java - 添加 JPanel 时仅显示 JFrame 并且不绘制任何内容

Question

我在其中创建了一个 JFrame 和一个 JPanel 。我已经实现了paintComponent，但它什么也没显示，只出现一个空白的JFrame我对此感到困惑。 The picture when the program runs

这是 JPanel 代码

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);
    }

}

这是 JFrame 代码:

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();
    }
}

这是机器人类代码

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 类代码:

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;
    }
}

他们怎么了？

Answer 1

代码按原样“工作”(但仍有很多需要更改的地方)。为了证明这一点，请将机器人颜色更改为 Color.RED 并将大小从 4 增加到 40。