Abstract methods in Java

Question

I want to write an abstract method but the compiler persistently gives this error:

abstract methods cannot have a body

I have a method like this:

public abstract boolean isChanged() {     return smt else... } 

What is wrong here?

Answer 1

Abstract methods means there is no default implementation for it and an implementing class will provide the details.

Essentially, you would have

abstract class AbstractObject {    public abstract void method(); }  class ImplementingObject extends AbstractObject {   public void method() {     doSomething();   } } 

So, it's exactly as the error states: your abstract method can not have a body.

There's a full tutorial on Oracle's site at: http://download.oracle.com/javase/tutorial/java/IandI/abstract.html

The reason you would do something like this is if multiple objects can share some behavior, but not all behavior.

A very simple example would be shapes:

You can have a generic graphic object, which knows how to reposition itself, but the implementing classes will actually draw themselves.

(This is taken from the site I linked above)

abstract class GraphicObject {     int x, y;     ...     void moveTo(int newX, int newY) {         ...     }     abstract void draw();     abstract void resize(); }  class Circle extends GraphicObject {     void draw() {         ...     }     void resize() {         ...     } } class Rectangle extends GraphicObject {     void draw() {         ...     }     void resize() {         ...     } }