Java JPanel显示在JFrame的右下角

我正在尝试添加一个MessagePanel，它将JPanel扩展到JFrame中，这样它就简单地位于中间，占据了整个内容，我以前做过很多次，但现在由于某种原因，它出现在JFrame的右下角，我不知道为什么。我在它周围加了一个边框，这样我就可以看到它在哪里了。有人知道怎么了吗

import javax.swing.*;
import java.awt.*;

public class Book extends JFrame{

    public Book(){
        setLayout(new BorderLayout());
        add(new MessagePanel("My Message"), BorderLayout.CENTER);
    }

    public static void main(String[] args){
        Book f = new Book();
        f.setDefaultCloseOperation(Book.EXIT_ON_CLOSE);
        f.setTitle("Title");
        f.setSize(500, 500);
        f.setLocationRelativeTo(null);
        f.setVisible(true);
    }
}

//

不要重写getX（）、setX（…）、getY（）和setY（…）

这些是组件类的方法，用于控制组件的位置

如果要控制文本的位置，请使用不同的变量

import java.awt.Color;
import java.awt.Font;
import java.awt.FontMetrics;
import java.awt.Graphics;
import javax.swing.JPanel;
import javax.swing.border.LineBorder;


public class MessagePanel extends JPanel{
    private String message = "";
    private boolean centered;
    private int x = 20;
    private int y = 20;
    private int interval = 10;

    public MessagePanel(){
    }

    public MessagePanel(String message){
        this.message = message;
        this.centered = true;
        setBorder(new LineBorder(Color.BLACK, 2));
    }

    public void setMessage(String message){
        this.message = message;
        repaint();
    }

    public void setCentered(boolean centered){
        this.centered = centered;
        repaint();
    }

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

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

    public int getX(){
        return x;
    }

    public int getY(){
        return y;
    }

    public void setInterval(int interval){
        this.interval = interval;
        repaint();
    }

    @Override
    protected void paintComponent(Graphics g){
        super.paintComponent(g);

        if(centered){
            FontMetrics fontMetrics = g.getFontMetrics();
            x = (getWidth() / 2) - (fontMetrics.stringWidth(message) / 2);
            y = (getHeight() / 2) + (fontMetrics.getAscent() / 2);

        }
        g.drawString(message, x, y);

    }

    public void moveUp(){
        this.y -= interval;
        repaint();
    }

    public void moveDown(){
        this.y += interval;
        repaint();
    }

    public void moveLeft(){
        this.x -= interval;
        repaint();
    }

    public void moveRight(){
        this.x += interval;
        repaint();
    }
}