JAVA Swing GUI-CRUD 项目

Question

我是从头开始做 JAVA Swing GUI 而不是拖放的新手。您能告诉我如何调整代码以显示我想要的输出吗？

import javax.swing.*;
import static javax.swing.GroupLayout.Alignment.*;
import java.awt.*;
import java.awt.event.*;

public class GUI extends JFrame implements ActionListener {

    private JFrame mainFrame;
    private JPanel upperPanel, lowerPanel;
    private JLabel header, lblName, lblPrice, lblQty;
    private JTextField txtName, txtPrice, txtQty;
    
    public GUI(){
        prepareGUI();
    }
    private void prepareGUI(){
        mainFrame = new JFrame("CRUD PROJECT");
        mainFrame.setSize(750, 750);
        mainFrame.setLayout(null);
        mainFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        mainFrame.pack();
        mainFrame.setVisible(true);
        mainFrame.setLayout(new GridLayout(3, 1));
        
        upperPanel= new JPanel();
        upperPanel.setBackground(Color.red);
        upperPanel.setBounds(0,0, 750, 360);
        

        lowerPanel= new JPanel();
        lowerPanel.setBackground(Color.blue);
        lowerPanel.setBounds(0,360, 750, 360);

        header = new JLabel();
        header.setText("JAVA CRUD");
        header.setFont(new Font("Tahoma", Font.BOLD, 30)); 
        header.setBounds(300, 3, 250, 100); 
        //header.setOpaque(true);
        //header.setBackground(Color.blue);

        lblName = new JLabel();
        lblName.setText("Product Name");
        lblName.setFont(new Font("Tahoma", Font.PLAIN, 18)); 
        lblName.setBounds(10, 110, 200, 50); 
       // lblName.setOpaque(true);
        //lblName.setBackground(Color.blue);

        txtName= new JTextField();
        lblName.setBounds(25, 110, 200, 50); 
        txtName.setPreferredSize(new Dimension(250,40));
     
        upperPanel.add(header);
        upperPanel.add(lblName);
        upperPanel.add(txtName);

        mainFrame.add(upperPanel);
        mainFrame.add(lowerPanel);
    }
    public static void main(String[] args) throws Exception {
        new GUI();  
    }
    @Override
    public void actionPerformed(ActionEvent e) {
        // TODO Auto-generated method stub
        
    }
}

我正在尝试创建一个 CRUD GUI，其中 JLabel 旁边会有一个文本字段。由于某种原因，它没有显示。