Topic  –  Abstract class – very important topic in Java.

Pre-requisite : Abstract method

Any method that does not have a body(open and closing curly brace) instead ends with a semicolon  is called Abstract method.

Ex: public void display();

But how do you tell this to the compiler , very simple

Public  abstract void display();

Abstract  class – any class in java which has at least one  abstract method must be declared as Abstract class.

public abstract class Employee {

Public  abstract void display();

}

What happens if I don’t declare as Abstract in that case ?

Simple – compile time error.

Whenever we  declare any class as an abstract it means we cant instantiate that class.

Instantiate –creating an object for the class.

So Employee emp = new Employee(); - is wrong. Compile time error.

So what is the use of abstract class if we are not able to instantiate it ?

Answer is Polymorphism.

Polymorphism – methods which behaves differently in different situations.

Any class which extends abstract class and implements all the unimplemented  methods of abstract class becomes concrete class.

We can  now store the object of concrete class inside a reference variable of Abstract class.

Yes …got confused  how  its polymorphism?  Analyze  the simple  below program carefully.

package pack1;

import sun.security.action.GetLongAction;

abstract class Employee {

     

public abstract int getLeaves();

}

class PermanentEmployee extends Employee{

static int leaves = 20;

      public int getLeaves() {

           

            return leaves;

      }

     

}

class ContractorEmployee extends Employee{

      static int leaves = 10;

     

      public int getLeaves() {

           

            return leaves;

      }

}

public class EmployeeMain{

      public static void main(String[] args) {

            Employee pEmp = new PermanentEmployee();

            Employee cEmp = new ContractorEmployee();

            System.out.println("permanent employee leaves");

            System.out.println(pEmp.getLeaves());

           

            System.out.println("Contractor employee leaves");

            System.out.println(cEmp.getLeaves());

           

           

      }

     

     

}

so same method  getLeaves() behaves differently in different situations in the above program.

Both pEmp and cEmp are of type Employee and we are calling getLeaves() method, but they are printing different values. How ?

Compiler looks at the type of a variable at the compile time but at run time it looks for the variable content(ie where it is pointing).

Since here both pEmp and cEmp are both of type Employee at compile time but at run time both are pointing to different objects.

This behavior is called polymorphism.

Quiz  ??

1)can we declare class as an abstract if it does not contain even a single abstract method ?

2)in the above program can we write like this

PermanentEmployee pEmp = new Employee ();