UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());

这是设置图形界面外观的.java的图形界面外观有3种,默认是java的金属外观,还有就是windows系统,motif系统外观.
UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
这是把外观设置成你所使用的平台的外观,也就是你这个程序在哪个平台运行,显示的窗口,对话框外观将是哪个平台的外观.

程序中添加代码“UIManager.setLookAndFeel(lookAndFeel);”即可，

JDK自带的有如下几个：
1、Metal风格 (默认)
String lookAndFeel = "javax.swing.plaf.metal.MetalLookAndFeel";
UIManager.setLookAndFeel(lookAndFeel);
2、Windows风格
String lookAndFeel = "com.sun.java.swing.plaf.windows.WindowsLookAndFeel";
UIManager.setLookAndFeel(lookAndFeel);
3、Windows Classic风格
String lookAndFeel = "com.sun.java.swing.plaf.windows.WindowsClassicLookAndFeel";
UIManager.setLookAndFeel(lookAndFeel);
4、Motif风格
String lookAndFeel = "com.sun.java.swing.plaf.motif.MotifLookAndFeel";
UIManager.setLookAndFeel(lookAndFeel);
5、Mac风格 (需要在相关的操作系统上方可实现)
String lookAndFeel = "com.sun.java.swing.plaf.mac.MacLookAndFeel";
UIManager.setLookAndFeel(lookAndFeel);
6、GTK风格 (需要在相关的操作系统上方可实现)
String lookAndFeel = "com.sun.java.swing.plaf.gtk.GTKLookAndFeel";
UIManager.setLookAndFeel(lookAndFeel);
7、可跨平台的默认风格
String lookAndFeel = UIManager.getCrossPlatformLookAndFeelClassName();
UIManager.setLookAndFeel(lookAndFeel);
8、当前系统的风格
String lookAndFeel = UIManager.getSystemLookAndFeelClassName();
UIManager.setLookAndFeel(lookAndFeel);