16.20

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

public class Test_16_20 extends JFrame{
    
    public Test_16_20(){
        add(new JP());
    }
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Test_16_20 frame = new Test_16_20();        
        frame.setSize(300,300);
        frame.setTitle("Test_16_20");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLocationRelativeTo(null); // Center the frame        
        frame.setVisible(true); 
    }
    
    static class JP extends JPanel{
        private int x_getX,y_getY,xCenter,yCenter;
        private String str="";
        public JP(){
            addMouseMotionListener(new MouseMotionAdapter(){
                public void mouseMoved(MouseEvent e){
                    x_getX = e.getX();
                    y_getY = e.getY();
                    if(x_getX > xCenter - 50 && x_getX < xCenter + 50
                    && y_getY >yCenter-20 && y_getY < yCenter+20)
                    str = "in the rect";
                    else str = "out of the rect";
                    repaint();
                } 
            });
        }
        protected void paintComponent(Graphics g){
            xCenter = getWidth()/2;
            yCenter = getHeight()/2;
            
            super.paintComponent(g);
            g.drawRect(xCenter - 50, yCenter - 20, 100, 40);
            g.drawString(str, x_getX, y_getY);
        }
    }
}

效果图：