Created constructor for graph and made Line immutable.

src/doubleset/DoubleComparator.java

@@ -1,4 +0,0 @@
-package doubleset;
-
-public class DoubleComparator {
-}

src/doubleset/DoubleSet.java

@@ -1,5 +0,0 @@
-package doubleset;
-
-public class DoubleSet {
-
-}

src/graphs/Graph.java

@@ -3,21 +3,31 @@ package graphs;
 import shapes.Line;
 import shapes.Point;
 
-import java.util.ArrayList;
+import java.util.HashMap;
 import java.util.List;
+import java.util.Map;
 
 
 public class Graph {
-  private List<Node> nodes;
-  private List<Point> locations;
+  private Map<Point, Node> nodes;
 
   public Graph(List<Line> lines) {
-    this.nodes = new ArrayList<>();
-    this.locations = new ArrayList<>();
+    this.nodes = new HashMap<>();
 
     for (Line line : lines) {
-      nodes.add(new Node(line.getA()));
-      nodes.add(new Node(line.getB()));
+      if (!nodes.containsKey(line.getA())) {
+        nodes.put(line.getA(), new Node(line.getA()));
+      }
+
+      if (!nodes.containsKey(line.getB())) {
+        nodes.put(line.getB(), new Node(line.getB()));
+      }
+
+      Node nodeA = nodes.get(line.getA());
+      Node nodeB = nodes.get(line.getB());
+
+      nodeA.addAdjacent(nodeB);
+      nodeB.addAdjacent(nodeA);
     }
   }
 }

src/graphs/Node.java

@@ -2,17 +2,26 @@ package graphs;
 
 import shapes.Point;
 
+import java.util.ArrayList;
+import java.util.List;
+
 public class Node {
   private Point location;
+  private List<Node> adjacentNodes;
 
   public Node(Point location) {
     this.location = location;
+    this.adjacentNodes = new ArrayList<>();
   }
 
   public Point getLocation() {
     return location;
   }
 
+  public void addAdjacent(Node node) {
+    adjacentNodes.add(node);
+  }
+
   @Override
   public boolean equals(Object obj) {
     if (obj == this) {

src/shapes/Line.java

@@ -1,9 +1,9 @@
 package shapes;
 
 public class Line {
-  private Point a;
-  private Point b;
-  private double weight;
+  private final Point a;
+  private final Point b;
+  private final double weight;
 
   public static final double DEFAULT_WEIGHT = 100;
 
@@ -61,28 +61,28 @@ public class Line {
     return b.getY();
   }
 
-  public void setX1(double x1) {
-    a.setX(x1);
+  public Line setX1(double x1) {
+    return new Line(x1, getY1(), getX2(), getY2());
   }
 
-  public void setY1(double y1) {
-    a.setY(y1);
+  public Line setY1(double y1) {
+    return new Line(getX1(), y1, getX2(), getY2());
   }
 
-  public void setX2(double x2) {
-    b.setX(x2);
+  public Line setX2(double x2) {
+    return new Line(getX1(), getY1(), x2, getY2());
   }
 
-  public void setY2(double y2) {
-    b.setY(y2);
+  public Line setY2(double y2) {
+    return new Line(getX1(), getY1(), getX2(), y2);
   }
 
   public double getWeight() {
     return weight;
   }
 
-  public void setWeight(double weight) {
-    this.weight = weight;
+  public Line setWeight(double weight) {
+    return new Line(getX1(), getY1(), getX2(), getY2(), weight);
   }
 
   @Override

src/shapes/Point.java

@@ -6,7 +6,7 @@ public class Point {
   private double x;
   private double y;
 
-  private static final double EPSILON = 0.0001;
+  private static final double EPSILON = 0.001;
 
   public Point(double x, double y) {
     this.x = x;
@@ -42,6 +42,15 @@ public class Point {
     }
   }
 
+  @Override
+  public int hashCode() {
+    return Objects.hash(round(getX()), round(getY()));
+  }
+
+  private double round(double d) {
+    return Math.round(d * 1000) / 1000d;
+  }
+
   public Point copy() {
     return new Point(x, y);
   }