09. Abstract Classes and Methods in Java

A class that is declared using “abstract” keyword is known as abstract class. 

It may or may not include abstract methods which means in abstract class you can have concrete methods (methods with body) as well along with abstract methods ( without an implementation, without braces, and followed by a semicolon).

 An abstract class can not be instantiated (you are not allowed to create object of Abstract class).

Abstract class declaration

Specifying abstract keyword before the class during declaration, makes it abstract. Have a look at below code:

// Declaration using abstract keyword

abstract class AbstractDemo{

   // Concrete method: body and braces

   public void myMethod(){

      //Statements here

   }

   // Abstract method: without body and braces

   abstract public void anotherMethod();

}

Since abstract class allows concrete methods as well, it does not provide 100% abstraction. You can say that it provides partial abstraction. Interfaces are used for 100% abstraction (full abstraction)

Remember two rules:

1) If the class is having few abstract methods and few concrete methods: declare it as abstract class.

2) If the class is having only abstract methods: declare it as interface.

Error!! – Object creation of abstract class is not allowed

As discussed above, we cannot instantiate an abstract class. The following code throws an error.

abstract public class AbstractDemo{

   public void myMethod(){

      System.out.println("Hello");

   }

   abstract public void anotherMethod();

}

public class ConcreteDemo{

   public void anotherMethod() {

        System.out.print("Abstract method");

   }

   public static void main(String args[])

   {

      //Can't create object of abstract class - error!

      AbstractDemo obj = new AbstractDemo();

      obj.display();

   }

}

Output:

Unresolved compilation problem: Cannot instantiate the type AbstractEx1

Note: The class that extends the abstract class, have to implement all the abstract methods of abstract class, else they can be declared abstract in the class as well.

Abstract methods

Well, we already discussed about abstract methods in the above section. Lets take few examples to understand it better.

syntax:

public abstract void display();

Points to remember about abstract method:

1) Abstract method has no body.

2) Always end the declaration with a semicolon(;).

3) It must be overridden. An abstract class must be extended and in a same way abstract method must be overridden.

4) Abstract method must be in a abstract class.

Note: The class which is extending abstract class must override (or implement) all the abstract methods.

Example of Abstract class and method

abstract class Demo1{

   public void disp1(){

     System.out.println("Concrete method of abstract class");

   }

   abstract public void disp2();

}

class Demo2 extends Demo1{

   /* I have given the body to abstract method of Demo1 class

   It is must if you don't declare abstract method of super class

   compiler would throw an error*/ 

   public void disp2()

   {

       System.out.println("I'm overriding abstract method");

   }

   public static void main(String args[]){

       Demo2 obj = new Demo2();

       obj.disp2();

   }

}

Output:

I'm overriding abstract method