BFS: From [North Gate] to [Canteen]:
North Gate
Square
Canteen

public interface Algorithm {
  /**
   * 执行算法
   */
  void perform(Graph g, String sourceVertex);
  /**
   * 得到路径
   */
  Map<String, String> getPath();
}

/**
 * (无向)图
 */
public class Graph {
  // 图的起点
  private String firstVertax;
  // 邻接表
  private Map<String, List<String>> adj = new HashMap<>();
  // 遍历算法
  private Algorithm algorithm;
  public Graph(Algorithm algorithm) {
    this.algorithm = algorithm;
  }
  /**
   * 执行算法
   */
  public void done() {
    algorithm.perform(this, firstVertax);
  }
  /**
   * 得到从起点到{@code vertex}点的最短路径
   * @param vertex
   * @return
   */
  public Stack<String> findPathTo(String vertex) {
    Stack<String> stack = new Stack<>();
    stack.add(vertex);
    Map<String, String> path = algorithm.getPath();
    for (String location = path.get(vertex) ; false == location.equals(firstVertax) ; location = path.get(location)) {
      stack.push(location);
    }
    stack.push(firstVertax);
    return stack;
  }
  /**
   * 添加一条边
   */
  public void addEdge(String fromVertex, String toVertex) {
    if (firstVertax == null) {
      firstVertax = fromVertex;
    }
    adj.get(fromVertex).add(toVertex);
    adj.get(toVertex).add(fromVertex);
  }
  /**
   * 添加一个顶点
   */
  public void addVertex(String vertex) {
    adj.put(vertex, new ArrayList<>());
  }
  public Map<String, List<String>> getAdj() {
    return adj;
  }
}

public Graph(Algorithm algorithm) {
    this.algorithm = algorithm;
  }

// 邻接表
  private Map<String, List<String>> adj = new HashMap<>();

/**
 * 封装BFS算法
 */
public class BroadFristSearchAlgorithm implements Algorithm {
  // 保存已经访问过的地点
  private List<String> visitedVertex;
  // 保存最短路径
  private Map<String, String> path;
  @Override
  public void perform(Graph g, String sourceVertex) {
    if (null == visitedVertex) {
      visitedVertex = new ArrayList<>();
    }
    if (null == path) {
      path = new HashMap<>();
    }
    BFS(g, sourceVertex);
  }
  @Override
  public Map<String, String> getPath() {
    return path;
  }
  private void BFS(Graph g, String sourceVertex) {
    Queue<String> queue = new LinkedList<>();
    // 标记起点
    visitedVertex.add(sourceVertex);
    // 起点入列
    queue.add(sourceVertex);
    while (false == queue.isEmpty()) {
      String ver = queue.poll();
      List<String> toBeVisitedVertex = g.getAdj().get(ver);
      for (String v : toBeVisitedVertex) {
        if (false == visitedVertex.contains(v)) {
          visitedVertex.add(v);
          path.put(v, ver);
          queue.add(v);
        }
      }
    }
  }
}

String[] vertex = {"North Gate", "South Gate", "Classroom", "Square", "Toilet", "Canteen"};
  Edge[] edges = {
      new Edge("North Gate", "Classroom"),
      new Edge("North Gate", "Square"),
      new Edge("Classroom", "Toilet"),
      new Edge("Square", "Toilet"),
      new Edge("Square", "Canteen"),
      new Edge("Toilet", "South Gate"),
      new Edge("Toilet", "South Gate"),
  };
@Test
  public void testBFS() {
    Graph g = new Graph(new BroadFristSearchAlgorithm());
    addVertex(g);
    addEdge(g);
    g.done();
    Stack<String> result = g.findPathTo("Canteen");
    System.out.println("BFS: From [North Gate] to [Canteen]:");
    while (!result.isEmpty()) {
      System.out.println(result.pop());
    }
  }