使用ActionListener時內部類中的靜態靜態聲明

Question

我正在嘗試編寫一個用於家庭作業的基本計算器GUI程序。 我正在嘗試為計算器上按下的每個按鈕定義一個變量，並在聲明所有變量時進行計算。 現在，它只能將兩個數字0-9相加/相減/相除/相乘，但是我想確保在擴展之前可以正常工作。 我的問題是我得到了錯誤代碼“內部類Calculator.sign中的非法靜態聲明”。 我想知道如何克服這個錯誤。

謝謝

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

我想知道如何通過此錯誤。

您可以先確定錯誤的性質。 它抱怨靜態聲明，那么它在說什么聲明？ 那不應該很難。 編譯器會為您提供錯誤的行號，但是即使沒有該行號，整個程序中也只有一個靜態聲明：

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

那是在Calculator.sign類中嗎？ 是的，如果仔細看，您會發現事實確實如此。 那是應該在的地方嗎？ 我傾向於不考慮，但是如果這確實是您的意圖，那么您將需要再考慮一下，因為內部類（與靜態嵌套類相對）可能沒有任何靜態成員。

您可能還想查看在Calculator.sign類中是否存在其他實際上不應該存在的內容。