Jarvis March Implementation in Java (#354)

Author: crafter312

CONTRIBUTORS.md

@@ -1,4 +1,4 @@
-This file lists everyone, who contributed to this repo and wanted to show up here. If you're looking for information on contribution, please check the `CONTRIBUTING.md` out.
+This file lists everyone, who contributed to this repo and wanted to show up here. If you're looking for information on contribution, please check the `CONTRIBUTING.md` out.
 
 # Contributors
 
@@ -37,3 +37,4 @@ This file lists everyone, who contributed to this repo and wanted to show up her
 - Ken Power
 - PaddyKe
 - nic-hartley
+- crafter312

contents/jarvis_march/code/java/JarvisMarch.java

@@ -0,0 +1,111 @@
+import java.util.*;
+
+public class JarvisMarch {
+    
+    static class Point {
+        private double x;
+        private double y;
+        
+        public Point(double a, double b) {
+            x = a;
+            y = b;
+        }
+        
+        public double getX() {
+            return x;
+        }
+        public double getY() {
+            return y;
+        }
+        
+        public boolean equals(Point p) {
+            if (p.getX() == x && p.getY() == y) {
+                return true;
+            } else {
+                return false;
+            }
+        }
+        public double magnitude() {
+            return Math.sqrt(Math.pow(x, 2) + Math.pow(y, 2));
+        }
+    }
+    
+    //find the angle by creating two vectors and then using a property of dot products
+    private static double angle(Point a, Point b, Point c) {
+        Point ab = new Point(b.getX() - a.getX(), b.getY() - a.getY());
+        Point bc = new Point(c.getX() - b.getX(), c.getY() - b.getY());
+        return Math.acos(-1 * ((ab.getX() * bc.getX()) + (ab.getY() * bc.getY())) /
+                               (ab.magnitude() * bc.magnitude()));
+    }
+    
+    public static ArrayList<Point> jarvisMarch(ArrayList<Point> arr) {
+        ArrayList<Point> hull = new ArrayList<Point>();
+        Point pointOnHull = new Point(Double.MAX_VALUE, 0);
+        
+        //find leftmost point
+        for (Point p: arr) {
+            if (p.getX() < pointOnHull.getX()) {
+                pointOnHull = p;
+            }
+        }
+        hull.add(pointOnHull);
+        
+        //look for the rest of the points on the hull
+        Point ref;
+        while (true) {
+            if (hull.size() == 1) {
+                ref = new Point(pointOnHull.getX(), pointOnHull.getY() + 1); //finds a third point to use in calculating the angle
+            } else {
+                ref = hull.get(hull.size() - 2);
+            }
+            Point endpoint = arr.get(0); //initial canidate for next point in hull
+            for (Point p: arr) {
+                if (angle(p, pointOnHull, ref) > angle(endpoint, pointOnHull, ref)) { //found a point that makes a greater angle
+                    endpoint = p;
+                }
+            }
+            pointOnHull = endpoint;
+            if (pointOnHull.equals(hull.get(0))) { //add next point to hull if not equal to the leftmost point
+                break;
+            } else {
+                hull.add(pointOnHull);
+            }
+        }
+        return hull;
+    }
+    
+    public static void main(String[] args) {
+        
+        //test array setup
+        ArrayList<Point> gift = new ArrayList<Point>();
+        gift.add(new Point(-5, 2));
+        gift.add(new Point(5, 7));
+        gift.add(new Point(-6, -12));
+        gift.add(new Point(-14, -14));
+        gift.add(new Point(9, 9));
+        gift.add(new Point(-1, -1));
+        gift.add(new Point(-10, 11));
+        gift.add(new Point(-6, 15));
+        gift.add(new Point(-6, -8));
+        gift.add(new Point(15, -9));
+        gift.add(new Point(7, -7));
+        gift.add(new Point(-2, -9));
+        gift.add(new Point(6, -5));
+        gift.add(new Point(0, 14));
+        gift.add(new Point(2, 8));
+        
+        //print initial array of points
+        System.out.println("Gift:");
+        for (Point p: gift) {
+            System.out.println("[" + p.getX() + ", " + p.getY() + "]");
+        }
+        
+        //find and print the array of points in the hull
+        ArrayList<Point> hull = jarvisMarch(gift);
+        System.out.println("Wrapping:");
+        for (Point p: hull) {
+            System.out.println("[" + p.getX() + ", " + p.getY() + "]");
+        }
+    }
+    
+}

contents/jarvis_march/jarvis_march.md

@@ -40,6 +40,8 @@ Since this algorithm, there have been many other algorithms that have advanced t
 [import, lang:"python"](code/python/jarvisMarch.py)
 {% sample lang="cpp" %}
 [import, lang:"c_cpp"](code/c++/jarvis_march.cpp)
+{% sample lang="java" %}
+[import, lang:"java"](code/java/JarvisMarch.java)
 {% sample lang="go" %}
 [import, lang:"go"](code/golang/jarvis.go)
 {% endmethod %}