import java.util.Collection;

public interface Graph {

  // returns a Collection (e.g. List, but could be anything iterable, such 
  // as a set) of the vertices of the Graph
  public Collection vertices();

  // returns a Collection (e.g. List, but could be anything iterable, such 
  // as a set) of the edges of the Graph
  public Collection edges();

  // returns a Collection (e.g. List, but could be anything iterable, such 
  // as a set) of the vertices incident to v, i.e. w is in it if v->w is 
  // an edge
  public Collection incidentEdges(Vertex v);

  // returns true if there is an edge from v to w, false otherwise
  public boolean isAdjacent(Vertex v, Vertex w);
}