]){
System.out.println(Integer.toBinaryString(4));
System.out.println(Integer.toHexString(16));

}
}

• Wrapper class names differ from the primitives only in the initial upper case letter.
Exceptions are Integer, which wraps int and Character, which wraps char values.
• All wrapper objects can be constructed by passing a string EXCEPT Character wrapper class for char data type. Most of these constructor throws NumberFormatException, which is a runtime exception.
• Wrapper classes override equals() method. equals() on wrappers return false if both the objects are not instances of the same class. This statement is true even when the value they are wrapping has the same numerical value.
• All wrapper objects are immutable. Once an object is created, the wrapped primitive value cannot be changed.
• Wrapper classes are final and hence cannot be sub classed.
• Byte, Double, Float, Integer and Short extend the abstract Number class
• all are public final ie cannot be extended
• get around limitations of primitive types
• allow objects to be created from primitive types
• all the classes have two constructor forms
• a constructor that takes the primitive type and creates an object eg Character(char), Integer(int)
• a constructor that converts a String into an object eg Integer("1"). Throws a NumberFormatException if the String cannot be converted to a number
Note
• The Character class does not have a constructor that takes a String argument
• all, except Character, have a valueOf(String s) method which is equivalent to new Type(String s)
• all have a typeValue() method which returns the value of the object as it's primitive type. These are all abstract methods defined in Number and overridden in each class
• public byte byteValue()
• public short shortValue()
• public int intValue()
• public long longValue()
• public float floatValue()
• public double doubleValue()
• all the classes override equals(), hashCode() and toString() in Object
• equals() returns true if the values of the compared objects are the same
• hashCode() returns the same hashcode for objects of the same type having the same value
• toString() returns the string representation of the objects value
• all have a public static final TYPE field which is the Class object for that primitive type
• all have two static fields MIN_VALUE and MAX_VALUE for the minimum and maximum values that can be held by the type
Void
• There is also a wrapper class for Void which cannot be instantiated.
• The constructors and methods described above do NOT exist for the Void class although it does have the TYPE field.
Character
• contains two methods for returning the numeric value of a character in the various number systems
• public static int digit(char ch, int radix)
• public static int getNumber(char ch)
• and one method to return the character value of a number
• public static char forDigit(int digit, int radix)
• has two case conversion methods
• public static char toLowerCase(char ch)
• public static char toUpperCase(char ch)
• also contains a variety of other methods to test whether a character is of a specific type eg isLetter(), isDefined(), isSpaceChar(), etc
• getType() returns an int that defines a character's Unicode type
Integer, Short, Byte and Long
• all have parseType methods e.g. parseInt(), parseShort, etc that take a String and parse it into the appropriate type
• the Integer and Long classes also have the static methods toBinaryString(), toOctalString() and toHexString() which take an integer value and convert it to the appropriate String representation
Float and Double
• both classes have static fields which define POSITIVE_INFINITY, NEGATIVE_INFINITY, and NaN
• and the following methods to test a value
• public boolean isNan()
• public static boolean isNaN(type value)
• public boolean isInfinite()
• public static boolean isInfinite(type value)
• Float also has a constructor that takes a double value
• both classes have methods to convert a value into a bit pattern or vice versa
• public static int floatToIntBits(float value)
• public static float intBitsToFloat(int bits)
• public static long doubleToLongBits(double value)
• public static double longBitsToDouble(long bits)
parseInt
public static int parseInt(String s) throws NumberFormatException
 Parses the string argument as a signed decimal integer.
 The characters in the string must all be decimal digits, except that the first character may be an ASCII minus sign '-' ('\u002D') to indicate a negative value.
 The resulting integer value is returned, exactly as if the argument and the radix 10 were given as arguments to the parseInt(java.lang.String, int) method

