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