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AWT-EventQueue-0 exepction

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我正在研究关于线程的一个小例子,示例的程序如下,编译好,但是当我按下按钮开始绘制圆圈Netbeans给我一个例外,有人可以帮助我 .

在线程 “AWT-EventQueue的-0” 在javax.swing.AbstractButton.fireActionPerformed在feedingschedule.FillDemo.actionPerformed(FillDemo.java:56)显示java.lang.NullPointerException在feedingschedule.FillDemo.fill(FillDemo.java:63)异常(AbstractButton.java:2022)在javax.swing.AbstractButton中$ Handler.actionPerformed(AbstractButton.java:2348)在javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)在javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel . java:259)javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)at java.awt.Component.processMouseEvent(Component.java:6535)at javax.swing.JComponent.processMouseEvent(JComponent.java) :3324)java.awt.Component.processEvent(Component.java:6300)at java.awt.Container.processEvent(Container.java:2236)at java.awt.Component.dispatchEventImpl(Component.java:4891)at java java.awt.Component.dispatchEvent中的.awt.Container.dispatchEventImpl(Container.java:2294)(Component.java: 4713)在java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4888)在java.awt.LightweightDispatcher.processMouseEvent(Container.java:4525)在java.awt.LightweightDispatcher.dispatchEvent(Container.java:4466)在java的 . awt.Container.dispatchEventImpl(Container.java:2280)at java.awt.Window.dispatchEventImpl(Window.java:2750)at java.awt.Component.dispatchEvent(Component.java:4713)at java.awt.EventQueue.dispatchEventImpl (EventQueue.java:758)java.awt.EventQueue.access $ 500(EventQueue.java:97)java.awt.EventQueue $ 3.run(EventQueue.java:709)at java.awt.EventQueue $ 3.run(EventQueue)的.java:703)在java.security.AccessController.doPrivileged(本机方法)在java.security.ProtectionDomain $ JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)在java.security.ProtectionDomain $ JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:86 )java.awt上的java.awt.EventQueue $ 4.run(EventQueue.java:731)java.awt.EventQueue $ 4.run(EventQueue.java:729)在java.security.ProtectionDomain $ JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)ity.AccessController.doPrivileged(本机方法)在java.awt.EventQueue.dispatchEvent(EventQueue.java:728)在java.awt.EventDispatchThread.pumpOneEventForFilters( EventDispatchThread.java:201)在java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)在java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)在java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101 )java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)BUILD SUCCESSFUL(总时间:3秒)

谢谢 .

package feedingschedule;

import java.awt.BorderLayout;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
/**
 *
 * @author ManagerCompufi
 */
public class FillDemo extends JFrame implements ActionListener {
  public static final int WIDTH_S = 300;
  public static final int HEIGHT_S = 200;
  public static final int FILL_WITH = 300;
  public static final int FILL_HEIGHT = 100;
  public static final int CIRCLE_SIZE = 10;
  public static final int PAUSE = 100;

  private final JPanel box;  

 public static void main(String[] args) {

   FillDemo gui = new FillDemo();
   gui.setVisible(true);

 }


 public FillDemo(){
   setSize(WIDTH_S, HEIGHT_S);
   setTitle("FillDemo");
   setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
   setLayout(new BorderLayout());
   box = new JPanel();

   JPanel buttonPanel = new JPanel();
   buttonPanel.setLayout(new FlowLayout());
   JButton startButton = new JButton();
   startButton.addActionListener(this);
   buttonPanel.add(startButton);
   add(buttonPanel, "South");
 }


  @Override
  public void actionPerformed(ActionEvent e) {
    fill();
  }

  public void fill(){
    Graphics g = box.getGraphics();
      for (int y = 0; y < FILL_HEIGHT; y = y + CIRCLE_SIZE)
        for (int x = 0; x < FILL_WITH; x = x + CIRCLE_SIZE){
        g.fillOval(x, y, CIRCLE_SIZE, CIRCLE_SIZE);
        doNothing(PAUSE);

      }
  }

  public void doNothing(int milliseconds){
    try {
      Thread.sleep(milliseconds);
    }
    catch (InterruptedException e) {
      System.out.println("Unexpected interrup");
      System.exit(0);
    }

  }
}

1 回答

  • 0

    进入你的程序,

    Graphics g = box.getGraphics(); // which returns null
    

    然后,你试着对它进行一些操作,

    g.fillOval(x, y, CIRCLE_SIZE, CIRCLE_SIZE); // which leads to NullPointerException
    

    那么,最好同样把支票放在那里,

    Graphics g = box.getGraphics(); 
    if(g != null){
            for (int y = 0; y < FILL_HEIGHT; y = y + CIRCLE_SIZE)
                for (int x = 0; x < FILL_WITH; x = x + CIRCLE_SIZE){
                    ......
                }
        }
    

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