public static int parseInt(String s, int radix) throws NumberFormatException
 Parses the string argument as a signed integer in the radix specified by the second argument.
 The characters in the string must all be digits of the specified radix (as determined by whether Character.digit(char, int) returns a nonnegative value), except that the first character may be an ASCII minus sign '-' ('\u002D') to indicate a negative value.
 The resulting integer value is returned.
An exception of type NumberFormatException (during RUNTIME )is thrown if any of the following situations occurs:
• The first argument is null or is a string of length zero.
• The radix is either smaller than Character.MIN_RADIX or larger than Character.MAX_RADIX. ( radix >= 2 , <=36 )
• Any character of the string is not a digit of the specified radix, except that the first character may be a minus sign '-' ('\u002D') provided that the string is longer than length 1.
• The value represented by the string is not a value of type int.
Examples:
parseInt("0", 10) returns 0
parseInt("473", 10) returns 473
parseInt("-0", 10) returns 0
parseInt("-FF", 16) returns -255
parseInt("1100110", 2) returns 102
parseInt("2147483647", 10) returns 2147483647
parseInt("-2147483648", 10) returns -2147483648
parseInt("2147483648", 10) throws a NumberFormatException // size is more than integer
parseInt("99", 8) throws a NumberFormatException // 9 is not there in octal system
parseInt("99", 37) throws a NumberFormatException // radix is > 36
parseInt("99", 1) throws a NumberFormatException // radix is <2
parseInt("Kona", 10) throws a NumberFormatException
parseInt("Kona", 27) returns 411787 // please note this..
getBoolean
public static boolean getBoolean(String name)
 Returns true if and only if the system property named by the argument exists and is equal to the string "true".
 (Beginning with version 1.0.2 of the JavaTM platform, the test of this string is case insensitive.)
 A system property is accessible through getProperty, a method defined by the System class.
 If there is no property with the specified name, or if the specified name is empty or null, then false is returned.
 Parameters: name - the system property name.
 Returns: the boolean value of the system property.
getInteger
 public static Integer getInteger(String nm, int val)
 Determines the integer value of the system property with the specified name.
 The first argument is treated as the name of a system property. System properties are accessible through the System.getProperty(java.lang.String) method. The string value of this property is then interpreted as an integer value and an Integer object representing this value is returned. Details of possible numeric formats can be found with the definition of getProperty.
 The second argument is the default value. An Integer object that represents the value of the second argument is returned if there is no property of the specified name, if the property does not have the correct numeric format, or if the specified name is empty or null.
 Parameters: nm - property name. val - default value.
 Returns: the Integer value of the property.



===============IMP POINTS FROM JAVARANCH==================================

 Boolean b1 = new Boolean("123");
Boolean b2 = new Boolean("123");
b1.toString() == b2.toString() // - Returns true
 Byte by = new Byte(“123”)
by.toString() == by.toString() // - Returns false . Same for other wrapper classes except boolean
Byte class ( and Other wrappers except Boolean )
----------
.toString() method creates a new String object and returns this every time you call this method.
Boolean class
-------------
.toString() method returns a String literal "true" or "false" depending upon the value of this Boolean object. This String literal will be in String pool and every time you call, the same ref. value (memory address) is returned
 Primitive type wrapper classes
a)allow primitive types to behave as reference types.
Some loose language here but the intention is TRUE, IMO.
b)allow operator overloading to be implemented for primitive numeric types.
NO!!
c)provide string parsing and conversion methods
YES as in Integer.parseInt(String s)
d)provide arithmetic operators such as '+','-'
NO!!
 Float f1 = new Float("10.0f"); //fine
Double f2 = new Double("10.0d"); // fine
Long f2 = new Long("10L"); //  NumberFormatException at RUNTIME
 "Neither L nor l is permitted to appear at the end of the string as a type indicator, as would be permitted in Java programming language source code".

SCJP-THREADS

Threads Important Points from Whizlabs:

