尝试使用自定义 JPanel 中的方法时出现 NullPointerException

Question

我正在尝试为我不久前创建的游戏制作 GUI，但我运行在运行制作它时遇到了一个小问题。

我想将输出打印到扩展 JPanel 中的 JTextField。但是，当我运行它时，它出现了这个错误：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at Classic.print(Classic.java:509)
    at Classic.play(Classic.java:43)
    at Karma.actionPerformed(Karma.java:134)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2346)
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:6525)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
at java.awt.Component.processEvent(Component.java:6290)
at java.awt.Container.processEvent(Container.java:2234)
at java.awt.Component.dispatchEventImpl(Component.java:4881)
at java.awt.Container.dispatchEventImpl(Container.java:2292)
at java.awt.Component.dispatchEvent(Component.java:4703)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4898)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4533)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4462)
at java.awt.Container.dispatchEventImpl(Container.java:2278)
at java.awt.Window.dispatchEventImpl(Window.java:2750)
at java.awt.Component.dispatchEvent(Component.java:4703)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
at java.awt.EventQueue.access0(EventQueue.java:97)
at java.awt.EventQueue.run(EventQueue.java:709)
at java.awt.EventQueue.run(EventQueue.java:703)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain.doIntersectionPrivilege(ProtectionDomain.java:75)
at java.security.ProtectionDomain.doIntersectionPrivilege(ProtectionDomain.java:86)
at java.awt.EventQueue.run(EventQueue.java:731)
at java.awt.EventQueue.run(EventQueue.java:729)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain.doIntersectionPrivilege(ProtectionDomain.java:75)
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)

这里是适用的代码：

（我将跳过 Karma.java:134，因为这只是播放按钮。）

Classic.java

import static javax.swing.SwingUtilities.invokeLater; //This is to show where invokeLater comes from
...
public class Classic extends Game {
    private static JFrame gui;
    private static GUIClassic newContentPane;
    ...
    public void play() {
        invokeLater(Classic::startGUI);
        //The next line is "Classic.java:43
        //length, difficulty, and log are all strings that were initialized when the game was instantiated
        print("Selected Options:\nLength: " + length + "\nDifficulty: " + difficulty + "\nOutput Log? " + log + "\n");
        ...
    }
    private static void startGUI() {
        gui = new JFrame("Karma :: Classic Mode");
        gui.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        newContentPane = new GUIClassic();
        newContentPane.setOpaque(true);
        gui.setContentPane(newContentPane);
        gui.pack();
        gui.setVisible(true);
    }
    private static void print(String text) {
        newContentPane.appendOutput(text);
    }

GUIClassic.java（用于内容面板的 class） [编辑：这是上下文的完整 class 内容。]

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
public class GUIClassic extends JPanel implements ActionListener {
    private JTextArea output;
    private JTextField input;
    private boolean inputReady;
    private String inputText;
    public GUIClassic() {
        super();
        setLayout(new BoxLayout(this, BoxLayout.PAGE_AXIS));
        output = new JTextArea(15, 15);
        output.setEditable(false);
        JScrollPane outputScroll = new JScrollPane(output);
        input = new JTextField("",40);
        add(outputScroll);
        add(Box.createRigidArea(new Dimension(0,5)));
        add(input);
    }
    public void actionPerformed(ActionEvent e) {
        inputText = input.getText();
        inputReady = true;
        input.setText("");
    }
    public boolean isInputReady() {
        return inputReady;
    }
    public String getInput() {
        if (!isInputReady())
            return null;
        inputReady = false;
        return inputText;
    }
    public void appendOutput(String addition) {
        output.append(addition + "\n");
    }
}

有意思的是，GUI在异常发生后确实会弹出并一直存在。它只是不将输出打印到 JTextArea 并卡住。

如果您需要更多上下文代码，请告诉我，我会添加它。

编辑：有些人指出某些字段可能未初始化。我相应地解决了这些问题。我试图找出导致异常的确切原因，但失败了。它不应该是 'newContentPane' 或 'output'，因为它们都会在程序启动时出现在屏幕上。

Answer 1

您在创建 JTextArea 之前调用了 print。 invokeLater 在 print 调用之前运行。修复 (IMO) 的最佳方法是在 Runnable.run 方法末尾打印输出语句，该方法首先调用 Classic.startGUI，通过 invokeLater 启动，而不是 Classic::startGUI 构造函数本身

    invokeLater(Classic::startGUI);
    //The next line is "Classic.java:43
    //length, difficulty, and log are all strings that were initialized when the game was instantiated
    print("Selected Options:\nLength: " + length + "\nDifficulty: " + difficulty + "\nOutput Log? " + log + "\n");

Answer 2

检查一下，这是你的错误：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException at Classic.print(Classic.java:509)

这是你的 Classic#print

private static void print(String text) {
    newContentPane.appendOutput(text);
}

Here, the only scenario that would throw a NullPointerException is when newContentPane equalsnull.

是的，您可能有一些初始化 newContentPane 的代码，但它没有在您调用 Classic#print 之前及时发生。

那么，为什么没有及时发生呢？就像每个人一直试图告诉你的那样，它与 invokeLater.

的使用有关

尝试使用自定义 JPanel 中的方法时出现 NullPointerException

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