Question

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

线程中的异常＆＃34; AWT-EventQueue-0＆＃34;显示java.lang.NullPointerException 在feedingschedule.FillDemo.fill（FillDemo.java:63）at feedingschedule.FillDemo.actionPerformed（FillDemo.java:56）at javax.swing.AbstractButton.fireActionPerformed（AbstractButton.java:2022）在 javax.swing.AbstractButton中的$ Handler.actionPerformed（AbstractButton.java:2348）在 javax.swing.DefaultButtonModel.fireActionPerformed（DefaultButtonModel.java:402）在 javax.swing.DefaultButtonModel.setPressed（DefaultButtonModel.java:259）在 javax.swing.plaf.basic.BasicButtonListener.mouseReleased（BasicButtonListener.java:252）在java.awt.Component.processMouseEvent（Component.java：6535）at javax.swing.JComponent.processMouseEvent（JComponent.java:3324）at 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.awt.Container.dispatchEventImpl（Container.java:2294）at java.awt.Component.dispatchEvent（Component.java:4713）at java.awt.LightweightDispatcher.retargetMouseEvent（Container.java:4888）在 java.awt.LightweightDispatcher.processMouseEvent（Container.java:4525） at java.awt.LightweightDispatcher.dispatchEvent（Container.java:4466）在java.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）at at java.awt.EventQueue.access $ 500（EventQueue.java:97）at java.awt.EventQueue $ 3.run（EventQueue.java:709）at java.awt.EventQueue $ 3.run（EventQueue.java:703）at java.security.AccessController.doPrivileged（Native Method）at java.security.ProtectionDomain $ JavaSecurityAccessImpl.doIntersectionPrivilege（ProtectionDomain.java:76）在 java.security.ProtectionDomain $ JavaSecurityAccessImpl.doIntersectionPrivilege（ProtectionDomain.java:86）在java.awt.EventQueue $ 4.run（EventQueue.java:731）at java.awt.EventQueue $ 4.run（EventQueue.java:729）at java.security.AccessController.doPrivileged（Native Method）at java.security.ProtectionDomain $ JavaSecurityAccessImpl.doIntersectionPrivilege（ProtectionDomain.java:76）在java.awt.EventQueue.dispatchEvent（EventQueue.java:728）at 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）在 java.awt.EventDispatchThread.pumpEvents（EventDispatchThread.java:93） at java.awt.EventDispatchThread.run（EventDispatchThread.java:82）建立成功（总时间：3秒）

感谢。

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);
    }

  }
}

Answer 1

进入你的程序，

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

之后，您将尝试对其执行某些操作，

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

所以，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){
                ......
            }
    }

AWT-EventQueue-0 exepction

1 个答案: