说明：  设计一个用户登录界面，输入用户名和密码后，点击登录按钮，若用户名为”JavaUser”，密码为”Java1234”，则允许登录，显示“登录成功!”，否则显示“登录失败！”。程序文件命名为Exe8_4.java。
(Design a user login screen, enter your user name and password, click the login button, if a user named JavaUser , password is Java1234 , is allowed to login, Login successful! , Otherwise Login failed!. Program file named Exe8_4.java.)