重力移動球

Question

以下程序很簡單，應該畫出一個落在重力影響下的球，然後彈回。該程序似乎編譯，但我不斷收到異常錯誤。

在類球中，函數運動包含計算速度和位移的物理公式。

包重力;

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); 
    } 

} 

---

編譯器不斷添加以下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

在你start方法，在您重新申報Ball ...

Ball ball = new Ball(x,y); 

這意味着當調用paint時，實例字段ball仍爲null。

相反，在你start方法，初始化實例字段...

ball = new Ball(x,y); 

Applet是非常過時的，你應該有很多的小程序重繪閃爍的準備。

如果可以，我建議使用JApplet，但使用JPanel作爲主要油漆表面。通過覆蓋JPanel小號paintComponent方法，你會得到默認情況下，它會停止閃爍，沒有你做任何額外的工作雙緩衝支持，只是不要忘記調用super.paintComponentŸ做任何自定義塗裝前...

Answer 2

paint被稱爲是指你的球之前，只需更改您的油漆：中

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