为什么JTextField的setText在actionPerformed方法中不起作用？

Question

即时制作一个货币计算器，我有一个问题，无法将计算结果设置为结果JTextField，这是对象otherTextField。

这里是我的类：

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

public class ValutaKalkulator implements ActionListener { 
    private double nokValue; 
    private double otherValue; 
    private JButton buttonRemoveNok; 
    private JButton removeOther; 
    private JButton removeBoth; 
    private JButton exitButton; 
    private JButton usdButton; 
    private JButton sekButton; 
    private JButton gbpButton; 
    private JButton eurButton; 
    private JLabel nokLabel; 
    private JTextField nokTextField; 
    private JLabel otherLabel; 
    private JTextField otherTextField; 

    public ValutaKalkulator() 
    { 
     JFrame frame = new JFrame(); 
     frame.setTitle("VALUTAKALKULATOR"); 
     buttonRemoveNok = new JButton("Fjern NOK"); 
     removeOther = new JButton("Fjern annen valuta"); 
     removeBoth = new JButton("Fjern begge"); 
     exitButton = new JButton("Avslutt"); 
     usdButton = new JButton("USD"); 
     sekButton = new JButton("SEK"); 
     gbpButton = new JButton("GBP"); 
     eurButton = new JButton("EUR"); 
     nokLabel = new JLabel("NOK"); 
     JTextField nokTextField = new JTextField(); 
     otherLabel = new JLabel("Annen valuta"); 
     otherTextField = new JTextField(); 

     buttonRemoveNok.addActionListener(this); 
     removeOther.addActionListener(this); 
     removeBoth.addActionListener(this); 
     exitButton.addActionListener(this); 
     usdButton.addActionListener(this); 
     sekButton.addActionListener(this); 
     gbpButton.addActionListener(this); 
     eurButton.addActionListener(this); 

     JPanel pnlSouth = new JPanel(new GridLayout(1, 4)); 
     JPanel pnlCenter = new JPanel(new GridLayout(2, 2)); 
     JPanel pnlNorth = new JPanel(new GridLayout(1, 4)); 

     pnlNorth.add(nokLabel); 
     pnlNorth.add(nokTextField); 
     pnlNorth.add(otherLabel); 
     pnlNorth.add(otherTextField); 

     pnlCenter.add(gbpButton); 
     pnlCenter.add(eurButton); 
     pnlCenter.add(usdButton); 
     pnlCenter.add(sekButton); 

     pnlSouth.add(buttonRemoveNok); 
     pnlSouth.add(removeOther); 
     pnlSouth.add(removeBoth); 
     pnlSouth.add(exitButton); 

     frame.add(pnlNorth, BorderLayout.NORTH); 
     frame.add(pnlSouth, BorderLayout.SOUTH); 
     frame.add(pnlCenter, BorderLayout.CENTER); 

     frame.setVisible(true); 
     frame.pack(); 

    } 

    public String calculateFromNok(String nokValueString, String toValue) 
    { 
     double result = 0; 
     double nokValue = Double.valueOf(nokValueString); 
     switch(toValue) 
     { 
      case "GBP": 
      result = nokValue * 10.8980/100; 
      break; 

      case "EUR": 
      result = nokValue * 9.2450/100; 
      break; 

      case "USD": 
      result = nokValue * 8.5223/100; 
      break; 

      case "SEK": 
      result = nokValue * 96.48/100; 
      break; 
     } 
     String resultString = Double.toString(result); 
     return resultString; 
    } 

    public void actionPerformed(ActionEvent event) 
    { 
     String text = event.getActionCommand(); 

     switch(text) 
     { 
      case "USD": 
      String txt = nokTextField.getText(); 
      String txt2 = calculateFromNok(txt, "USD"); 
      otherTextField.setText(txt2); 
      break; 

     } 
    } 
} 

我知道货币是不正确的，还有其他的逻辑上的缺陷和不良变量名，但我现在的问题是，为什么我不能从方法把结果calculateFromNok到我的JTextField otherTextField？

帮助赞赏thx！

Answer 1

您正在创建初始化局部变量JTextField nokTextField。
因此private JTextField nokTextField;未初始化。这就是为什么它给你nullpointerexception。

只是改变行号37：

JTextField nokTextField = new JTextField(); 

要

nokTextField = new JTextField(); 

Answer 2

你的问题是与线：

JTextField nokTextField = new JTextField(); 

将其更改为：

nokTextField = new JTextField(); 

发生这种情况的原因是，您之前在顶部定义了nokTextField，然后在构造函数中使用相同名称初始化了一个单独的变量。使用诸如Eclipse之类的IDE可以很容易地捕捉到这些类型的错误。我建议你使用一个。