04-02-25
----------
Q)final modifier can be applied to which type of variables?
    1)local variables
    2)instance variables
    3)static variables
For eg.
class  Test{
	final int b=10;//final instance variable
        static final int c=20;//final static variable
	void m(){
	}

	public static void main(String[] args) {
		final int a=10;//a is a local final variable		
	}
}
Note:- Once a final variable is initilized, its value can't be changed programmatically
in the later part of the program.

final  method example
-------------------------
class  A{
	final void x(){
	}
}
class B extends A{
	void x(){ //error.final method can't be overridden.
	}
}
class Test{

	  public static void main(String args[]){
		   B b=new B();
		   b.x();//OK. final method can be inherited
	  }
}
Q)Which class is generally declared final?
    =>In a heirarchy of user defined classes, the most specifinalized class
    is declared final to prevent further specialization.
    =>Some times, to make a Java class immutable(non modifiable),
     it can be declared final.
        For eg.     String class.

       class MyString extends String{//error
       }

Q)Explain about "abstract" modifier.
=>An abstract keyword can be associated  only with a class and a method but not
with variables. I.e. we don't have abstract variables.

=>If a class is declared abstract, it can't be instantiated. But its reference
can be created.

For eg.
  abstract class A{
}
class Test{
	public static void main(String args[]){
	   A a=new A();//error
	}
}

=>We can create a reference of an abstract class.
=>An abstract class can be inherited.
=>An abstract class can have a constructor(for child sake)
=>The most generalised class in a hierarchy of user defined classes
is declared "abstract"

   For eg.
              abstract class Vehicle{
	      }
=>Vehicle is too generalised and incomplete  to create an instance of it.

=>An abstract class can have zero or more abstract methods.
=>An abstract class can have non-abstract methods also

Q) What is an abstract method?
 =>bodyless method is an abstract method.
    
   For eg.
              
abstract class A{
	abstract void x(); //no  body
	void m(){
	} //m has null body
}