以作备注在问题区域，请看。

package test;

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
public class Ex127 {
    public Ex127(){
        new LoginWindow();
    }
    public static void main(String[] args){
        new Ex127();
    }
}
class LoginWindow extends JFrame implements ActionListener
{
    JLabel bq1=new JLabel("请输入用户名");
    JLabel bq2=new JLabel("请输入密码");
    JTextField wbk=new JTextField(10);
    JPasswordField mmk=new JPasswordField(10);
    JButton an1=new JButton("登录");
    JButton an2=new JButton("重置");
    JButton an3=new JButton("注册");

    public LoginWindow(){
        this.setLayout(null);
        this.add(bq1);
        this.add(bq2);
        this.add(wbk);
        this.add(mmk);
        this.add(an1);
        an1.addActionListener(this);
        an1.setActionCommand("denglu");
        this.add(an2);
        an2.addActionListener(this);
        an2.setActionCommand("chongzhi");
        this.add(an3);
        bq1.setBounds(20,10,80,20);
        bq2.setBounds(20,50,100,20);
        wbk.setBounds(100,10,100,20);
        mmk.setBounds(100,50,100,20);
        an1.setBounds(20,100,60,25);
        an2.setBounds(100,100,60,25);
        an3.setBounds(180,100,60,25);
        this.setSize(380,250);
        //不允许用户改变窗口的大小
        this.setResizable(false);
        //让窗口在屏幕的正中间显示
        this.setVisible(true);
        this.setSize(300,200);
        this.setLocationRelativeTo(null);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
    public void actionPerformed(ActionEvent e) {
        if(e.getActionCommand().equals("denglu"))
        {
            if(wbk.getText()==null){                                    //这里没有输入用户名，点登陆按钮为什么不弹出提示"请输入用户名！"，下面也是？
                JOptionPane.showMessageDialog(null,"请输入用户名！", "提示", JOptionPane.INFORMATION_MESSAGE);
            }
            else if(mmk.getText()==null)
            {
                JOptionPane.showMessageDialog(null,"请输入密码！", "提示", JOptionPane.INFORMATION_MESSAGE);
            }
            else
            {
                //this.dispose();
                JOptionPane.showMessageDialog(null, wbk.getText()+" 已成功登陆！", "提示", JOptionPane.INFORMATION_MESSAGE);
            }
        }
        else if(e.getActionCommand().equals("chongzhi"))
        {
            wbk.setText(null);
            mmk.setText(null);

        }
    }

}

[ 本帖最后由 暂不存在 于 2013-12-3 18:45 编辑 ]

对了，怎么怎么粗心