Java MessagePanel

一个小测试，MessagePanel继承于Panel,显示结果为：

import java.awt.*;

import javax.swing.*;

public class hello extends JFrame{
    public hello(){
        MessagePanel mp1=new MessagePanel("Welcome to Java");
        MessagePanel mp2=new MessagePanel("Java is fun");
        MessagePanel mp3=new MessagePanel("Java is cool");
        MessagePanel mp4=new MessagePanel("I love Java");
        
        mp1.setFont(new Font("SansSerif",Font.ITALIC,20));
        mp2.setFont(new Font("Courier",Font.BOLD,20));
        mp3.setFont(new Font("Times",Font.ITALIC,20));
        mp4.setFont(new Font("Californian FB",Font.PLAIN,20));
        
        mp1.setBackground(Color.red);
        mp2.setBackground(Color.cyan);
        mp3.setBackground(Color.green);
        mp4.setBackground(Color.white);
        mp1.setCentered(true);
        
        setLayout(new GridLayout(2,2));
        add(mp1);
        add(mp2);
        add(mp3);
        add(mp4);
    }
    
    public static void main(String[] args){
        hello frame=new hello();
        frame.setSize(300,200);
        frame.setTitle("Test MessagePanel");
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
}


class MessagePanel extends JPanel{
    private String message="Welcome to Java";
    private int xCoordinate=20;
    private int yCoordinate=20;
    private boolean centered;
    private int interval=10;
    public MessagePanel(){}
    public MessagePanel(String message){
        this.message=message;
    }
    public String getMessage(){
        return message;
    }
    public void getMessage(String message){
        this.message=message;
    }
    public int getXCoordinate(){
        return xCoordinate;
    }
    public void setXCoordinate(int x){
        this.xCoordinate=x;
        repaint();
    }
     public int getYCoordinate(){
        return yCoordinate;
    }
    public void setYCoordinate(int y){
        this.yCoordinate=y;
        repaint();
    }
    public boolean isCentered(){
        return centered;
    }
    public void setCentered(boolean centered){
        this.centered=centered;
        repaint();
    }
    
    public int getInterval(){
        return interval;
    }
    
    public void setinterVal(int interval){
        this.interval=interval;
        repaint();
    }
    protected void paintComponent(Graphics g){
        super.paintComponent(g);
        if(centered){
            FontMetrics fm = g.getFontMetrics();
            int stringWidth=fm.stringWidth(message);
            int stringAscent=fm.getAscent();
            xCoordinate=getWidth()/2-stringWidth/2;
            yCoordinate=getHeight()/2-stringAscent/2;
        }
        g.drawString(message, xCoordinate, yCoordinate);
    }
    public void moveLeft(){
        xCoordinate-=interval;
        repaint();
    }
    public void moveRight(){
        xCoordinate+=interval;
        repaint();
    }

    public void moveUp(){
        yCoordinate-=interval;
        repaint();
    }
    public void moveDown(){
        yCoordinate+=interval;
        repaint();
    }
    public Dimension getPreferredSize(){
        return new Dimension(200,30);
    }
    
}