I've assigned another class that draws images to a JPanel in the main JFrame, but it doesn't work

I tried to create a JPanel and apply graphics to it, but only a blank screen appears.

I created a new class that extends JPanel and assigned it to an arbitrary panel in the main JFrame.
(inGamePanel is JPanel in main JFrame)

Below is the code in main JFrame

inGamePanel = new GameStart(songList.get(currentSong), inGamePanel.getGraphics());
inGamePanel.setVisible(true);

Below is the another class that extends JPanel

package RhythmGame;

import java.awt.Graphics;
import java.awt.Graphics2D;
import java.io.ObjectInputStream.GetField;
import java.util.ArrayList;

import javax.swing.ImageIcon;
import javax.swing.JPanel;

public class GameStart extends JPanel {
	
	private Song currentSong;
	private Graphics gameGraphics;
	private ImageIcon inGameImage;
	
	public GameStart(Song currentSong, Graphics gameGraphics) {
		this.currentSong = currentSong;
		this.gameGraphics = gameGraphics;
		paintComponent(gameGraphics);
	}
	
	public void paintComponent(Graphics g) {
		super.paintComponent(g);
		
		inGameImage = currentSong.getSongBackgroundImage();
		
		g.drawImage(inGameImage.getImage(), 0, 0, null);
	}
}

The main Jframe has a CardLayout applied to it. What's the matter?

I want to display a JPanel with an image normally applied to the screen.

The problem is that you're trying to use the Graphics object from the old panel (inGamePanel.getGraphics()) in your constructor for the new GameStart panel. This is incorrect because the Graphics object is associated with the component from which it was obtained, and each component will have its own Graphics object.

Instead, allow Swing to provide the Graphics object via the paintComponent() method in your GameStart class. This method is automatically called by Swing every time the component needs to be painted (for example, when it is resized or becomes visible).

Secondly, you need to assign the newly created GameStart panel to the inGamePanel variable in the JFrame. The way you've written your code, you've created a new GameStart object but haven't attached it to anything.

Here is how you should modify your code:

In the JFrame:

    // create a new GameStart panel
GameStart gamePanel = new GameStart(songList.get(currentSong));
// assign the new panel to inGamePanel
inGamePanel = gamePanel;
inGamePanel.setVisible(true);

In the GameStart class:

package RhythmGame;

import java.awt.Graphics;
import java.awt.Image;
import java.util.ArrayList;

import javax.swing.ImageIcon;
import javax.swing.JPanel;

public class GameStart extends JPanel {
    
    private Song currentSong;
    private Image inGameImage;
    
    public GameStart(Song currentSong) {
        this.currentSong = currentSong;
        this.inGameImage = currentSong.getSongBackgroundImage().getImage();
    }
    
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawImage(inGameImage, 0, 0, this);
    }
}

Note that I've removed the Graphics object from the constructor, and instead load the image in the constructor. The Graphics object g that is passed to paintComponent is used to draw the image.

Also, make sure to revalidate and repaint your JFrame after changing its content.

frame.revalidate();
frame.repaint();

This ensures that the JFrame updates its layout manager, which could be necessary after adding or removing components, and then repaints itself to show the updated components.