JPanel不会出现在JFrame（主类） - 初学者Java中

Question

I'm just starting out swing in Java... I have this problem about creating a separate JPanel class to be put in the main class (the one with JFrame), that the JPanel won't show up in the main class. The program can run but only the frame would show up. I was hoping that I would see the panel with the 'hallo' label but no.

I know I've been looking for the other solutions in this site, but I didn't really get some of it.

This is my JPanel class:

import javax.swing.*;

public class CreatePanel extends JPanel
{
    private JPanel panel = new JPanel();
    private JLabel narrate;

    public void setNarrate(String label)
    {
        narrate = new JLabel(label);
        panel.add(narrate);
        panel.setVisible(true);
    }

    public JPanel getPanel()
    {
        return panel;
    }
}

This is the main class with the JFrame:

import javax.swing.*;

public class Maine extends JFrame
{
    private static JFrame frame = new JFrame();

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

        CreatePanel panel1 = new CreatePanel();

        panel1.setNarrate("Hallo");
        panel1.getPanel();
        frame.add(panel1);
    }

    public Maine()
    {
        frame.setTitle("Detective Game");
        frame.setSize(500,500);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
        frame.setLocationRelativeTo(null);
        frame.setIconImage(new ImageIcon("agent.png").getImage());
    }
}

Answer 1

You're confusing inheritance and composition. Your CreatePanel class extends JPanel. So it is a JPanel. But you don't have any component inside this panel, because what you did is create another JPanel and add a label to this other JPanel.

In short, your CreatePanel class should be:

public class CreatePanel extends JPanel {
    private JLabel narrate;

    public CreatePanel(String label) {
        narrate = new JLabel(label);
        this.add(narrate);
    }
}

That said, using a panel to only contain a label is useless. You could add the label to the frame directly.

Answer 2

You are calling panel1.getPanel() , but that doesn't do anything except take up memory. Doing frame.add(panel1.getPanel()) , you are getting the panel and then adding it. In your code you are getting the panel but not assigning it to anything.