» What is Abstraction in java - Java Abstract Class, Method and example

An abstract class is something which is incomplete and you can not create instance of abstract class. If you want to use it you need to make it complete or concrete by extending it. A class is called concrete if it does not contain any abstract method and implements all abstract method inherited from abstract class or interface it has implemented or extended. By the way Java has concept of abstract classes, abstract method but a variable can not be abstract in Java. Popular example of abstract class in Java is ActionListener which has abstract method called actionPerformed(ActionEvent ae). This method is called when an ActionEvent is fired like when you click on JButton. Its common in java to attach ActionListener with JButton by implementing abstract method actionPerformed(ActionEvent ae) using Anonymous class, as shown in below Example :

JButton  ok = new JButton("OK");
ok.addActionListener(new ActionListener(){
           public void  actionPerformed(ActionEvent ae){
               //code to handle event
           }
});


An abstract method in Java doesn't have body , its just a declaration. In order to use abstract method you need to override that method in SubClass.

so when do you use abstraction ? 

when Yo  know something needs to be there but  not sure how exactly it should look like.

e.g. when I am creating a class called Vehicle, I know there should be methods like start() and Stop() but don't know start and stop mechanism of every vehicle since they could have different start and stop mechanism e..g some can be started by kick or some can be by pressing buttons .