26. What are wrapped classes?

26. What are wrapped classes?
Wrapped classes are classes that allow primitive types to be accessed as objects.

25. Is sizeof a keyword?

25. Is sizeof a keyword?
The sizeof operator is not a keyword.

24. Which java.util classes and interfaces support event handling?

24. Which java.util classes and interfaces support event handling?
The EventObject class and the EventListener interface support event processing.

23What is the difference between yielding and sleeping?

23What is the difference between yielding and sleeping?
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.

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

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

21. Which method of the Component class is used to set the position and

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

setBounds()

20. What is the difference between the >> and >>> operators?

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

19. What is an Iterator interface?

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

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

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

17. What is the Vector class?

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

16. How does Java handle integer overflows and underflows?

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

15. What is the List interface?

The List interface provides support for ordered collections of objects.

14. Which characters may be used as the second character of an identifier,

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.

13. What is the Collections API?

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

12. What state does a thread enter when it terminates its processing?

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

11. Which containers use a FlowLayout as their default layout?

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

10. What method is used to specify a container's layout?

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

9. What is the preferred size of a component?

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

8. Is null a keyword?

The null value is not a keyword.

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

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

6. Can a lock be acquired on a class?

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

5. What is synchronization and why is it important?

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.

3. Why do threads block on I/O?

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

2. Which containers use a border Layout as their default layout?

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

1. What is a transient variable?

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

What are different types of inner classes?

They are Nested -level classes, Member classes, Local classes, Anonymous classes

Nested -level classes- If you declare a class within a class and specify the static modifier, the compiler treats the class just like any other -level class. Any class outside the declaring class accesses the nested class with the declaring class name acting similarly to a package. eg, outer.inner. -level inner classes implicitly have access only to static variables.There can also be inner interfaces. All of these are of the nested -level variety.

Member classes - Member inner classes are just like other member methods and member variables and access to the member class is restricted, just like methods and variables. This means a public member class acts similarly to a nested -level class. The primary difference between member classes and nested -level classes is that member classes have access to the specific instance of the enclosing class.

Local classes - Local classes are like local variables, specific to a block of code. Their visibility is only within the block of their declaration. In order for the class to be useful beyond the declaration block, it would need to implement a more publicly available interface. Because local classes are not members; the modifiers public, protected, private, and static are not usable.

Anonymous classes - Anonymous inner classes extend local inner classes one level further. As anonymous classes have no name, you cannot provide a constructor.

What is Overriding?

When a class defines a method using the same name, return type, and arguments as a method in its superclass, the method in the class overrides the method in the superclass. When the method is invoked for an object of the class, it is the new definition of the method that is called, and not the method definition from superclass. Methods may be overridden to be more public, not more private.

What are Checked and UnChecked Exception?

A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses. Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown. eg, IOException thrown by java.io.FileInputStream's read() method

checked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String's charAt() method Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.

Will the JVM load the package twice at runtime?

One can import the same package or same class multiple times. Neither compiler nor JVM complains abt it. And the JVM will internally load the class only once no matter how many times you import the same class.

Do I need to import java.lang package any time? Why ?

No. It is by default loaded internally by the JVM.

Can I have multiple main methods in the same class?

No the program fails to compile. The compiler says that the main method is already defined in the class.

Can an application have multiple classes having main method?

Yes it is possible. While starting the application we mention the class name to be run. The JVM will look for the Main method only in the class whose name you have mentioned. Hence there is not conflict amongst the multiple classes having main method.

What environment variables do I need to set on my machine in order to be able to run Java programs?

CLASSPATH and PATH are the two variables.

How can one prove that the array is not null but empty?

Print args.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print args.length.

If I do not provide any arguments on the command line, then the String array of Main method will be empty of null?

It is empty. But not null.

What is the first argument of the String array in main method?

The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name.

What if I do not provide the String array as the argument to the method?

Program compiles but throws a runtime error "NoSuchMethodError".

What if I write static public void instead of public static void?

Program compiles and runs properly.

What if the static modifier is removed from the signature of the main method?

Program compiles. But at runtime throws an error "NoSuchMethodError".

What if the main method is declared as private?

The program compiles properly but at runtime it will give "Main method not public." message.

What one should take care of while serializing the object?

One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a Not Serializable Exception.

What happens to the object references included in the object?

The serialization mechanism generates an object graph for serialization. Thus it determines whether the included object references are serializable or not. This is a recursive process. Thus when an object is serialized, all the included objects are also serialized along with the original object.

What is Externalizable interface?

Externalizable is an interface which contains two methods read External and write External. These methods give you a control over the serialization mechanism. Thus if your class implements this interface, you can customize the serialization process by implementing these methods.

what is the common usage of serialization?

Whenever an object is to be sent over the network, objects need to be serialized. Moreover if the state of an object is to be saved, objects need to be serialized.

What is an abstract class?

Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract may not be instantiated (ie, you may not call its constructor), abstract class may contain static data. Any class with an abstract method is automatically abstract itself, and must be declared as such. A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.

How can I customize the serialization process?

i.e. how can one have a control over the serialization process?
Yes it is possible to have control over serialization process. The class should implement Externalizable interface. This interface contains two methods namely readExternal and writeExternal. You should implement these methods and write the logic for customizing the serialization process.

Which methods of Serializable interface should I implement?

The serializable interface is an empty interface; it does not contain any methods. So we do not implement any methods.

How do I serialize an object to a file?

The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a fileoutputstream. This will save the object to a file.

Java n J2ee faqs

Java & J2ee faqs