Q) What is volatile keyword?

Ans) In general each thread has its own copy of variable, such that one thread is not concerned with the value of same variable in the other thread. But sometime this may not be the case. Consider a scenario in which the count variable is holding the number of times a method is called for a given class irrespective of any thread calling, in this case irrespective of thread access the count has to be increased. In this case the count variable is declared as volatile. The copy of volatile variable is stored in the main memory, so every time a thread access the variable even for reading purpose the local copy is updated each time from the main memory. The volatile variable also have performance issues. 

Q) What is a transient variable?

Ans) If some of the properties of a class are not required to be serialized then the varaibles are marked as transient. 

Q) What is the use of final keyword? 
Ans) The final keyword can be assigned to 

Class level variable 
method 
class 
Objects 
If a final is assigned to a variable, the variable behaves as a constant. It means that the value of variable once set cannot be changed. 

final int i=1;
i =5; // error

If a final is assigned to a method then it cannot be overridden in its child class.

class Parent {

public final void print(){

            System.out.println(“Inside”);
} 

} 

class Child extends Parent{

public final void print(){             // error cannot override final method

            System.out.println(“Inside”);
} 

}

If a class is made as final, then no other class can extend it and make it as parent class. E.g. String Class.

Final objects are instantiated only once. i.e 

final Map map = new HashMap();

map.put(“key”,”value”);

map = new HashMap();  // error

Q) What is use of synchronized keyword? 

Ans) This keyword is used to prevent mulitple access. synchronized keyword can be applied to static/non-static methods or a block of code. Only one thread at a time can access synchronized methods and if there are multiple threads trying to access the same method then other threads have to wait for the execution of method by one thread. Synchronized keyword provides a lock on the object and thus prevents race condition. E.g.

public void synchronized method(){} 
public void synchronized staticmethod(){}
public void myMethod(){

            synchronized (this){             // synchronized keyword on block of  code
            }

}

Relevant Java Topics:

• Java and EJB - Interview Questions
• Interview Questions in Core Java - Part One
• Interview Questions in Core Java - Part Two

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Java Certification, Programming, JavaBean and Object Oriented Reference Books

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