NullPointerException when trying to use JButton

Question

This is the code:

    addCube = new JButton("Add Cube");
    addCube.addKeyListener(kl);
    addCube.addActionListener(new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent ae) {
            cubes.add(cube);
        }
    });
    panel.add(addCube);
    frame.add(panel, BorderLayout.NORTH);

Where panel is an JPanel and frame is an JFrame. When I execute the code I got the following error message:

Exception in thread "main" java.lang.NullPointerException at cg2k15.CG2K15.main(CG2K15.java:91)

Where line 91 is this: panel.add(addCube);

What am I doing wrong? :/

Possible duplicate of [What is a Null Pointer Exception, and how do I fix it?](http://stackoverflow.com/questions/218384/what-is-a-null-pointer-exception-and-how-do-i-fix-it) — ΦXocę 웃 Пepeúpa ツ, Jan 21 '16 at 09:55

score 1 · Answer 1 · answered Jan 21 '16 at 09:54

1

Your addCube object is clearly not null since it has already been initialized, looks like you forgot to initialize your panel.

answered Jan 21 '16 at 09:54

Carlos Albaladejo

66
8

even if addCube was null ... as long as you're not actually trying to call one of it's (instance) members, I doubt it would thrown an NPE – Stultuske Jan 21 '16 at 10:03

score 0 · Answer 2 · answered Jan 21 '16 at 12:07

Thanks guys. It ended up that the problem was that I was doing everything directly under the main method. I read that a constructor shouldn't initialize a static field. The buttons should be private instance fields of the frame and the main method shouldn't deal with them. After I created another class just to deal with the frame, it solved the problem! :)

NullPointerException when trying to use JButton

2 Answers2