使用 ActionListener 时内部类中的非法静态声明

Question

我正在尝试为家庭作业编写一个基本的计算器 GUI 程序。我试图为计算器上按下的每个按钮定义一个变量，并在声明所有变量时进行计算。现在，它只能加/减/除/乘两个数字 0-9，但我想确保在扩展它之前我可以让它工作。我的问题是我收到错误代码“内部类 Calculator.sign 中的非法静态声明”。我想知道如何才能克服这个错误。

谢谢

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
/*
This program will display a calculator by GUI application. It accepts
floating point numbers.
*/
public class Calculator extends JFrame
{
   private final int WINDOW_WIDTH = 350;
   private final int WINDOW_HEIGHT = 350;
   private double num1 = 0;
   private double num2 = 0;
   private int sign;
   private double answer;

   JButton seven = new JButton("7");
   JButton eight = new JButton("8");
   JButton nine = new JButton("9");
   JButton mult = new JButton("x");
   JButton four = new JButton("4");
   JButton five = new JButton("5");
   JButton six = new JButton("6");
   JButton min = new JButton("-");
   JButton one = new JButton("1");
   JButton two = new JButton("2");
   JButton three = new JButton("3");
   JButton plus = new JButton("+");
   JButton zero = new JButton("0");
   JButton point = new JButton(".");
   JButton equ = new JButton("=");
   JButton div = new JButton("/");


   //contructor
   public Calculator()
   {
      setTitle("Calculator");

      setSize(WINDOW_WIDTH, WINDOW_HEIGHT);

      setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

      setLayout(new GridLayout(4,4));

      add(seven);
      add(eight);
      add(nine);
      add(mult);
      add(four);
      add(five);
      add(six);
      add(min);
      add(one);
      add(two);
      add(three);
      add(plus);
      add(zero);
      add(point);
      add(equ);
      add(div);

      seven.addActionListener(new ButtonListener());
      eight.addActionListener(new ButtonListener());
      nine.addActionListener(new ButtonListener());
      mult.addActionListener(new ButtonListener());
      four.addActionListener(new ButtonListener());
      five.addActionListener(new ButtonListener());
      six.addActionListener(new ButtonListener());
      min.addActionListener(new ButtonListener());
      one.addActionListener(new ButtonListener());
      two.addActionListener(new ButtonListener());
      three.addActionListener(new ButtonListener());
      plus.addActionListener(new ButtonListener());
      zero.addActionListener(new ButtonListener());
      point.addActionListener(new ButtonListener());
      equ.addActionListener(new ButtonListener());
      div.addActionListener(new ButtonListener());

      setVisible(true);
   }

   private class ButtonListener implements ActionListener
   {
      public void actionPerformed(ActionEvent e)
      {
         if(e.getSource() == seven)
        {
            num1 = 7;
        }
        else if(e.getSource() == eight)
        {
            num1 = 8;
        }
        else if(e.getSource() == nine)
        {
            num1 = 9;
        }
        else if(e.getSource() == four)
        {
            num1 = 4;
        }
        if(e.getSource() == five)
        {
            num1 = 5;
        }
        else if(e.getSource() == six)
        {
            num1 = 6;
        }
        else if(e.getSource() == one)
        {
            num1 = 1;
        }
        else if(e.getSource() == two)
        {
            num1 = 2;
        }
        if(e.getSource() == three)
        {
            num1 = 3;
        }
        else if(e.getSource() == zero)
        {
            num1 = 0;
        }
      }
   }
   private class sign implements ActionListener
   {
      public void actionPerformed(ActionEvent e)
      {
         if(e.getSource() == mult)
         {
            sign = 1;
         }
         else if(e.getSource() == div)
         {
            sign = 2;
         }
         else if(e.getSource() == plus)
         {
            sign = 3;
         }
         else if(e.getSource() == min)
         {
            sign = 4;
         }
      }
      private class ButtonListener2 implements ActionListener
      {
         public void actionPerformed(ActionEvent e)
         {
            if(e.getSource() == seven)
        {
            num2 = 7;
        }
        else if(e.getSource() == eight)
        {
            num2 = 8;
        }
        else if(e.getSource() == nine)
        {
            num2 = 9;
        }
        else if(e.getSource() == four)
        {
            num2 = 4;
        }
        if(e.getSource() == five)
        {
            num2 = 5;
        }
        else if(e.getSource() == six)
        {
            num2 = 6;
        }
        else if(e.getSource() == one)
        {
            num2 = 1;
        }
        else if(e.getSource() == two)
        {
            num2 = 2;
        }
        if(e.getSource() == three)
        {
            num2 = 3;
        }
        else if(e.getSource() == zero)
        {
            num2 = 0;
        }

        }
      }
      private class equals implements ActionListener
      {
         public void actionPerformed(ActionEvent e)
         {
            if(sign == 1)
            {
               answer = num1*num2;
               JOptionPane.showMessageDialog(null, answer);
            }
            else if(sign == 2)
            {
               answer = num1/num2;
               JOptionPane.showMessageDialog(null, answer);
            }
            else if(sign == 3)
            {
               answer = num1 + num2;
               JOptionPane.showMessageDialog(null, answer);
            }
            else if(sign == 4)
            {
               answer = num1 - num2;
               JOptionPane.showMessageDialog(null, answer);
            }
         }

   }

   /*
   Main Method
   */
   public static void main(String[] args)
   {
      new Calculator();
   }



}
}

Answer 1

我想知道如何才能通过此错误。

您可以从识别错误的性质开始。它在抱怨静态声明，那么它可以谈论什么声明呢？那应该不难。编译器会给你一个错误的行号，但即使没有这个，整个程序中也只有一个静态声明：

   public static void main(String[] args) {
      new Calculator();
   }

这是在课堂上Calculator.sign 吗？为什么是的，如果你仔细看，你会发现它是。那是它应该在的地方吗？我倾向于不这么认为，但如果这确实是您的意图，那么您需要再考虑一下，因为内部类（与静态嵌套类相反）可能没有任何静态成员。

您可能还想查看Calculator.sign 类中是否还有其他实际上不应该存在的内容。