Question

我有一个java程序，可以创建一个像这样的JFrame：

        javax.swing.SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            gui.setFrame(new gui(), 1000, 300);
        }
    });

我还有一个实现setFrame的类（gui.java）：

  public static void setFrame(final JFrame frame, final int width, final int height) {
 SwingUtilities.invokeLater(new Runnable() {
 public void run() {
   f1=frame;
 frame.setTitle("Testing");
 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 frame.setSize(width, height);
 frame.setVisible(true);   
}
});
}

如果用户尝试单击“提交”（我创建的按钮）并且未填充JFrame中的字段，则会引发错误。错误消息的代码是：

     submit.addMouseListener(new MouseListener(){

@Override
public void mouseClicked(MouseEvent e) {}
@Override
public void mouseEntered(MouseEvent e) {}

@Override
public void mouseExited(MouseEvent e) {}

@Override
public void mousePressed(MouseEvent e) {}

@Override
public void mouseReleased(MouseEvent e) {
    //check to make sure all values filled in
          if(chooser.getSelectedFile().toString()!=null&&saveChooser.getSelectedFile().toString()!=null)
        parseFile.readFile(chooser.getSelectedFile(),saveChooser.getSelectedFile(),startSpanText.getText(),(String)col2.getSelectedItem(),(String)col3.getSelectedItem(),(String)col4.getSelectedItem(),(String)col5.getSelectedItem(),(String)col6.getSelectedItem());     
    else
        JOptionPane.showMessageDialog(f1,"Bad");
}

}）;

//注意：f1是我最初收到的帧的静态版本

我得到的错误是：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at gui$3.mouseReleased(gui.java:133)
at java.awt.AWTEventMulticaster.mouseReleased(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$000(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

我做错了什么？

Answer 1

其中一个变量为null。它可能是：

选择器
chooser.getSelectedFile（）
saveChooser
saveChooser.getSelectedFile（）
parseFile
startSpanText
col2 to col6

使用堆栈跟踪中的行号，以及代码中的调试器或跟踪来了解哪一个。我的猜测将是所选文件之一，因为调用

是没有意义的

chooser.getSelectedFile().toString() != null

要么没有选定的文件，它会抛出NPE，因为getSelectedFile()返回null，或者有一个，而toString()永远不会为空。

此外，当按下按钮时，您不应该使用鼠标监听器来执行某些操作。这就是ActionListener的用途。它会更简单，并且当用户用键盘按下按钮时也可以工作。

Answer 2

您的异常消息应指向您的答案。您在mouseReleased方法中使用的其中一项是null：

@Override
public void mouseReleased(MouseEvent e) {
  //check to make sure all values filled in
  if(chooser.getSelectedFile().toString()!=null && 
      saveChooser.getSelectedFile().toString()!=null)
    parseFile.readFile(chooser.getSelectedFile(), saveChooser.getSelectedFile(),
        startSpanText.getText(), (String)col2.getSelectedItem(), 
        (String)col3.getSelectedItem(), (String)col4.getSelectedItem(),
        (String)col5.getSelectedItem(), (String)col6.getSelectedItem());     
  else
    JOptionPane.showMessageDialog(f1,"Bad");
}

使用调试器检查设置了哪些变量。使用例外消息中的行号来帮助您。

Java显示消息问题

2 个答案: