JavaAlgorithms/DataStructures/Graphs/Graphs.java

package DataStructures.Graphs;

import java.util.ArrayList;

class AdjacencyListGraph<E extends Comparable<E>> {

  ArrayList<Vertex> verticies;

  public AdjacencyListGraph() {
    verticies = new ArrayList<>();
  }

  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;
    }
  }

  /**
   * this method removes an edge from the graph between two specified verticies
   *
   * @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);
  }

  /**
   * this method adds an edge to the graph between two specified verticies
   *
   * @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);
  }

  /**
   * this gives a list of verticies in the graph and their adjacencies
   *
   * @return returns a string describing this graph
   */
  @Override
  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 {

  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);
  }
}