这左边的Java程序代码代表一份完整的源文件。你的任务是要扮

import javax.swing.*;
imoprt java.awt.event.*;
import java.awt.*;
class InnerButton {
  JFrame frame;
  JButton b;
  public static void main(String [] args) {
    InnerButton gui = new InnerButton();
    gui.go();
  }
  public void go() {
    frame = new JFrame();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    b = new JButton("A");
    b.addActionListener();
    frame.getContentPane().add(BorderLayout.SOUTH,b);
    frame.setSize(200,100);
    frame.setVisible(true);
  }
  class BListener extends ActionListener {
    public void actionPerformed(ActionEvent e) {
      if (b.getText().equals("A")) {
        b.setText("B");
      } else {
        b.setText("A");
      }
    }
  }
}