16.19

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

public class Test_16_19 extends JFrame{
    
    public Test_16_19(){
        JP jp1 = new JP();        
        add(jp1);
    }
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Test_16_19 frame = new Test_16_19();        
        frame.setSize(300,300);
        frame.setTitle("Test_16_19");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLocationRelativeTo(null); // Center the frame        
        frame.setVisible(true); 
    }
    static class JP extends JPanel{
        private int x_distance, x_co;
        private int y_distance, y_co;
        private double distance;
        private String str="";
        public JP(){
            addMouseMotionListener(new MouseMotionAdapter(){
                public void mouseMoved(MouseEvent e){
                    x_co = e.getX();y_co = e.getY();                    
                    distance =Math.sqrt(x_distance*x_distance+y_distance*y_distance);
                    
                    if(distance>50)
                        str = "out of the circle";
                    else str = "in the circle";
                    repaint();
                }
            });
        }
    protected void paintComponent(Graphics g){
        int xCenter = getWidth()/2;
        int yCenter = getHeight()/2;
        x_distance = Math.abs(x_co - xCenter);
        y_distance = Math.abs(y_co - yCenter);
        
        super.paintComponent(g);
        g.drawOval(xCenter -50, yCenter - 50, 100, 100);
        g.drawString(str, x_co, y_co);
        g.drawString(x_co+","+y_co, x_co, y_co-10);
//        System.out.println(getX() + " " + getY());
    }
    }
}

效果图：