爲什麼JTextField的setText在actionPerformed方法中不起作用？

Question

即時製作一個貨幣計算器，我有一個問題，無法將計算結果設置爲結果JTextField，這是對象otherTextField。

這裏是我的類：

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; 

     } 
    } 
} 

我知道貨幣是不正確的，還有其他的邏輯上的缺陷和不良變量名，但我現在的問題是，爲什麼我不能從方法把結果calculateFromNok到我的JTextField otherTextField？

幫助讚賞thx！

Answer 1

您正在創建初始化局部變量JTextField nokTextField。
因此private JTextField nokTextField;未初始化。這就是爲什麼它給你nullpointerexception。

只是改變行號37：

JTextField nokTextField = new JTextField(); 

要

nokTextField = new JTextField(); 

Answer 2

你的問題是與線：

JTextField nokTextField = new JTextField(); 

將其更改爲：

nokTextField = new JTextField(); 

發生這種情況的原因是，您之前在頂部定義了nokTextField，然後在構造函數中使用相同名稱初始化了一個單獨的變量。使用諸如Eclipse之類的IDE可以很容易地捕捉到這些類型的錯誤。我建議你使用一個。