Java在JFrame中添加了2个对象，但只显示了1个

Question

我知道在stackoverflow中经常会问这个问题。我知道有一个关于LayoutManagers的oracle页面，但我不明白如何使用它？有人能用我的代码帮助我吗？我试图用2个方块做一个试图互相捕捉的游戏。它尚未完成。一旦我解决了图形问题，我将完成它。我需要一个JPanel来做吗？提前谢谢！

JFrame类

package catchmev2;

import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Toolkit;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JFrame;
import static javax.swing.JFrame.EXIT_ON_CLOSE;

/**
 *
 * @author MertKarakas
 */
public class CatchMeV2 implements ActionListener{

    public static void main(String[] args) {
        Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
        int width = (int) screenSize.getWidth();
        int height = (int) screenSize.getHeight();
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(EXIT_ON_CLOSE);
        frame.setResizable(true);
        frame.setVisible(true);
        frame.setSize(width, height);
        frame.setTitle("CatchMe.V2");
        frame.getContentPane().setLayout(new FlowLayout());
        //Adding square 1
        RedSquare r = new RedSquare();
        frame.getContentPane().add(r);
        r.setFocusable(true);
        r.requestFocusInWindow();
        //Adding square 2
        BlueSquare b = new BlueSquare();
        frame.getContentPane().add(b);
        b.setFocusable(true);
        b.requestFocusInWindow();


    }

    @Override
    public void actionPerformed(ActionEvent e) {
        throw new UnsupportedOperationException("Not supported yet."); 
    }
}

第一个方格

package catchmev2;

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Toolkit;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import javax.swing.JPanel;
import javax.swing.Timer;

/**
 *
 * @author MertKarakas
 */
public class RedSquare extends JPanel implements ActionListener, KeyListener {

int x = 20; int y = 20;
int velX = 0; int velY = 0;
Timer tm = new Timer(5, this);
Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
int width = (int) screenSize.getWidth();
int height = (int) screenSize.getHeight();

public RedSquare(){
    addKeyListener(this);      
    tm.start();
    setFocusable(true);
    setFocusTraversalKeysEnabled(false);
}
public void paintComponent(Graphics g) {
    super.paintComponent(g);
    g.setColor(Color.red);
    g.fillRect(x, y, 25, 25);
    repaint();
}

@Override
public void actionPerformed(ActionEvent e) {
    x = x + velX;
    y = y + velY;
    if (x < 0) {
        velX = 0;
        x = 0;
    }
    if (x > width - 50) {
        velX = 0;
        x = width - 50;
    }
    if (y < 0) {
        velY = 0;
        y = 0;
    }
    if (y > height - 40) {
        velY = 0;
        y = height - 40;
    }
    repaint();
}

public void keyTyped(KeyEvent e) {
    throw new UnsupportedOperationException("Not supported yet."); 
}

@Override
public void keyPressed(KeyEvent e) {
    int code = e.getKeyCode();
    if (code == KeyEvent.VK_LEFT) {
        velX = -4;
        velY = 0;
        System.out.println("sex1");
    }
    if (code == KeyEvent.VK_UP) {
        velX = 0;
        velY = -4;
        System.out.println("sex2");
    }
    if (code == KeyEvent.VK_RIGHT) {
        velX = 4;
        velY = 0;
        System.out.println("sex3");
    }
    if (code == KeyEvent.VK_DOWN) {
        velX = 0;
        velY = 4;
        System.out.println("sex4");
    }
}

@Override
public void keyReleased(KeyEvent e) {
    velX = 0;
    velY = 0;
    }
}

第二方格

package catchmev2;

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Toolkit;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import javax.swing.JPanel;
import javax.swing.Timer;

/**
 *
 * @author MertKarakas
 */
 public class BlueSquare  extends JPanel implements ActionListener, KeyListener {

    int x = 700;
    int y = 600;
    int velX = 0;
    int velY = 0;
    Timer tm = new Timer(5, this);
    Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
    int width = (int) screenSize.getWidth();
    int height = (int) screenSize.getHeight();

    public BlueSquare() {
        addKeyListener(this);
        tm.start();
        setFocusable(true);
        setFocusTraversalKeysEnabled(false);
    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.blue);
        g.fillRect(x, y, 25, 25);
        repaint();
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        x = x + velX;
        y = y + velY;
        if (x < 0) {
            velX = 0;
            x = 0;
        }
        if (x > width - 50) {
            velX = 0;
            x = width - 50;
        }
        if (y < 0) {
            velY = 0;
            y = 0;
        }
        if (y > height - 40) {
            velY = 0;
            y = height - 40;
        }
        repaint();
    }

    public void keyTyped(KeyEvent e) {
        throw new UnsupportedOperationException("Not supported yet."); 
    }

    @Override
    public void keyPressed(KeyEvent e) {
        int code = e.getKeyCode();
        if (code == KeyEvent.VK_A) {
            velX = -4;
            velY = 0;
            System.out.println("sex1");
        }
        if (code == KeyEvent.VK_W) {
            velX = 0;
            velY = -4;
            System.out.println("sex2");
        }
        if (code == KeyEvent.VK_D) {
            velX = 4;
            velY = 0;
            System.out.println("sex3");
        }
        if (code == KeyEvent.VK_S) {
            velX = 0;
            velY = 4;
            System.out.println("sex4");
        }
    }

    @Override
    public void keyReleased(KeyEvent e) {
        velX = 0;
        velY = 0;
    }
}