Java Swing - 如何使JProgressBar可见按钮点击

import java.awt.Color;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JProgressBar;

public class Main {

  public static void main(String arg[]) {
    MyFrame m = new MyFrame();
    m.setSize(330, 100);
    m.setVisible(true);
  }
}

class MyFrame extends JFrame implements ActionListener {
  JProgressBar pb;
  JButton b1 = new JButton("LOGIN");

  MyFrame() {
    setLayout(null);    
    b1.setBackground(Color.yellow);
    pb = new JProgressBar(1, 100);
    pb.setValue(0);
    pb.setStringPainted(true);
    b1.setBounds(20, 20, 80, 25);
    pb.setBounds(110, 20, 200, 25);
    pb.setVisible(false);
    add(b1);
    add(pb);
    b1.addActionListener(this);
    setResizable(false);
    setDefaultCloseOperation(EXIT_ON_CLOSE);
  }
  public void actionPerformed(ActionEvent e) {
    int i = 0;
    if (e.getSource() == b1) {
      pb.setVisible(true);
      try {
        while (i <= 100) {
          Thread.sleep(50);
          pb.paintImmediately(0, 0, 200, 25);
          pb.setValue(i);
          i++;
        }
      } catch (Exception e1) {
        System.out.print("Caughted exception is =" + e1);
      }
    }
  }
}