java Null Pointer Exception传递对象

Question

I have some class which creates a Ship (class Ship extends GameObject) and attempts to add it to a gameBoard.
To do this, it tells the gameFrame to add the object as follows:

public void startNewGame() {
    Ship myShip = new Ship(GAME_BOARD_WIDTH / 2, GAME_BOARD_HEIGHT-1, SHIP_WIDTH,
            SHIP_HEIGHT);
    SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            gameFrame = new InvadersGameFrame();
        }
    });
    gameFrame.addGameObject(myShip); //Problem line
    gameFrame.repaint();
}

The gameFrame then calls:

GameBoard gameBoard = new GameBoard();
...
...
public void addGameObject(GameObject ob) {
    gameBoard.addGameObject(ob);
}

Which in turn calls:

public class GameBoard extends JPanel implements GameData{
    private JPanel gameBoard;
    private List<GameObject> objects = new ArrayList<>();

    public GameBoard() {
        gameBoard = new JPanel();
    }

    @Override
    protected void paintComponent(Graphics g)
    {
        super.paintComponent(g);
        setBackground(Color.black);
        g.setColor(Color.RED);
        for(GameObject ob : objects){
            g.drawOval(ob.x, ob.y, ob.width, ob.height);
        }
    }

    //Places object into list for drawing upon next repaint.
    public void addGameObject(GameObject ob) {
        objects.add(ob);
    }
}

Now my problem is that I receive a Null Pointer Exception when I gameFrame.addGameObject(myShip);
The tricky thing is when I run via debugger I do not receive the NPE at all (but my objects list still seems empty).
Also, I can follow into each of these and still see myShip, so am I just referencing my GameObject (Ship) wrong?
Should my parameters for addGameObject somehow be more abstract?

Answer 1

The problem is that you are assigning gameFrame inside of the Runnable , which does not get run until later. So gameFrame may be null at the point you call gameFrame.addGameObject(myShip)

Answer 2

The problem probably is the invokeLater you use, that is scheduled whenever Swing feels like it and probably happens after gameFrame.addGameObject(myShip);

This doesnt happen in the debugger because it is somewhat of a race condition, in the debugger Swing invokes the runmethod before gameFrame.addGameObject(myShip) because you can't click fast enough to preemt that statement ;)

Answer 3

The best solution to this problem is to move those statements inside the run method, like this:

public void startNewGame() {
    SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            Ship myShip = new Ship(GAME_BOARD_WIDTH / 2, GAME_BOARD_HEIGHT-1, 
                SHIP_WIDTH, SHIP_HEIGHT);
            gameFrame = new InvadersGameFrame();
            gameFrame.addGameObject(myShip); //Problem line
            gameFrame.repaint();
        }
    });
}

Doing it this way, the 4 statements will be run in the expected order, and the attribute will not be null when required.

Answer 4

check the scope where you initialized that variable. ie the curly braces { } around the new InvadersGameFrame(). Make sure you are using the object after creating it