Abstract Methods don't have body?

I am new to Java (reading books for 4 months now). So probably my question can appear too simple. My understanding is that abstract methods don't have a body and can't provide implementation

So how does this works?

public abstract void fillRect (int x, int y, with, height);

I did't point the question clearly. We've got abstract method. Why does it draw a rectangle if I doesn't provide a body, just parameters.

For example

public void paint (Graphics g) {

g.fillRect (5, 5, 30, 30);

}

There are two things you need to know

-declaration : The prototype or structure of the method. e.g:

    public int add(int a, int b);

-definition : The implementation of the method

    public int add(int a, int b) {
        this.a = a;
        this.b = b;
        return a + b;
    }

Now an abstract method can have a declaration i.e a structure or prototype. But it cannot have a definition. The definition should be done in the class which extends the class containing the abstract method:

    class A {
        public abstract int add(int a,int b); //just declaration- no body
    }
    class B extends A {
        /*must override add() method because it is abstract in class A i.e class B must have a body or definition of add()*/ 
        int a, b;
        public int add(int a,int b) {
            this.a = a;
            this.b = b;
            return a + b;
        }
    }