 If class A extends Thread class and doesn't override the run method, calling new A ().start  No compile and run time errors. Thread class run is called, which does nothing.
 The run method can be overloaded. But the run method with
o public void run () {} is called when the start () is called on the thread.
 Sleep method must be enclosed in try-catch blocks as it throws Interrupted Exception. Same is the case with wait and join methods also. Otherwise it gives compile time error
 MyTherad mt = new MyThread (); mt.run ()  calls run method like any other method. It doesn’t throw compile or runtime errors.
 Every Java Thread is a member of a Thread group.  If you do not specify the group of a thread, it will be a member of the group of the thread that started it, or the member of the current thread’s group. So if you start a thread from main thread without specifying the thread group, this thread will be member of the main thread group.
 A group of threads can’t be assigned a priority applicable for all the threads of that group. In other words there is no setPriority () method in ThreadGroup class. Rather a ThradGroup can be assigned the maximum priority by calling the setMaxPriority () method indicating that no thread of that group can have more priority than the set value of the group.
 A thread group can contain any number of threads.
 You can’t differentiate between a new Thread and a dead thread because calling isAlive() method in both the situations will return false. Method isAlive() returns true if a thread has been started but not yet died i.e. a thread is either runnable, running or non-running state.
 If a Thread is assigned a priority more than Thread.MAX_PRIORITY, it will compile fine but it will throw IllegalArgumentException during runtime.
 If a thread’s priority is set within range of Thread.MIN_PRIORITY and Thread.MAX_PRIORITY but higher than the Thread Group’s Max Priority, it will compile fine but at run time the scheduling system will assign the thread’s priority the is lower of the two priorities.
 Thread may share data between one another
 wait(), suspend(), stop(), sleep()  will definitely stop a thread from executing
 yield(), setPriority(), notify(), notifyAll()  may not stop a running thread
 The yield() method is used to call from the currently running thread to allow another thread of the same or higher priority to run. It is not guaranteed that the Thread that grabs the CPU will be of the higher priority. It will depend on the underlying OS or the threading algorithm of the JVM
 A subclass decides whether an inherited method should be marked synchronized or not. A synchronized method can be inherited to be non-synchronized or vice versa.
 Synchronized methods can call methods which are not synchronized; there is no restriction on it.
 Static methods can be synchronized; in this case the lock is obtained on the Class object of the class.
 Directly sub classing Thread class does NOT give access to more functionality than using the Runnable interface
 It is very important to remember that synchronized modifier can NOT be applied to constructor; else it will give compiler error. However block synchronization can be used within the constructors.
 MyThread implements Runnable; if Mythread doesn’t implement the run method correctly, it will give compile error.
 Thread type (Daemon thread / User Thread) can be changed by calling setDaemon(boolean), as many times you wish BUT only before the thread has been started. Attempt to change the thread type after start(), will compile fine but will throw IllegalThreadStateException when run, which is an indirect subclass of RuntimeException.
 Thread name doesn’t need to be unique, so two threads with the same name can execute separately.
 public class Thread extends Object implements Runnable
 A thread is a thread of execution in a program.
 The Java Virtual Machine allows an application to have multiple threads of execution running concurrently.
 Every thread has a priority.
 Threads with higher priority are executed in preference to threads with lower priority.
 Each thread may or may not also be marked as a daemon.
 When code running in some thread creates a new Thread object, the new thread has its priority initially set equal to the priority of the creating thread, and is a daemon thread if and only if the creating thread is a daemon.
 When a Java Virtual Machine starts up, there is usually a single non-daemon thread (which typically calls the method named main of some designated class).
 The Java Virtual Machine continues to execute threads until either of the following occurs:
o The exit method of class Runtime has been called and the security manager has permitted the exit operation to take place.
o All threads that are not daemon threads have died, either by returning from the call to the run method or by throwing an exception that propagates beyond the run method.
 Every thread has a name for identification purposes. More than one thread may have the same name. If a name is not specified when a thread is created, a new name is generated for it.

public interface Runnable:
 The Runnable interface should be implemented by any class whose instances are intended to be executed by a thread.
 The class must define a method of no arguments called run.
 This interface is designed to provide a common protocol for objects that wish to execute code while they are active. For example, Runnable is implemented by class Thread.
 Being active simply means that a thread has been started and has not yet been stopped.
 In addition, Runnable provides the means for a class to be active while not subclassing Thread.
 A class that implements Runnable can run without subclassing Thread by instantiating a Thread instance and passing itself in as the target.
 In most cases, the Runnable interface should be used if you are only planning to override the run() method and no other Thread methods. This is important because classes should not be subclassed unless the programmer intends on modifying or enhancing the fundamental behavior of the class.
Method Description
public void run() When an object implementing interface Runnable is used to create a thread, starting the thread causes the object's run method to be called in that separately executing thread
The general contract of the method run is that it may take any action whatsoever.

