java - 使用.getWidth()时找不到空指针的原因

我很难找到为什么我在这段代码上返回空指针：

import java.awt.Graphics2D;
import java.awt.Rectangle;

public class Move {

  private static final int DIAMETER = 30;

  int x = 0;
  int y = 0;
  int x_1 = 1;
  int y_1 = 1;


  private GameStart game;

  public void Ball(GameStart game) {
    this.game = game;
  }

  void moveBall() {
    if (x + x_1 < 0) {
        x_1 = 1;
    }

    if (x + x_1 > game.getWidth() - DIAMETER) {
        x_1 = -1;
    }

    if (y + y_1 < 0) {
        y_1 = 1;
    }

    if (y + y_1 > game.getHeight() - DIAMETER) {
        game.gameOver();
    }

    if (collision()) {
        y_1 = -1;
        y = game.racquet.getTopY() - DIAMETER;
    }

    x = x + x_1;
    y = y + y_1;
  }

  private boolean collision() {
    return game.racquet.getBounds().intersects(getBounds());
  }

  public void paint(Graphics2D g) {
    g.fillOval(x, y, 30, 30);
  }

  public Rectangle getBounds() {
    return new Rectangle(x, y, DIAMETER, DIAMETER);
  }

}

GameStart类在这里：

import javax.swing.JFrame;
import javax.swing.JOptionPane;
import javax.swing.JPanel;

import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.event.KeyEvent;

@SuppressWarnings("serial")
public class GameStart extends JPanel {

  Move move_ball = new Move();
  Racquet racquet = new Racquet(this);

  public GameStart() {

    addKeyListener(new MyKeyListener() {
        public void keyTyped(KeyEvent e) {
        }

        public void keyReleased(KeyEvent e) {
            racquet.keyReleased(e);
        }

        public void keyPressed(KeyEvent e) {
            racquet.keyPressed(e);
        }

    });

    setFocusable(true);

  }

  private void move() {
    move_ball.moveBall();
    racquet.move();
  }

  public void paint(Graphics g) {
    super.paint(g);
    Graphics2D graphics = (Graphics2D) g;
    graphics.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                RenderingHints.VALUE_ANTIALIAS_ON);
    move_ball.paint(graphics);
    racquet.paint(graphics);
  }

  public void gameOver() {
    JOptionPane.showMessageDialog(this, "Spil slut", "Du tabte", JOptionPane.YES_NO_OPTION);
    System.exit(ABORT);
  }

  public static void main (String[] args) throws InterruptedException {

    JFrame mainWindow = new JFrame("Matematikken");
    GameStart game = new GameStart();
    mainWindow.add(game);
    mainWindow.setSize(300, 400);
    mainWindow.setVisible(true);
    mainWindow.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    while (true) {

        game.move();
        game.repaint();
        Thread.sleep(10);

    }

  }

}

我在if (x + x_1 > game.getWidth() - DIAMETER) {行上得到了一个空指针。我在另一个类中以samme方式使用.getWidth()，它可以正常工作-但由于某种原因，它不会在此使用。我试过打印结果，但这只是另一个nullpointer。我知道我缺少某些东西，但找不到。

最佳答案

你不打电话

public void Ball(GameStart game) {
    this.game = game;
}

因此，在您的Move类中，GameStart仍然为空。

关于java - 使用.getWidth()时找不到空指针的原因，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/31053708/