基本Java GUI兴趣计算器中的线程“ AWT-EventQueue-0”中的异常java.lang.NullPointerException

Question

我是一位相当新手的程序员，她试图学习swing和GUI构建的基础知识。尝试构建一个Java Gui，在该Java Gui中获得最终的双倍余额和初始余额，并在按下按钮时为其添加最终的双倍5％利息。我不断收到错误

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at interest.Balance$ButtonListener.actionPerformed(Balance.java:30)
    at java.desktop/javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1967)
    at java.desktop/javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2308)
    at java.desktop/javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:405)
    at java.desktop/javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:262)
    at java.desktop/javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:279)
    at java.desktop/java.awt.Component.processMouseEvent(Component.java:6636)
    at java.desktop/javax.swing.JComponent.processMouseEvent(JComponent.java:3342)
    at java.desktop/java.awt.Component.processEvent(Component.java:6401)
    at java.desktop/java.awt.Container.processEvent(Container.java:2263)
    at java.desktop/java.awt.Component.dispatchEventImpl(Component.java:5012)
    at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2321)
    at java.desktop/java.awt.Component.dispatchEvent(Component.java:4844)
    at java.desktop/java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4918)
    at java.desktop/java.awt.LightweightDispatcher.processMouseEvent(Container.java:4547)
    at java.desktop/java.awt.LightweightDispatcher.dispatchEvent(Container.java:4488)
    at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2307)
    at java.desktop/java.awt.Window.dispatchEventImpl(Window.java:2762)
    at java.desktop/java.awt.Component.dispatchEvent(Component.java:4844)
    at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:772)
    at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:721)
    at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:715)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
    at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
    at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:95)
    at java.desktop/java.awt.EventQueue$5.run(EventQueue.java:745)
    at java.desktop/java.awt.EventQueue$5.run(EventQueue.java:743)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
    at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
    at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:742)
    at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:203)
    at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124)
    at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113)
    at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109)
    at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
    at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90

而且似乎无法弄清楚我在哪里出错了。我没有null，并且在eclipse中没有错误。我已经搜索了此问题，但情况视情况而异。我认为当我尝试将JLabel的文本设置为JLabel时会出现问题，但是即使这样我也不确定如何解决。

这是我的代码，我在GUI中使用了两个不同的类，BalancePane.java和Balance.java

import javax.swing.JFrame;

public class BalancePane {

    public static void main(String[] args) {
        JFrame frame = new JFrame();
        Balance panel = new Balance();

        frame.setDefaultCloseOperation(frame.EXIT_ON_CLOSE);    
        frame.getContentPane().add(panel);  
        frame.setSize(300, 500);    
        frame.pack();
        frame.setVisible(true);
    }
}

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

public class Balance extends JPanel {
    private JTextField balanceTextField;
    private final double INITIAL_BALANCE = 1000;
    private final double INTEREST_RATE = 5; 

    public Balance() {
        JButton interestButton = new JButton("Add interest");
        JTextField balanceTextField = new JTextField(10);

        interestButton.addActionListener(new ButtonListener());

        add(interestButton);
        add(balanceTextField);

        balanceTextField.setText(Double.toString(INITIAL_BALANCE));
    }

    private class ButtonListener implements ActionListener {
        public void actionPerformed(ActionEvent event) {

            double balance = INITIAL_BALANCE * (INTEREST_RATE)/100;
            balanceTextField.setText(Double.toString(balance));
        }
    }
}

Answer 1

public class Balance extends JPanel {
    private JTextField balanceTextField;
    // ...

[balanceTextField是一个字段，最初是null。

    public Balance() {
        // ...
        JTextField balanceTextField = new JTextField(10);

糟糕。 balanceTextField是局部变量。永远不会设置相同名称的字段。

        // ...
            balanceTextField.setText(Double.toString(balance));

并且这将取消引用从未分配的balanceTextField字段。