Error with timer and JFrame

问题

I'm making a game with a timer and a JFrame (and many other things but only these 2 are causing problems), and after running the segments below, I got a weird error. At least for me who has never used these classes prior to this.

Start executing this

private static GameView window;
private static Timer time;
public static void main(String args[])
{
    window = new GameView(800,600);
    window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    window.setVisible(true);

    time = new Timer();
    time.schedule( new TimerTask(){
        public void run(){GameState.update(); 
        window.paintComponents(null);}
        },0, 40);

}

which calls this:

public void paintComponents (Graphics g)
{

    for(Bullet j : GameState.getEnBullets()){
        g.drawImage(j.getImage(),j.getX(), j.getY(), null);}
    for(Enemy j : GameState.getEnemies()){
        g.drawImage(j.getImage(),j.getX(), j.getY(), null);}
    for(Bullet j : GameState.getPlayBullets()){
        g.drawImage(j.getImage(),j.getX(), j.getY(), null);}
    this.paint(g);
}

And here's the error:

Exception in thread "Timer-0" java.lang.NullPointerException
    at sun.awt.SunGraphicsCallback.runComponents(Unknown Source)
    at java.awt.Container.paint(Unknown Source)
    at java.awt.Window.paint(Unknown Source)
    at Game.GameView.paintComponents(GameView.java:59)
    at Game.GameController$1.run(GameController.java:39)
    at java.util.TimerThread.mainLoop(Unknown Source)
    at java.util.TimerThread.run(Unknown Source)

I also get a blank JFrame window (GameView extends JFrame).

回答1:

You get NPE because you are passing null as graphics in window.paintComponents(null); And then you are calling g.drawImage(j.getImage(),j.getX(), j.getY(), null); where g is null.

回答2:

You shouldn't be painting direclty in a JFrame at all but rather a JPanel or JComponent that is held by the JFrame. You should override the JPanel's paintComponent method as noted above (not the JFrame since it doesn't even have this method) and paint in there. Another thing, don't use a java.util.Timer but rather a javax.swing.Timer better known as a Swing Timer since this is a Swing application. Also you shouldn't call paint/paintComponent directly but rather have your GUI update class fields then call repaint() on the JPanel that you're doing your drawing on and then paintComponent will (usually) be called by the JVM. There are many examples of Swing animation here in this forum and I suggest you search out these examples and learn from them as I think that they can help you.

Edit: heck, you've already been told all of this in your previous threads. Why should we help you if you ignore our advice?