java小程序，关于布局管理器的，编译时出现了错误，求教

import java.awt.*;
public class BorderLayout {
public static void main (String []args) {
Frame frame = new Frame("this is a test");

  Button b1 = new Button("north");
  Button b2 = new Button("west");
  Button b3 = new Button("south");
  Button b4 = new Button("east");
  Button b5 = new Button("center"); 
  frame.add(b1,BorderLayout.NORTH);
  frame.add(b2,BorderLayout.WEST);
  frame.add(b3,BorderLayout.SOUTH);
  frame.add(b4,BorderLayout.EAST);
  frame.add(b5,BorderLayout.CENTER);

  frame.setSize(400,400);
  frame.setLocation(0,0);
  frame.setBackground(Color.red);
  frame.setVisible(true);

}
}

在命令行提示符中编译时提示变量NORTH,SOUTH,WEST,EAST,CENTER找不到符号，是怎么回事？