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() {
...
}
}