Methods of Thread Class:
Method Signature Description
public static Thread currentThread() Returns a reference to the currently executing thread object.
public static void yield() Causes the currently executing thread object to temporarily pause and allow other threads to execute.
public static void sleep(long millis) throws InterruptedException Causes the currently executing thread to sleep (temporarily cease execution) for the specified number of milliseconds. The thread does not lose ownership of any monitors.
InterruptedException - if another thread has interrupted the current thread. The interrupted status of the current thread is cleared when this exception is thrown.
public static void sleep(long millis, int nanos) throws InterruptedException Causes the currently executing thread to sleep (cease execution) for the specified number of milliseconds plus the specified number of nanoseconds. The thread does not lose ownership of any monitors.
IllegalArgumentException - if the value of millis is negative or the value of nanos is not in the range 0-999999.
public void start() Causes this thread to begin execution; the Java Virtual Machine calls the run method of this thread.
The result is that two threads are running concurrently: the current thread (which returns from the call to the start method) and the other thread (which executes its run method).
IllegalThreadStateException - if the thread was already started.
public void run() If this thread was constructed using a separate Runnable run object, then that Runnable object's run method is called; otherwise, this method does nothing and returns.
Subclasses of Thread should override this method.
public final void stop() Forces the thread to stop executing.
It is permitted to stop a thread that has not yet been started. If the thread is eventually started, it immediately terminates.
public void interrupt() Interrupts this thread.
First the checkAccess method of this thread is invoked, which may cause a SecurityException to be thrown.
If this thread is blocked in an invocation of the wait(), wait(long), or wait(long, int) methods of the Object class, or of the join(), join(long), join(long, int), sleep(long), or sleep(long, int), methods of this class, then its interrupt status will be cleared and it will receive an InterruptedException.
If this thread is blocked in an I/O operation upon an interruptible channel then the channel will be closed, the thread's interrupt status will be set, and the thread will receive a ClosedByInterruptException.
If this thread is blocked in a Selector then the thread's interrupt status will be set and it will return immediately from the selection operation, possibly with a non-zero value, just as if the selector's wakeup method were invoked.
If none of the previous conditions hold then this thread's interrupt status will be set.
public static boolean interrupted() Tests whether the current thread has been interrupted. The interrupted status of the thread is cleared by this method. In other words, if this method were to be called twice in succession, the second call would return false (unless the current thread were interrupted again, after the first call had cleared its interrupted status and before the second call had examined it).
Returns: true if the current thread has been interrupted; false otherwise.
public boolean isInterrupted() Tests whether this thread has been interrupted. The interrupted status of the thread is unaffected by this method.
Returns: true if this thread has been interrupted; false otherwise.
public void destroy() Destroys this thread, without any cleanup. Any monitors it has locked remain locked. (This method is not implemented.)
public final boolean isAlive() Tests if this thread is alive. A thread is alive if it has been started and has not yet died.
public final void setPriority(int newPriority) Changes the priority of this thread.
First the checkAccess method of this thread is called with no arguments. This may result in throwing a SecurityException.
Otherwise, the priority of this thread is set to the smaller of the specified newPriority and the maximum permitted priority of the thread's thread group.
Throws: IllegalArgumentException - If the priority is not in the range MIN_PRIORITY to MAX_PRIORITY.
SecurityException - if the current thread cannot modify this thread.
public final int getPriority() Returns this thread's priority.
public final void setName(String name) Changes the name of this thread to be equal to the argument name.
public final String getName() Returns this thread's name.
public final ThreadGroup getThreadGroup() Returns the thread group to which this thread belongs. This method returns null if this thread has died (been stopped).
public static int activeCount() Returns the number of active threads in the current thread's thread group.
public int countStackFrames() Deprecated
public final void join(long millis) throws InterruptedException Waits at most millis milliseconds for this thread to die. A timeout of 0 means to wait forever.
public final void join(long millis, int nanos) throws InterruptedException Waits at most millis milliseconds plus nanos nanoseconds for this thread to die.
IllegalArgumentException - if the value of millis is negative the value of nanos is not in the range 0-999999.
public final void join() throws InterruptedException Waits for this thread to die.
public static void dumpStack() Prints a stack trace of the current thread. This method is used only for debugging.
public final void setDaemon(boolean on) Marks this thread as either a daemon thread or a user thread. The Java Virtual Machine exits when the only threads running are all daemon threads.
This method must be called before the thread is started.
This method first calls the checkAccess method of this thread with no arguments. This may result in throwing a SecurityException (in the current thread).
Parameters:
on - if true, marks this thread as a daemon thread.
Throws: IllegalThreadStateException - if this thread is active.

