add adjacency list implementation for graphs

data_structures/Graphs/Graphs.java

@@ -1,32 +1,122 @@
+import java.util.ArrayList;
+import java.lang.StringBuilder;
 
-/**
+class AdjacencyListGraph<E extends Comparable<E>> {
- * This class implements the Graph data structure
- * using the classes Graph and Graphs.
- * 
- * @author Zachary Jones
- *
- */
-class Graph {
 	
-}
+    ArrayList<Vertex> verticies;
 
-/**
+    public AdjacencyListGraph() {
- * This class is used to test the Graph
+        verticies = new ArrayList<>();
- * class above.
+    }
- * 
+
- * @author Zachary Jones
+    private class Vertex {
- *
+        E data;
- */
+        ArrayList<Vertex> adjacentVerticies;
+
+        public Vertex(E data) {
+            adjacentVerticies = new ArrayList<>();
+            this.data = data;
+        }
+
+        public boolean addAdjacentVertex(Vertex to) {
+            for (Vertex v: adjacentVerticies) {
+                if (v.data.compareTo(to.data) == 0) {
+                    return false; // the edge already exists
+                }
+            }
+            return adjacentVerticies.add(to); // this will return true;
+        }
+
+        public boolean removeAdjacentVertex(E to) {
+            // use indexes here so it is possible to 
+            // remove easily without implementing 
+            // equals method that ArrayList.remove(Object o) uses
+            for (int i = 0; i < adjacentVerticies.size(); i++) {
+                if (adjacentVerticies.get(i).data.compareTo(to) == 0) {
+                    adjacentVerticies.remove(i);
+                    return true;
+                }
+            }
+            return false;
+        }
+    }
+
+    /**
+     * @param from the data of the vertex the edge is from
+     * @param to the data of the vertex the edge is going to
+     * @return returns false if the edge doesn't exist, returns true if the edge exists and is removed
+     */
+    public boolean removeEdge(E from, E to) {
+        Vertex fromV = null;
+        for (Vertex v: verticies) {
+            if (from.compareTo(v.data) == 0) {
+                fromV = v;
+                break;
+            }
+        }
+        if (fromV == null) return false;
+        return fromV.removeAdjacentVertex(to);
+    }
+    /**
+     * @param from the data of the vertex the edge is from
+     * @param to the data of the vertex the edge is going to
+     * @return returns true if the edge did not exist, return false if it already did
+     */
+    public boolean addEdge(E from, E to) {
+        Vertex fromV = null, toV = null;
+        for (Vertex v: verticies) {
+            if (from.compareTo(v.data) == 0) { // see if from vertex already exists
+                fromV = v;
+            } else if (to.compareTo(v.data) == 0) { // see if to vertex already exists
+                toV = v;
+            }
+            if (fromV != null && toV != null) break; // both nodes exist so stop searching
+        }
+        if (fromV == null) {
+            fromV = new Vertex(from);
+            verticies.add(fromV);
+        }
+        if (toV == null) {
+            toV = new Vertex(to);
+            verticies.add(toV);
+        }
+        return fromV.addAdjacentVertex(toV);
+    }
+
+    /**
+     * 
+     * @return returns a string describing this graph
+     */
+    public String toString() {
+        StringBuilder sb = new StringBuilder();
+        for (Vertex v: verticies) {
+            sb.append("Vertex: ");
+            sb.append(v.data);
+            sb.append("\n");
+            sb.append("Adjacent verticies: ");
+            for (Vertex v2: v.adjacentVerticies) {
+                sb.append(v2.data);
+                sb.append(" ");
+            }
+            sb.append("\n");
+        }
+        return sb.toString();
+    }
+}
 
 public class Graphs {
 	
-	/**
-	 * Main method
-	 * 
-	 * @param args Command line arguments
-	 */
 	public static void main(String args[]) {
-		
+		AdjacencyListGraph<Integer> graph = new AdjacencyListGraph<>();
-	}
+        assert graph.addEdge(1, 2);
+        assert graph.addEdge(1, 5);
+        assert graph.addEdge(2, 5);
+	    assert !graph.addEdge(1, 2);
+        assert graph.addEdge(2, 3);
+        assert graph.addEdge(3, 4);
+        assert graph.addEdge(4, 1);
+        assert !graph.addEdge(2, 3);
+        System.out.println(graph);
+    }
 	
 }

data_structures/Graphs/makefile

@@ -0,0 +1,3 @@
+test:
+	javac Graphs.java
+	java -ea Graphs