一个java课后练习，哪位大侠帮忙看一下，为什么没有显示

package programmingExercises;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class Exercise14_3 extends JFrame{
public Exercise14_3(){
WhichClicked who=new WhichClicked();
}
public static void main(String[] args) {
Exercise14_3 frame=new Exercise14_3();
frame.setSize(300,200);
frame.setTitle("Exercise14_3");
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
}
class WhichClicked extends JPanel{
public WhichClicked(){
this.setLayout(new BorderLayout());

JLabel label=new JLabel("THE RESULT WILL BE PLAYED HERE");
this.add(label,BorderLayout.NORTH);

JPanel panelbuttons=new JPanel();
JButton jbtok=new JButton("OK");
JButton jbtcancel=new JButton("cancel");
panelbuttons.add(jbtok);
panelbuttons.add(jbtcancel);
this.add(panelbuttons,Border

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