来自另一个类的JPanel不会出现在主JFrame中

Question

This is my main frame code

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

public class MainGUI extends JFrame {

    cPanel cP = new cPanel();

    public static void main(String[] args) {
        MainGUI main = new MainGUI();
        main.setVisible(true);
    }

    private MainGUI(){
        setTitle("X");
        setResizable(true);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setLayout(null);
        add(cP);
    }   


}

Then my JPanel code

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

public class cPanel extends JPanel{

//cPanel
    JPanel cP1 = new JPanel();

//Panel 1
    JLabel lb1 = new JLabel("dx");
    JLabel lb1dx = new JLabel("dx (pixel)");
    JTextField tf1dx = new JTextField(5);
    JLabel lb1dy = new JLabel("dy (pixel)");
    JTextField tf1dy = new JTextField(5);
    JButton btn1 = new JButton("move");


    public void cPanel(){
        setBounds(0, 0, 600, 190);
        setLayout(new GridLayout(1, 8));
        add(cP1);

    //Panel 1   
        cP1.add(lb1);
        cP1.add(lb1dx);
        cP1.add(tf1dx);
        tf1dx.setText("10");
        cP1.add(lb1dy);
        cP1.add(tf1dy);
        tf1dy.setText("10");
        cP1.add(btn1);
    }

}

The Panel doesn't seem to appear. The frame appeared with the title and no size, adding pack() to the frame does nothing. I've tried a lot of ways including making a new class to try, what's wrong with my code?

Answer 1

public void cPanel(){ //get rid of void as it is a constructor not a method.
//public cPanel(){ instead
    setBounds(0, 0, 600, 190);
    setLayout(new GridLayout(1, 8));
    add(cP1);

private MainGUI(){
    setTitle("X");
    setResizable(true);
    setDefaultCloseOperation(EXIT_ON_CLOSE);
    setLayout(null); //change this to setSize(500, 200) or something.
    add(cP);
}   

Answer 2

The issue is because of the wrong implementation of the cPanel class. You have created a method in cPanel class rather then constructor. The constructor must not have a return type, make the below changes and it should work.

public cPanel(){
    setBounds(0, 0, 600, 190);
    setLayout(new GridLayout(1, 8));
    add(cP1);

//Panel 1   
    cP1.add(lb1);
    cP1.add(lb1dx);
    cP1.add(tf1dx);
    tf1dx.setText("10");
    cP1.add(lb1dy);
    cP1.add(tf1dy);
    tf1dy.setText("10");
    cP1.add(btn1);
}

Answer 3

As Berger suggests: use getContentPane()

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

public class MainGUI extends JFrame {

cPanel cP = new cPanel();

public static void main(String[] args) {
    MainGUI main = new MainGUI();
    main.setVisible(true);
}

private MainGUI(){
    setTitle("X");
    setResizable(true);
    setDefaultCloseOperation(EXIT_ON_CLOSE);
    setSize(800,600);
    getContentPane().add(cP);
}   

}