如何从另一个类访问java字段值？

Question

null

class Main {
  public static void main(String[] args) {
    JFrame frame = new CurrencyConversionFrame();
  } 
}

public class CurrencyConversionFrame extends JFrame {
  private JTextField currencyAmountField;

  private JButton createConvertButton() {
    JButton button = new JButton("Convert");
    ActionListener listener = new ConvertListener();
    button.addActionListener(listener);
    return button;
  }
}

class ConvertListener implements ActionListener {
  public void actionPerformed(ActionEvent action) {
    double balance = 0;
    try {
      balance = Double.parseDouble(currencyAmountField.getText());
    }
  }
}

null

ConvertListener.java:9: error: cannot find symbol
      balance = Double.parseDouble(currencyAmountField.getText());
                                   ^
  symbol:   variable currencyAmountField
  location: class ConvertListener

Answer 1

为了访问类字段（按照惯例），您应该声明getter和setter，所以在您的情况下，它将是

public void setCurrencyAmountField(JTextField currencyAmountField) {
    this.currencyAmountField = currencyAmountField;
}

public JTextField getCurrencyAmountField() {
    return this.currencyAmountField;
}

这样可以通过getCurrencyAmountField（）访问变量；

还有一个可能有帮助的方法是直接将ActionListener实现到帧中，并通过switch语句确定实际执行了哪种类型的操作（如果帧有多种可能的话）

public class CurrencyConversionFrame extends JFrame implements ActionListener {
   
   ...
   
    public void actionPerformed(ActionEvent action) {
        switch(action.getActionCommand()) {
            case "Name Of the button":
                balance = Double.parseDouble(currencyAmountField.getText());
                break;
            default:
                System.out.println("The action is not yet defined");
        }
    }
}