Java显示消息问题

Question

我有一个java程序，它创建一个像这样的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 JFrame 中的字段未填写，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

其中一个变量为空。可能是:

• 选择器
• chooser.getSelectedFile()
• 保存选择器
• saveChooser.getSelectedFile()
• 解析文件
• startSpanText
• 第 2 列到第 6 列

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是哪一个。我的猜测是选定的文件之一，因为调用

没有意义

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

要么没有选定的文件，并且它会抛出一个 NPE，因为 getSelectedFile() 返回 null，要么有一个，并且它的 toString() 永远不会为 null .

此外，您不应该在按下按钮时使用鼠标监听器执行某些操作。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。