Java ActionListener

Question

试图创建动作侦听器来执行乘法任务。它似乎有效，但它似乎忽略了我输入的最后一个数字。我一直在想，我需要一个第二个变量来跟踪最后一个命令，这样当按下“相等”按钮时，它就会将当前的数字添加到前面的命令中。但是，如果按下相等的按钮，我就不用去执行任务了？

class ButtonListener implements ActionListener {        
    public void actionPerformed(ActionEvent e) {
        String numbers = e.getActionCommand();
        if (begin) {
            textField.setText(numbers);
            begin = false;
        }

        else if (numbers.equals("C")) {
            textField.setText("0.0");
            action.reset();
        }

        else 
            textField.setText(textField.getText() + numbers);
    }
}

class OperatorListener implements ActionListener {  

    public void actionPerformed(ActionEvent e) {
        String command = e.getActionCommand();
        String text = textField.getText();

        if (begin) {
            textField.setText("0.0");
            action.setTotal("0.0");
        }
        else {
            if (command.equals("+")) {
                action.add(text);
                begin = true;
            }
            else if (command.equals("=")) {
                textField.setText(text);
                System.out.println(action.getTotal());
            }
            textField.setText(action.getTotal());
        }
    }
}

对变量的一些解释。begin只需检查JTextField的当前状态是否为空。action只是用来“添加”的方法，它还有我希望它做的其他调用。

对于如何跟踪最后一条命令或绕过以避免忽略最后一位数，有什么建议吗？这些任务类似于计算器的任务。

编辑:对于那些感兴趣的人来说，这是我最后得到的结果。

class ButtonListener implements ActionListener {        
    public void actionPerformed(ActionEvent e) {
        String numbers = e.getActionCommand();

        // check if a number has been pressed
        if (begin) {
            textField.setText(numbers);
            begin = false;
        }

        // if clear is checked
        else if (numbers.equals("C")) {
            action.reset();
            begin = true;
            operator = "=";
            textField.setText("0.0");
        }

        // Once a number is pressed keep adding it to the text field until an operator is pressed

        else 
            textField.setText(textField.getText() + numbers);
    }
}

/**
 * Action listener for Operators
 */

class OperatorListener implements ActionListener {  

    public void actionPerformed(ActionEvent e) {
        // String command = e.getActionCommand();

        // if nothing has been pressed
        if (begin) {
            textField.setText("0.0");
            begin = false;

        // else begin performing operation pressed
        } else {
            begin = true; 
            String text = textField.getText();

            // right away add the value of the text field
            if (operator.equals("=")) {
                action.setTotal(text);
            } 
            else if (operator.equals("+")) {
                action.add(text);
            } 
            // and so on for all the other operators.

            textField.setText("" + action.getTotal());

            // now capture the operator pressed.
            operator = e.getActionCommand();
        }
    }
}

Answer 1

附带注意:一般来说，为每个按钮创建一个单独的监听器要比尝试共享一个侦听器要容易。当您创建一个侦听器时，首先要将几个按钮放入一个函数，然后要求该函数将它们分开。为什么不把它们分开呢？如果有公共代码，请调用公共函数。

但对于您的问题:如果所有运算符都有相同的优先级，那么只需保存运算符。如：

class Calculator
{
  // Well, this would probably better be an enum, but whatever.
  public char operator=' ';
  public float lastNum;

  public void calc()
  {
    try
    {
      double currNum=Double.parseDouble(numfield.getText());
      if (operator==' ')
        lastNum=currNum;
      else if (operator=='+')
        lastNum+=currNum;
      else if (operator=='-')
        lastNum-=currNum;
      else if (operator=='*')
        lastNum*=currNum;
      else if (operator=='/')
        lastNum/=currNum;
    }
    catch (NumberFormatException panic)
    {
      ... whatever error handling ...
    }
  }
}
class OperatorListener implements ActionListener
{
   Calculator calc;
   public OpeatorListener(Calculator calc)
   {
     this.calc=calc;
   }
   public abstract void actionPerformed(ActionEvent e);
}
class PlusListener extends OperatorListener
{
  public void actionPeformed(ActionEvent e)
  {
    calc.calc();
    calc.operator='+';
  }
}
class MinusListener extends OperatorListener
{
  public void actionPerformed(ActionEvent e)
  {
    calc.calc();
    calc.operator='-';
  }
}
class EqualListener extends OperatorListener
{
  public void actionPerformed(ActionEvent e)
  {
    calc.calc();
  }
}

该代码给予所有运算符同等的优先权，例如"2+3*5=“将给出25。

如果您希望乘法和除法具有更高的优先级，例如"2+3*5=“给出17，则需要创建一个堆栈并保存操作符、临时中间值和一些其他复杂性。

Answer 2

在JTextField中存储操作数时，可以方便地侦听property changes、document changes或focus changes，以便决定采取什么操作。