java interview questions

                JAVA  Interview Questions

1.what  is a transient variable?

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


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.


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.


4. How are Observer and Observable used?

 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.

5. What is the difference between an Inner Class and a Sub-Class?

 An Inner class is a class which is nested within another class. An Inner class

has access rights for the class which is nesting it and it can access all variables and

methods defined in the outer class.

A sub-class is a class which inherits from another class called super class. Sub-

class can access all public and protected methods and fields of its super class.

6. What are the various access specifiers for Java classes?

 In Java, access specifiers are the keywords used before a class name

which defines the access scope. The types of access specifiers for classes are:

1. Public : Class,Method,Field is accessible from anywhere.

2. Protected:Method,Field can be accessed from the same class to which they

belong or from the sub-classes,and from the class of same package,but not from

outside.

3. Default: Method,Field,class can be accessed only from the same package and

not from outside of it’s native package.

4. Private: Method,Field can be accessed from the same class to which they belong.

7. What’s the purpose of Static methods and static variables?

 When there is a requirement to share a method or a variable between multiple

objects of a class instead of creating separate copies for each object, we use static

keyword to make a method or variable shared for all objects.

8 What is data encapsulation and what’s its significance?

 Encapsulation is a concept in Object Oriented Programming for combining

properties and methods in a single unit.

Encapsulation helps programmers to follow a modular approach for software

development as each object has its own set of methods and variables and serves its

functions independent of other objects. Encapsulation also serves data hiding

purpose.

9. What is a singleton class? Give a practical example of its usage.

A singleton class in java can have only one instance and hence all its methods and

variables belong to just one instance. Singleton class concept is useful for the

situations when there is a need to limit the number of objects for a class.