public final boolean isDaemon() Tests if this thread is a daemon thread.
public final void checkAccess() Determines if the currently running thread has permission to modify this thread.
public String toString() Returns a string representation of this thread, including the thread's name, priority, and thread group.
public static boolean holdsLock(Object obj) Returns true if and only if the current thread holds the monitor lock on the specified object.
This method is designed to allow a program to assert that the current thread already holds a specified lock: assert Thread.holdsLock(obj);
NullPointerException - if obj is null


Kai’s Notes:

(1) Stop a thread

Know what methods you can call to interrupt or suspend a thread.
Know which classes these methods reside in and whether they are staticor not.

* yield() and sleep() of Thread class are static.
* start(), run() and interrupt() of Thread class are non-static.
* wait(), notify() and notifyAll() of Object class are non-static.

When will the execution of a Thread eventually stop?
x a) In case another Thread with higher priority thread enters the runnable state.
x b) Through a call to suspend()
x c) Through a call to wait()
x d) Through a call to MediaTracker.waitforID()
e) If the current thread starts another thread.
I think the answer are a,b,c if you considering that stop means coming
back to ready state. What about choice d.

Reply#1: Choice d is also correct, because you can use MediaTracker to load one or more images and to wait until those images have been completely loaded and are ready to be used. So when waitForID() is called, the execution of the thread stops until all the images have been loaded. This is correct because I got this question in the exam, and I got 100% in threads.

Reply #2: One of the cloudy issues that I did not like about the test was the ambiguity of the word 'stop' on the test: do they mean entering the stop state or do they mean 'the thread ceases to execute, but can run in the future'. This question and the test will refer to the latter definition. Therefore:
(a) wrong, because of pre-emptive and time sharing OS differences.
(b) true, suspend() puts you in the blocked state so answer is true, BUT suspend is deprecated in java 1.2 so this should not be given as an alternative.
(c) true, wait() causes the thread execution to stop and enter the runnable queue for its time out period or being notified and obtaining a lock.
(d) false, MediaTracker works to keep track of image loading so the current thread can keep on doing whatever it does.
(e) false, how does a current thread start another thread? Yield() will cease the execution of the current thread to give another thread in the runnable queue a chance to run, and start() will put a newly created thread into the runnable queue, but nothing other than the OS can force
a thread to run.

Reply #3: These are the answers based on both the assumptions.
Assumption 1: Stop means the thread cannot be run again. In this case,
none of the answers are correct. Assumption 2: Stop means temporary
suspension of thread. In this case, a,b,c,d are correct.
a is correct, because the policy is governed by the underlying OS.
b is correct, coz u can call a suspend method to stop the thread temporarily.
c is correct, coz the thread releases the monitor and waits for another thread to issue a notify.(refer to JDK docs or API)
d is correct, coz the current thread waits until the mediatracker's done with its work.
e is false, coz starting another thread does not stop the currentthread.

Which methods or cases may cause a thread to stop executing:

x wait()

x sleep()

x suspend()

x stop()

x MediaTracker.waitForID() // Something about loading a graphic
image of MediaTracker class

x Interrupted()

x The program exits via a call to exit(0)

- If the current thread starts a new thread.

x In case another thread with higher priority enters the
runnable state.

Are we supposed to pick stop() and suspend(), because they are
deprecated? I didn't know about this question, but interrupting can't
stop a thread, it can only wake up sleeping or waiting thread.


Jyoti's notes: Conditions that might prevent a thread from executing :


x The thread is not the highest priority thread and so cannot get CPU time.

x The thread is waiting on a condition because someone invoked wait() for the thread.

x The thread has explicitly yielded control by invoking yield() to allow another thread of the same priority to run.

x The thread has been put to sleep using the sleep() method

x Someone has suspended the thread using the suspend() method. (deprecated in Java 2)

x It is blocked for file I/O

x There is more than one thread with the same highest priority and JVM is switching between these threads, at the moment, the thread in question is awaiting CPU time.

