java问题，为什么这段代码运行后窗体里面没有文本框显示？

import java.awt.*;
import javax.swing.*;
public class calculator extends JFrame{
String buttonname[] = {"0","1","2","3","4","5","6","7","8","9"};
String button[];
public calculator(){

//get a container and set its layout
Container box = getContentPane();

//now apply some attributes
setTitle("Calculator");
setSize(250,350);
setLayout(new GridLayout(3,1));
setResizable(false);
setVisible(true);
setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);

//begin to create components

//3+3+2 panels
JPanel p1 = new JPanel();
JPanel p2 = new JPanel();
JPanel p3 = new JPanel();

JPanel p31 = new JPanel();
JPanel p32 = new JPanel();
JPanel p33 = new JPanel();

JPanel p311 = new JPanel();
JPanel p312 = new JPanel();

//10 buttons with numbers 0-9
JButton[] btn = new JButton[10];
for(int i =0;i<10;i++){
btn[i] = new JButton(buttonname[i]);
}

//another 8 buttons with operators
JButton btnjia = new JButton("+");
JButton btnjian = new JButton("-");
JButton btnchen = new JButton("*");
JButton btnchu = new JButton("/");
JButton btndian = new JButton(".");
JButton btntui = new JButton("←");
JButton btnfu = new JButton("C");
JButton btndeng = new JButton("=");

//an text field to show the result
JTextField txtresult = new JTextField("0",230);

//components create over

//add the components to the panels and set layout

//add the first 3 panels
box.add(p1);
box.add(p2);
box.add(p3);
box.setLayout(new GridLayout(3,1,10,10));

//add the text field to p1 and set layout
p1.add(txtresult);
p1.setLayout(new GridLayout(1,1));

}

//create the main method
public static void main (String[] args){
new calculator();
}

}

1. JTextField txtresult = new JTextField("0",230);这里230改成30先，230太长了。（可以不用改）
2. 在 p1.setLayout(new GridLayout(1,1));这段后面加上pack();
搞定！