java - 尝试从文本字段获取文本以移动到文本区域

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 7 年前。

package edu.bsu.cs121.mamurphy;

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

public class InClass1Main extends JFrame {

    JButton button1;
    JTextField textField1;
    JTextArea textArea1;
    int buttonClicked;

    public static void main(String[] args) {
        new InClass1Main();
    }

    public InClass1Main() {

        Toolkit tk = Toolkit.getDefaultToolkit();

        Dimension dim = tk.getScreenSize();
        this.setSize((dim.width / 2), (dim.height / 2));
        int xPos = (dim.width / 2) - (this.getWidth() / 2);
        int yPos = (dim.height / 2) - (this.getHeight() / 2);

        JPanel panel = new JPanel();
        JLabel label1 = new JLabel("Translation Panel");
        JButton button1 = new JButton("Transfer");

        ListenForButton button = new ListenForButton();

        button1.addActionListener(button);

        JTextField textField1 = new JTextField();
        JTextArea textArea1 = new JTextArea(10, 50);
        JScrollPane scrollBar1 = new JScrollPane(textArea1, JScrollPane.VERTICAL_SCROLLBAR_AS_NEEDED,
                JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);

        textField1.setColumns(50);

        textArea1.setLineWrap(true);
        textArea1.setWrapStyleWord(true);

        panel.add(label1);
        panel.add(button1);
        panel.add(textField1);
        panel.add(scrollBar1);

        this.add(panel);
        this.setLocation(xPos, yPos);
        this.setResizable(false);
        this.setTitle("Translation Frame");
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setVisible(true);

        textField1.requestFocus();

    }

    private class ListenForButton implements ActionListener {

        @Override
        public void actionPerformed(ActionEvent ae) {
            textArea1.setText(textField1.getText());
        }
    }
}

现在，每当我尝试在文本字段中实际输入文本，然后将其放入文本区域时，都会收到错误。我无法理解这个错误到底有什么意义。

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at edu.bsu.cs121.mamurphy.InClass1Main$ListenForButton.actionPerformed(InClass1Main.java:66)
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
    at javax.swing.AbstractButton$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$500(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.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)

我真的不知道是什么导致了这个错误。它给我的很多原因(正如你在上面看到的)是(来源不明)。因此，任何关于为什么会发生这种情况的帮助将不胜感激。

Answer 1

从 ListenForButton 主体中删除 addActionListener 语句

private class ListenForButton implements ActionListener {

    @Override
    public void actionPerformed(ActionEvent ae) {
        textArea1.setText(textField1.getText());
    }
}