java - JButton 和 ActionListener 在 java.awt、安全性和事件中产生多个异常？

Question

我在东、西、北、南有四个按钮，我试图让按下的按钮更改其文本。在 Eclipse 中，编译前似乎没有问题，但是当我编译并尝试按四个按钮中的任何一个时，它会显示许多异常。

import java.awt.event.*;
import java.awt.*;
import javax.swing.*;

public class gui {
    JButton rightbutton;
    JButton leftbutton;
    JButton downbutton;
    JButton upbutton;

    public static void main(String[] args) {
        gui GUI = new gui();
        GUI.go();
    }

    public void go() {
        JFrame myFrame = new JFrame("The Gui");
        JPanel myPanel = new JPanel();

        JButton rightbutton = new JButton("Right Button");
        rightbutton.addActionListener(new rightListener());
        JButton leftbutton = new JButton ("Button of the west");
        leftbutton.addActionListener(new leftListener());
        JButton downbutton = new JButton ("Button on the bottom");
        downbutton.addActionListener(new downListener());
        JButton upbutton = new JButton ("Button in the sky");
        upbutton.addActionListener(new upListener());   

        myFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        myFrame.setSize(500,500);
        myFrame.setVisible(true);

        myFrame.getContentPane().add(myPanel);
        myFrame.getContentPane().add(BorderLayout.EAST,rightbutton);
        myFrame.getContentPane().add(BorderLayout.NORTH,upbutton);
        myFrame.getContentPane().add(BorderLayout.SOUTH,downbutton);
        myFrame.getContentPane().add(BorderLayout.WEST,leftbutton);
    }
        class rightListener implements ActionListener {
            public void actionPerformed(ActionEvent event) {
                rightbutton.setText("Pushed");
            }
        }
        class leftListener implements ActionListener {
            public void actionPerformed(ActionEvent event) {
                leftbutton.setText("Pushed");
            }
        }
        class downListener implements ActionListener {
            public void actionPerformed(ActionEvent event) {
                downbutton.setText("Pushed");
            }
        }
        class upListener implements ActionListener {
            public void actionPerformed(ActionEvent event) {
                upbutton.setText("Pushed");
            }
        }
}

以下是当我按下四个按钮中的任何一个时给出的所有异常(exception)情况。

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at gui$leftListener.actionPerformed(gui.java:46)
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
    at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
    at javax.swing.plaf.basic.BasicButtonListener.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$200(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$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at gui$upListener.actionPerformed(gui.java:56)
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
    at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
    at javax.swing.plaf.basic.BasicButtonListener.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$200(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)

Answer 1

您的问题是您声明了JButtons，但没有将它们放入为它们保留的实例变量中。您可以通过替换以下行来解决此问题:

JButton rightbutton = new JButton("Right Button");

与:

rightbutton = new JButton("Right Button");

第一种形式声明的局部变量掩盖了实例变量，因此实例变量消失了。当监听器尝试访问实例变量时，会发生空指针异常，该实例变量实际上为空，因为它从未被初始化。

解决此问题的另一种方法是使用 event.getSource() 而不是使用实例变量，在这种情况下实例变量就变得多余。