java.lang.NullPointerException when running program

Question

I have 4 classes. One is the abstract entity class, one is a ball class, one is the main class and the other is the screen class.

The ball class extends the entity class and has 5 variables.

The screen function has a paint method:

public void paint(Graphics g){
    super.paint(g);
    ball.paint(g);
}

And of course, to use ball.paint need to make an object for it. So, I make ball object:

Ball ball;

And then add this in the screen constructor, since I need to (Ball has a constructor which takes 5 variables):

public Screen(){
    ball = new Ball(ball.getWeight(), ball.getWidth(), ball.getHeight(), ball.getX(), ball.getX());
}

This comes up with no errors, but when I run the program, I get this error in the console:

Exception in thread "main" java.lang.NullPointerException
at h3x.engine.gfx.Screen.<init>(Screen.java:18)
at h3x.engine.Main.main(Main.java:16)

Line 16 of the main class is this:

frame.add(new Screen());

...and line 18 of the screen class is this:

ball = new Ball(ball.getWeight(), ball.getWidth(), ball.getHeight(), ball.getX(), ball.getX());

So my question is, why is this happening, and how can I fix it. I can put in the whole code in the classes if it is needed.

Thanks!

Answer 1

You call:

ball = new Ball(ball.getXxx()....)

which means you expect to retrieve values from the instance you initialize. And before initialization is complete, it is null. Hence the NPE (short for NullPointerException).