Does MediaTracker.waitForID() block the current thread?

Ans: If it didn't block the current thread, it could not do its job, right? :-) The objective is to WAIT UNTIL something happens. Of course, it doesn't stop the separate thread generated by the AWT to load the image. Or any other thread but its own. Also, the requirement that it be used in a "try/catch (InterruptedException e)" block suggests the same.

(2) Theory
The thread questions were tenuous and some took the same form as the garbage collection questions.

Threads is more theoretical.

Which is true about Threads:
A. If suspended, cannot be restarted.
B. Stop running, if another Thread with higher priority is getting runnable.
C. Stop running, if the Thread starts another Thread.
D. If dead, cannot be restarted.
E. If interrupted, stop running.
F. .. I cannot remember. :(

I choose B,D,E. (a classic question, but I had 2-3 question about threads in this style)

Is it true or false, i would appreciate the answer?

A) Java Interpreter exits normally after the non-daemon threads have stopped. (True?)

B) Current thread suspends, when a file is opened for reading. (True?)

a) A thread can stop execution, if it was performing an I/O task with a remote file. (True?)

b) It is possible to suspend a thread indefinitely. (?)

c) Only way to make a thread is to extend the Thread class. (False)

d) After the thread exit main(), it can still run. (related to garbage collection) (?)

When, exactly, does a Java application/applet stop execution?
Does it stop execution when there are no more Daemon threads running?
What is a Daemon thread? Does it stop execution when there are no more User threads?


It keeps running until all user threads have ended. Here is the definition of daemon thread from Bruce Eckels TIJ. A "daemon" thread is one that is supposed to provide a general service in the background as long as the program is running, but is not part of the essence of the program. Thus, when all of the non-daemon threads complete the program is terminated.

What's daemon or non-daemon threads?

Daemon threads are utility threads to service regular non-daemon threads. In Java, their only reason of being is "service to others". If ONLY daemon threads are left in a JVM, the program exits. I guess an implication would be that if you create some daemon threads in your program, you don't have to worry about killing them since they will die when the 'real' threads are through executing.... Any good book and/or chapter and/or on-line tutorial on Java threads should cover this.

(3) When to use "synchronize" keyword

What modifier to use, if a method needs to protect the lock of its object.

A. public
B. protected
C. private
D. final
E. synchronized

Threads, there were couple of questions on synchronized.

Synchronization was asked.

Which modifier can be used together with 'synchronized'?

(4) Implement Runnable and start a thread

Post from Maha Anna:

A thread stops permanently becoz of
1. finishes it 's run method --YES
2. interrupt is invoked --- NO
3. stop --is invoked but since it is –YES deprecated it is no longer in use. should we use if given in exam
Thread stops executing for a while becoz of
1. sleep -YES
2. I/O Operations YES
3. wait --YES
4. yield --YES
5. suspend --deprecated??? should we use if given in exam --YES
6. higher priority thread enters --YES
7. if a thread creates a new thread. --NO
When an Intertrupt is invoked, the waiting, blocked or sleeping threads moves to "ready" state and then throws interrupted exception
I think for both 6 and 7 the answer is maybe. Anytime other threads are present, this may cause the current thread to stop executing temporarily, but there are no guarantees. If another thread is higher priority than the current thread, it's more likely that the current thread will pause, but still no guarantees.
For the options
6. higher priority thread enters --YES
7. if a thread creates a new thread. --NO
I also thought like you. But from the Exam point of view (only yes or no, if at all they word the answers like this ) what shoud we do? We shd think of the closest YES/NO .So a Higher priority thread enters (I assume the Ready state) means it most likely going to enter into Running state than NOT. So I thought the answer is YES. If a thread creates a new thread means the new thread is going to have the SAME PRIORITY as the creating thread, in which case it has less chance to enter into a Running state ,unless the creater yield() than as in option 6. So I inclained towards NO.
That makes sense, and I agree. I just didn't want people to think those answers were absolute. Yeah, if you get an ambiguous question like that on the actual test (which I don't think is nearly as common as ambiguous sample questions are) then you must simply make your best guess.

