Java - JComboBox无法显示

Question

我只想创建一个JComboBox。并且，我将以下代码粘贴到https://www.compilejava.net/中，窗口中没有显示任何内容。那么，代码有什么问题？或者在线编译器有问题吗？如果是这样，有没有可用的在线编译器？

import java.awt.*;
import javax.swing.*;
import java.awt.event.*; 

public class test{

private JFrame f; //window
private JPanel p; //layout


private JComboBox<String> j; //pull down menu

public static void main(String[] args){
   new test();
}

public test(){
   f = new JFrame("title of window");
   f.setVisible(true); 
   f.setSize(600,400);
   f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

   p = new JPanel();
   p.setBackground(Color.YELLOW);

   String[] bookTitles = new String[] {"Effective Java", "Head First Java",
                                "Thinking in Java", "Java for Dummies"};
   JComboBox<String> bookList = new JComboBox<>(bookTitles);
   p.add(bookList);

   f.add(p);
}

}