import java.util.Collection;

/**
 * Interface specifying a graph
 */
public interface Graph {
  
  /** 
   * Return the collection of vertices of this graph
   * @return the vertices as a collection (which is anything iterable)
   */
  public Collection vertices();
 
  /**
   * Return a collection of vertices adjacent to a given vertex v.
   *   i.e., the set of all vertices w where edges v -> w exist in the graph.
   * @param v one of the vertices in the graph
   * @return an iterable collection of vertices adjacent to v in the graph
   */
  public Collection adjacentVertices(Vertex v);
  
  /**
   * Test whether vertex b is adjacent to vertex a (i.e. a -> b)
   * in a directed graph.
   * @param a one vertex
   * @param b another vertex
   * @return true if there is a directed edge from a to b in the graph 
   * Return false otherwise.
   * (Including returning false if one of the two vertices does not exist.)
   */
  public boolean isAdjacent(Vertex a, Vertex b);
}