Chapter 22 Exercise 15, Introduction to Java Programming, Tenth Edition Y. Daniel LiangY.

22.15 (Geometry: non-cross polygon) Write a program that enables the user to add/remove points by clicking the left/right mouse button, and displays a non-
crossed polygon that links all the points, as shown in Figure 22.11a. A polygon
is crossed if two or more sides intersect, as shown in Figure 22.11b. Use the
following algorithm to construct a polygon from a set of points.

Step 1: Given a set of points S, select the rightmost lowest
point p 0 in the set S.
Step 2: Sort the points in S angularly along the x-axis
with p 0 as the center. If there is a tie and two points have
the same angle, the one that is closer to p 0 is considered
greater. The points in S are now sorted as p 0 , p 1 , p 2 , ...,
p n-1 .
Step 3: The sorted points form a non-cross polygon.

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.GridLayout;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.util.ArrayList;
import java.util.Collections;
import javax.swing.*;
import javax.swing.border.LineBorder;

public class Exercise15 extends JApplet{
 private static final long serialVersionUID = 1L;
 private ArrayList<MyPoint> points = new ArrayList<>();
 private MousePanel mousePanel = new MousePanel();
 
 public Exercise15() {
  setLayout(new BorderLayout(5, 5));
  JPanel panel1 = new JPanel(new GridLayout(1, 5, 5, 5));
  panel1.setBorder(new LineBorder(Color.BLACK));
  panel1.add(new JLabel("INSTRUCTIONS"));
  panel1.add(new JLabel("Add: Left Click"));
  panel1.add(new JLabel("Remove: Right Click"));
  add(panel1, BorderLayout.NORTH);
  add(mousePanel, BorderLayout.CENTER);  
 }
 
 public static void main(String[] args) {
  JFrame frame = new JFrame();
  frame.add(new Exercise15());
  frame.setTitle("Exercise15");
  frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  frame.setSize(600, 600);
  frame.setMinimumSize(new Dimension(frame.getWidth(), frame.getHeight()));
  frame.setLocationRelativeTo(null);
  frame.setVisible(true);
 }

 
 class MousePanel extends JPanel {
  private static final long serialVersionUID = 1L;
  private final int RADIUS = 5;
  public MousePanel() {
   addMouseListener(new MouseAdapter() {
    @Override
    public void mouseReleased(MouseEvent e) {
     if (e.getButton() == MouseEvent.BUTTON1) {
      points.add(new MyPoint(e.getX(), e.getY()));
      repaint();
     } else if (e.getButton() == MouseEvent.BUTTON3) {
      for (int i = 0; i < points.size(); i++) {
       if (getSide(new MyPoint(e.getX(), e.getY()), points.get(i)) < RADIUS) {
        points.remove(i);
        repaint();
        break;
       }
      }
     }
    }
   });
  }
  
  @Override
  protected void paintComponent(Graphics g) {
   super.paintComponent(g);
   if(points.size() < 1) {
    return;
   }
   
   MyPoint h0 = points.get(0);
   for (int i = 1; i < points.size(); i++) {
    if(points.get(i).y > h0.y) {
     h0 = points.get(i);
    } else if(points.get(i).y == h0.y) {
     if(points.get(i).x > h0.x) {
      h0 = points.get(i);
     }
    }
   }
   for (MyPoint myPoint : points) {
    myPoint.setRightMostLowestPoint(h0);
   }
   
   Collections.sort(points);
   
   for (MyPoint myPoint : points) {
    g.fillOval((int)(myPoint.x - RADIUS), (int)(myPoint.y - RADIUS), 2 * RADIUS, 2 * RADIUS); 
   }
   for (int i = 0; i < points.size() - 1; i++) {
    g.drawLine((int)(points.get(i).x),(int)( points.get(i).y), (int)(points.get(i + 1).x), (int)(points.get(i + 1).y));    
   }
   if(points.size() > 1) {
    g.drawLine((int)(points.get(0).x),(int)( points.get(0).y), (int)(points.get(points.size() - 1).x), (int)(points.get(points.size() - 1).y));
   }

  }
  
 }
 

 
 private static class MyPoint implements Comparable<MyPoint> {
  double x, y;
  MyPoint rightMostLowestPoint;

  MyPoint(double x, double y) {
   this.x = x;
   this.y = y;
  }

  public void setRightMostLowestPoint(MyPoint p) {
   rightMostLowestPoint = p;
  }

  @Override
  public int compareTo(MyPoint o) {
   MyPoint virtualPoint = new MyPoint(rightMostLowestPoint.x + 1, rightMostLowestPoint.y);
   double a1 = getAngle(rightMostLowestPoint, virtualPoint, o);
   double a2 = getAngle(rightMostLowestPoint, virtualPoint, this);
   if(a1 > a2) {
    return -1;
   } else if(a2 > a1) {
    return 1;
   } else {
    double l1 = getSide(rightMostLowestPoint, o);
    double l2 = getSide(rightMostLowestPoint, this);
    if(l1 > l2) {
     return -1;
    } else if(l2 > l1) {
     return 1;
    } else {
     return 0;
    }
   }
  }
 }
 
 public static double getAngle(MyPoint p1, MyPoint p2, MyPoint p3) {
  double a = getSide(p2, p3);
  double b = getSide(p1, p3);
  double c = getSide(p1, p2);
  return Math.toDegrees(Math.acos((a * a - b * b - c * c) / (-2 * b * c)));
 }
 
 public static double getSide(MyPoint p1, MyPoint p2) {
  return Math.sqrt((p2.x - p1.x) * (p2.x - p1.x) + (p2.y - p1.y) * (p2.y - p1.y));
 }

}