Question

以下程序很简单，应该画一个落到重力影响的球然后反弹回来。该程序似乎编译但我不断收到异常错误。

在类球中，功能移动包含用于计算速度和位移的物理公式。

包装重力;

import java.awt.Color;
import java.awt.Graphics;

public class Ball extends StartingPoint{
/**
     * 
     */
    private static final long serialVersionUID = 1L;
public int x;
public int y;

private static final double dt = .2;
private static final double g = 9.8;
public double vy = 0;

Ball(int x, int y){
    this.x = x;
    this.y = y;
}

public void move(){
    vy += g*dt;
    y += (1/2)*g * dt*dt + vy * dt;
    if (y >= (this.getHeight() - 20 -1)){
    y = this.getHeight() - 20 -1;
    vy = -vy;}

    }


public void paintBALL(Graphics g){
g.setColor(Color.LIGHT_GRAY);
g.fillOval(x, y, 20, 20);
}
}

package gravity;

import java.applet.Applet;
import java.awt.Graphics;

public class StartingPoint extends Applet {

    /**
     * 
     */
    private static final long serialVersionUID = 1L;
    public int x = 400, y = 25;
    Ball ball;

    //private static final int nbBalls = 1;

    @Override
    public void init() {
        setSize(800, 600);

    }

    @Override
    public void start() {
        Ball ball = new Ball(x,y);
        Thread thread = new Thread(new RunBall(ball));
        thread.start();

    }

    public class RunBall implements Runnable{
        private Ball ball;
        RunBall(Ball ball){
            this.ball = ball;
        }
        @Override
        public void run() {
            while(true){

                ball.move();
                try {
                    Thread.sleep(17);
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
                repaint();
            }

        }

    }

    @Override
    public void stop() {

    }

    @Override
    public void paint(Graphics g) {
        ball.paintBALL(g);
    }

}

编译器不断添加以下exeption：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at gravity.StartingPoint.paint(StartingPoint.java:61)
at java.awt.Container.update(Container.java:1835)
at sun.awt.RepaintArea.updateComponent(RepaintArea.java:267)
at sun.awt.RepaintArea.paint(RepaintArea.java:233)
at apple.awt.ComponentModel.handleEvent(ComponentModel.java:263)
at java.awt.Component.dispatchEventImpl(Component.java:4852)
at java.awt.Container.dispatchEventImpl(Container.java:2142)
at java.awt.Component.dispatchEvent(Component.java:4604)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:717)
at java.awt.EventQueue.access$400(EventQueue.java:82)
at java.awt.EventQueue$2.run(EventQueue.java:676)
at java.awt.EventQueue$2.run(EventQueue.java:674)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:86)
at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:690)
at java.awt.EventQueue$3.run(EventQueue.java:688)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:86)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:687)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:296)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:211)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:196)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:188)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

Answer 1

在start方法中，您已宣布Ball ...

    Ball ball = new Ball(x,y);

这意味着在调用ball时，实例字段null仍为paint。

相反，在start方法中，初始化实例字段...

ball = new Ball(x,y);

Applet已经过时了，因为小程序重新绘制，你应该为很多闪烁做好准备。

如果可以，我建议使用JApplet，但使用JPanel作为主要油漆表面。通过覆盖JPanel paintComponent方法，您可以默认获得双缓冲支持，这样可以在不进行任何其他工作的情况下停止闪烁，只是不要忘记拨打super.paintComponent在做任何自定义绘画之前...

Answer 2

在定义Ball之前调用

paint，只需更改颜色：

@Override
public void paint(Graphics g) {
  if(ball != null)    
    ball.paintBALL(g);
}

重力移动球

2 个答案: