只需显示 JFrame 并在向其添加 JPanel 时不要绘制任何内容

Question

I create a JFrame and a JPanel inside it. I've implemented the paintComponent but it shows nothing, Just a blank JFrame appears I'm confused with it. The picture when the program runs

This the JPanel code

import java.awt.Graphics;
import java.awt.Graphics2D;
import javax.swing.JPanel;

/**
 *
 * @author nguyencong
 */
public class RobotWorld extends JPanel {
    public Robot robot;
    public PlayField field;

    public RobotWorld(Robot robot , PlayField field) {
        super();
        this.robot = robot;
        this.field = field;
        this.setSize(field.width , field.height);
        this.setVisible(true);
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D graphic = (Graphics2D)g;
        graphic.setBackground(field.fill_Color);
        graphic.setColor(robot.color);
        graphic.drawOval(robot.x, robot.y, 4, 4);
    }

}

And this is the JFrame code:

import java.awt.Color;
import javax.swing.JFrame;

/**
 *
 * @author nguyencong
 */
public class GameMain extends JFrame {
    public void Game_Start()
    {
        Robot a = new Robot(10, 10, Color.yellow);
        PlayField field = new PlayField(500, 500, Color.BLACK);
        RobotWorld world = new RobotWorld(a, field);
        this.setSize(field.width , field.height);
        this.setLayout(null);
        this.add(world);
        world.setBounds(0, 0, world.field.width, world.field.height);
        this.setVisible(true);
        world.repaint();
    }

    public static void main(String args[])
    {
        GameMain main = new GameMain();
        main.Game_Start();
    }
}

And this is Robot class code

import java.awt.Color;

/**
 *
 * @author nguyencong
 */
public class Robot {
    public int x;
    public int y;
    public Color color;
    public final int speed = 2;
    Robot(int x , int y , Color color)
    {
        this.x = x;
        this.y = y;
        this.color = color;
    }
    public void move()
    {

    }
}

Play Field class code:

import java.awt.Color;

/**
 *
 * @author nguyencong
 */
public class PlayField {
    public int width;
    public int height;
    public Color fill_Color;
    PlayField(int width , int height , Color fill_Color)
    {
        this.width = width;
        this.height = height;
        this.fill_Color = fill_Color;
    }
}

What's wrong with them??

Answer 1

The code 'works' as is (but still has much that needs changing). To prove this, change the robot color to Color.RED and increase the size from 4 to 40.