label标签的显示
import java.awt.*;
import java.awt.event.*;

public class TestKeyboardListener {
public static void main(String[] args){

Frame f = new Frame("KeyBoardListener");
Label l1 = new Label("UP");
Label l2 = new Label("DOWN");
TextField tf1 = new TextField(10);
TextField tf2 = new TextField(10);
f.add(l1,BorderLayout.NORTH);
f.add(tf1,BorderLayout.NORTH);
f.add(l2,BorderLayout.SOUTH);
f.add(tf2,BorderLayout.SOUTH);
f.pack();
f.setVisible(true);
}

}
请问为什么Label标签不显示？