java - 我的表达是非法的，我也不知道为什么

当我编译此行74作为非法的表达开始时，为什么会这样？
我一直无法弄清楚，任何帮助将不胜感激
谢谢。

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

public class NumericKeypadPanel2 extends JPanel
{
    public NumericKeypadPanel2()
    {
        String num = " ";


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

        JPanel keypad = new JPanel();
        keypad.setLayout(new GridLayout (4, 3));
        keypad.setBorder (BorderFactory.createLineBorder (Color.black, 3));

        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 ("#");

        NumListener listener = new NumListener();

        b1.addActionListener (listener);

        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);

        panel_main.add(keypad, BorderLayout.CENTER);

        JButton clear = new JButton ("Clear");

        JLabel display = new JLabel(num);

        panel_main.add(clear, BorderLayout.SOUTH);
        panel_main.add(display, BorderLayout.NORTH);
        display.setBorder (BorderFactory.createLineBorder (Color.black, 3));

        add(panel_main);

        private class NumListener implements ActionListener
        {
            public void actionPerformed (ActionEvent event)
            {
                Object source = event.getSource();

                switch (source)
                {
                case b1:
                    display.setText(display.getText()+"1");
                    break;

                case b2:
                    display.setText(display.getText()+"2");
                    break;

                case b3:
                    display.setText(display.getText()+"3");
                    break;

                case b4:
                    display.setText(display.getText()+"4");
                    break;

                case b5:
                    display.setText(display.getText()+"5");
                    break;

                case b6:
                    display.setText(display.getText()+"6");
                    break;

                case b7:
                    display.setText(display.getText()+"7");
                    break;

                case b8:
                    display.setText(display.getText()+"8");
                    break;

                case b9:
                    display.setText(display.getText()+"9");
                    break;

                case b10:
                    display.setText(display.getText()+"*");
                    break;

                case b11:
                    display.setText(display.getText()+"0");
                    break;

                case b12:
                    display.setText(display.getText()+"#");
                    break;
                }


            }
        }



    }
}

最佳答案

只是一个疯狂的猜测：

 Object source = event.getSource();
 switch (source){ <-- ERROR

据我所知，switch语句不适用于对象。