java - 到处都找不到符号错误

Question

这应该是一个由三部分组成的面板，顶部面板是一个标签，当按下中间面板中的按钮时，该标签会更新为数字。当出现以下情况时，标签将被清除: 按下底部的清除按钮。

问题是，几乎每次提到中间面板中的每个按钮都会导致“找不到符号”错误......

如有任何帮助，我们将不胜感激。

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class NumericKeypadPanel2 extends JPanel
{


//----------------------------------------------------------------------


    public NumericKeypadPanel2()
    {
        JButton b1 = new JButton ("1");
        JButton b2 = new JButton ("2");
        JButton b3 = new JButton ("3");
        JButton b4 = new JButton ("4");
        JButton b5 = new JButton ("5");
        JButton b6 = new JButton ("6");
        JButton b7 = new JButton ("7");
        JButton b8 = new JButton ("8");
        JButton b9 = new JButton ("9");
        JButton b10 = new JButton ("*");
        JButton b11 = new JButton ("0");
        JButton b12 = new JButton ("#");
        JButton clear = new JButton ("Clear");

        NumListener listener = new NumListener();

        b1.addActionListener (listener);
        b2.addActionListener (listener);
        b3.addActionListener (listener);
        b4.addActionListener (listener);
        b5.addActionListener (listener);
        b6.addActionListener (listener);
        b7.addActionListener (listener);
        b8.addActionListener (listener);
        b9.addActionListener (listener);
        b10.addActionListener (listener);
        b11.addActionListener (listener);
        b12.addActionListener (listener);
        clear.addActionListener (listener);

        String num = " ";

        JPanel panel_main = new JPanel();
        panel_main.setLayout(new BorderLayout());

        //top
        JLabel display = new JLabel(num);
        display.setBorder (BorderFactory.createLineBorder (Color.black, 3));
        panel_main.add(display, BorderLayout.NORTH);

        //mid
        JPanel keypad = new JPanel();
        keypad.setLayout(new GridLayout (4, 3));
        keypad.setBorder (BorderFactory.createLineBorder (Color.black, 3));
        //adding the buttons
        keypad.add (b1);
        keypad.add (b2);
        keypad.add (b3);
        keypad.add (b4);
        keypad.add (b5);
        keypad.add (b6);
        keypad.add (b7);
        keypad.add (b8);
        keypad.add (b9);
        keypad.add (b10);
        keypad.add (b11);
        keypad.add (b12);
        //adding the panel
        panel_main.add(keypad, BorderLayout.CENTER);

        //btm
        panel_main.add(clear, BorderLayout.CENTER);

    }

    private class NumListener implements ActionListener
    {
        public void actionPerformed (ActionEvent e)
        {
            if(e.getSource() == b1)
            {
                display.setText(display.getText()+"1");
            }
            else if(e.getSource() == b2)
            {
                display.setText(display.getText()+"2");
            }
            else if(e.getSource() == b3)
            {
                display.setText(display.getText()+"3");
            }
            else if(e.getSource() == b4)
            {
                display.setText(display.getText()+"4");
            }
            else if(e.getSource() == b5)
            {
                display.setText(display.getText()+"5");
            }
            else if(e.getSource() == b6)
            {
                display.setText(display.getText()+"6");
            }
            else if(e.getSource() == b7)
            {
                display.setText(display.getText()+"7");
            }
            else if(e.getSource() == b8)
            {
                display.setText(display.getText()+"8");
            }
            else if(e.getSource() == b9)
            {
                display.setText(display.getText()+"9");
            }
            else if(e.getSource() == b10)
            {
                display.setText(display.getText()+"*");
            }
            else if(e.getSource() == b11)
            {
                display.setText(display.getText()+"0");
            }
            else if(e.getSource() == b12)
            {
                display.setText(display.getText()+"#");
            }
            else if(e.getSource() == clear)
            {
                display.setText(" ");
            }

        }
    }

}

Answer 1

所有 JButtons 和 JLabels 都在构造函数中声明为局部变量，因此无法从构造函数外部访问它们。

要使它们可用于其他方法，请将这些变量声明为类中的实例变量(在任何方法或构造函数之外)。然后它们将在整个类(class)的范围内。