关于Java：如何将JPanel添加到JFrame的中心？

How to add a JPanel to the center of a JFrame?

我尝试制作一个JFrame，其中有一个JPanel(包含一个圆圈)，并且以四个按钮(北，南，东，西)为边界。圆圈将按所按按钮指示的方向移动。
我的问题是我无法将JPanel放在中心：

JFrame的类如下所示：

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import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;

@SuppressWarnings("serial")
public class Frame extends JFrame implements ActionListener {

JButton north, south, east, west;
int x = 10, y = 10;
MyPanel panel;

public Frame() {
setLayout(new BorderLayout());
panel = new MyPanel();
panel.setBackground(Color.MAGENTA);

north = new JButton("NORTH");
south = new JButton("SOUTH");
east = new JButton("EAST");
west = new JButton("WEST");

add(panel, BorderLayout.CENTER);
add(north, BorderLayout.NORTH);
add(south, BorderLayout.SOUTH);
add(east, BorderLayout.EAST);
add(west, BorderLayout.WEST);

north.addActionListener(this);
south.addActionListener(this);
east.addActionListener(this);
west.addActionListener(this);

setBounds(100, 100, 300, 300);
setVisible(true);
}

@Override
public void actionPerformed(ActionEvent e) {
if (e.getSource() == north) {
y -= 3;
panel.setY(y);
panel.repaint();
}

if (e.getSource() == south) {
y += 3;
panel.setY(y);
panel.repaint();
}

if (e.getSource() == east) {
x += 3;
panel.setX(x);
panel.repaint();
}

if (e.getSource() == west) {
x -= 3;
panel.setX(x);
panel.repaint();
}
}
}

MyPanel类如下所示：

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import java.awt.Color;
import java.awt.Graphics;

import javax.swing.JPanel;

@SuppressWarnings("serial")
public class MyPanel extends JPanel {
private Color color = Color.CYAN;
private int x = 10, y = 10;

public void paint(Graphics g) {
super.paintComponent(g);
g.setColor(color);
g.fillOval(x, y, 20, 20);
}

public int getX() {
return x;
}

public void setX(int x) {
this.x = x;
}

public int getY() {
return y;
}

public void setY(int y) {
this.y = y;
}
}