java - AWT-EventQueue-0 异常

Question

我正在研究一个关于线程的小例子，这个例子的程序如下，编译得很好，但是当我按下按钮开始画圆时，Netbeans给我抛出一个异常，有人可以帮助我。

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException at feedingschedule.FillDemo.fill(FillDemo.java:63) at feedingschedule.FillDemo.actionPerformed(FillDemo.java:56) at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022) at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348) at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402) at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259) at 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) 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) at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4525) at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4466) at 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 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) at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:86) at 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) at java.awt.EventQueue.dispatchEvent(EventQueue.java:728) at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201) at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116) at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105) at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101) at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93) at java.awt.EventDispatchThread.run(EventDispatchThread.java:82) BUILD SUCCESSFUL (total time: 3 seconds)

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);
    }

  }
}

Answer 1

进入你的程序，

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

然后，您尝试对其执行一些操作，

 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){
                ......
            }
    }