ConnectedComponent.java

package DataStructures.Graphs;

import java.util.ArrayList;
import java.util.HashSet;
import java.util.Set;

/**
 * A class that counts the number of different connected components in a graph
 *
 * @author Lukas Keul, Florian Mercks
 */
class Graph<E extends Comparable<E>> {

  class Node {
    E name;

    public Node(E name) {
      this.name = name;
    }
  }

  class Edge {
    Node startNode, endNode;

    public Edge(Node startNode, Node endNode) {
      this.startNode = startNode;
      this.endNode = endNode;
    }
  }

  ArrayList<Edge> edgeList;
  ArrayList<Node> nodeList;

  public Graph() {
    edgeList = new ArrayList<Edge>();
    nodeList = new ArrayList<Node>();
  }

  /**
   * Adds a new Edge to the graph. If the nodes aren't yet in nodeList, they will be added to it.
   *
   * @param startNode the starting Node from the edge
   * @param endNode the ending Node from the edge
   */
  public void addEdge(E startNode, E endNode) {
    Node start = null, end = null;
    for (Node node : nodeList) {
      if (startNode.compareTo(node.name) == 0) {
        start = node;
      } else if (endNode.compareTo(node.name) == 0) {
        end = node;
      }
    }
    if (start == null) {
      start = new Node(startNode);
      nodeList.add(start);
    }
    if (end == null) {
      end = new Node(endNode);
      nodeList.add(end);
    }

    edgeList.add(new Edge(start, end));
  }

  /**
   * Main method used for counting the connected components. Iterates through the array of nodes to
   * do a depth first search to get all nodes of the graph from the actual node. These nodes are
   * added to the array markedNodes and will be ignored if they are chosen in the nodeList.
   *
   * @return returns the amount of unconnected graphs
   */
  public int countGraphs() {
    int count = 0;
    Set<Node> markedNodes = new HashSet<Node>();

    for (Node n : nodeList) {
      if (!markedNodes.contains(n)) {
        markedNodes.add(n);
        markedNodes.addAll(depthFirstSearch(n, new ArrayList<Node>()));
        count++;
      }
    }

    return count;
  }

  /**
   * Implementation of depth first search.
   *
   * @param n the actual visiting node
   * @param visited A list of already visited nodes in the depth first search
   * @return returns a set of visited nodes
   */
  public ArrayList<Node> depthFirstSearch(Node n, ArrayList<Node> visited) {
    visited.add(n);
    for (Edge e : edgeList) {
      if (e.startNode.equals(n) && !visited.contains(e.endNode)) {
        depthFirstSearch(e.endNode, visited);
      }
    }
    return visited;
  }
}

public class ConnectedComponent {

  public static void main(String[] args) {
    Graph<Character> graphChars = new Graph<>();

    // Graph 1
    graphChars.addEdge('a', 'b');
    graphChars.addEdge('a', 'e');
    graphChars.addEdge('b', 'e');
    graphChars.addEdge('b', 'c');
    graphChars.addEdge('c', 'd');
    graphChars.addEdge('d', 'a');

    graphChars.addEdge('x', 'y');
    graphChars.addEdge('x', 'z');

    graphChars.addEdge('w', 'w');

    Graph<Integer> graphInts = new Graph<>();

    // Graph 2
    graphInts.addEdge(1, 2);
    graphInts.addEdge(2, 3);
    graphInts.addEdge(2, 4);
    graphInts.addEdge(3, 5);

    graphInts.addEdge(7, 8);
    graphInts.addEdge(8, 10);
    graphInts.addEdge(10, 8);

    System.out.println("Amount of different char-graphs: " + graphChars.countGraphs());
    System.out.println("Amount of different int-graphs: " + graphInts.countGraphs());
  }
}