The isAlive Method
 The isAlive method returns true if the thread has been started and not stopped.
 If the isAlive method returns false, you know that the thread either is a New Thread or is Dead.
 If the isAlive method returns true, you know that the thread is either Runnable or Not Runnable.
 Runnable State includes Ready and Running States.
 You cannot differentiate between a New Thread or a Dead thread. Nor can you differentiate between a Runnable thread and a Not Runnable thread.
Understanding Thread Priority
 When a Java thread is created, it inherits its priority from the thread that created it.
 You can also modify a thread's priority at any time after its creation using the setPriority method.
 Thread priorities are integers ranging between MIN_PRIORITY and MAX_PRIORITY (constants defined in the Thread class).
 The higher the integer, the higher the priority.
 At any given time, when multiple threads are ready to be executed, the runtime system chooses the runnable thread with the highest priority for execution.
 Only when that thread stops, yields, or becomes not runnable for some reason will a lower priority thread start executing.
 If two threads of the same priority are waiting for the CPU, the scheduler chooses one of them to run in a round-robin fashion. The chosen thread will run until one of the following conditions is true:
• A higher priority thread becomes runnable.
• It yields, or its run method exits.
• On systems that support time-slicing, its time allotment has expired.
Then the second thread is given a chance to run, and so on, until the interpreter exits.
 The Java runtime system's thread scheduling algorithm is also preemptive. If at any time a thread with a higher priority than all other runnable threads becomes runnable, the runtime system chooses the new higher priority thread for execution. The new higher priority thread is said to preempt the other threads.
 Rule of thumb: At any given time, the highest priority thread is running. However, this is not guaranteed. The thread scheduler may choose to run a lower priority thread to avoid starvation. For this reason, use priority only to affect scheduling policy for efficiency purposes. Do not rely on thread priority for algorithm correctness
Grouping Threads
 Every Java thread is a member of a thread group.
 Thread groups provide a mechanism for collecting multiple threads into a single object and manipulating those threads all at once, rather than individually.
 For example, you can start or suspend all the threads within a group with a single method call.
 Java thread groups are implemented by the ThreadGroup class in the java.lang package.
 The runtime system puts a thread into a thread group during thread construction.
 When you create a thread, you can either allow the runtime system to put the new thread in some reasonable default group or you can explicitly set the new thread's group.
 The thread is a permanent member of whatever thread group it joins upon its creation--you cannot move a thread to a new group after the thread has been created.

The Default Thread Group
 If you create a new Thread without specifying its group in the constructor, the runtime system automatically places the new thread in the same group as the thread that created it (known as the current thread group and the current thread, respectively).
 When a Java application first starts up, the Java runtime system creates a ThreadGroup named main. Unless specified otherwise, all new threads that you create become members of the main thread group. :
 If you create a thread within an applet, the new thread's group may be something other than main, depending on the browser or viewer that the applet is running in

Methods that Operate on All Threads within a Group
 The ThreadGroup class has three methods that allow you to modify the current state of all the threads within that group:
• resume
• stop
• suspend
 These methods apply the appropriate state change to every thread in the thread group and its subgroups.

Making a Thread Not Runnable
A thread becomes Not Runnable when one of these events occurs:
• Its sleep method is invoked.
• The thread calls the wait method to wait for a specific condition to be satisifed.
• The thread is blocking on I/O.
One Question:
Most of the test questions were easy, but there were some tough thread questions. For example, I had a question that was almost identical to this:
Thread b holds the lock to object a. Thread c is blocked inside a wait call on object a. What will allow c to continue running?
 Threads do not have parent-child relationships
Q8. Select most appropriate answer for the following code
class MyThread extends Thread
{
public void run()
{
try {
Thread.sleep(300);
}
catch (InterruptedException e)
{
System.err.println(e);
}

System.out.println("I am alive");
}
}
public class MyClass
{
public static void main(String args[])
{
new MyThread().start();

int arr[] = new int[20];

arr[20] = 1237;
}
}

Thread MyThread continues even after runtime exception is thrown in
main() and prints "I am alive"
Thread MyThread dies as soon as runtime exception is thrown in main() without printing "I am alive"
Will print "I am alive" without any runtime error
Will give compilation error
SKIP THE QUESTION
Thread MyThread continues even after runtime exception is thrown inThread MyThread continues even after runtime exception is thrown in main() and prints "I am alive" Correct