我的 JFrame 中什么都没有出现

I am practicing using GUIs. I have been following a set of instructions however when I try to run the program only an empty frame appears. No information can be seen inside the frame.

这是我的代码:

package practice528;

import java.util.Scanner;
import java.io.*;
import javax.swing.*;
import java.awt.*;

public class Practice528 
{
public static void main(String[] args) 
{
    JFrame frame = new JFrame("Rectangle Calculator");
    frame.setVisible(true);
    frame.setSize(400,300);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    JLabel lengthL, widthL, areaL, perimeterL;
    lengthL = new JLabel("Enter the length: ", SwingConstants.RIGHT);
    widthL = new JLabel("Enter the width: ", SwingConstants.RIGHT);
    areaL = new JLabel("The area is ", SwingConstants.RIGHT);
    perimeterL = new JLabel("The perimeter is ", SwingConstants.RIGHT);

    frame.setLayout(new GridLayout(5,2));


    System.out.println();
} //end main
} //end class

推荐答案

将组件添加到内容窗格:)

JPane panel = (JPanel) frame.getContentPane();
panel.setLayout(new MigLayout("fill, wrap 2", "[right][fill]"));

panel.add(lengthL);
panel.add(new JTextField());
panel.add(widthL);
panel.add(new JTextField());
panel.add(areaL);
panel.add(new JTextField());
panel.add(perimeterL);
panel.add(new JTextField());