import java.util.Collection;

/**
 * Abstract implementation of a graph.
 * Must be extended to create a particular kind of graph.
 */
public abstract class AbstractGraph implements Graph {
  
  // Graph representation - an iterable collection of vertices and edges
  protected Collection vertices;    
  protected Collection edges;
  
  /**
   * Construct a new abstract graph
   * @param vertices the initial set of verticies in the graph
   * @param edges the initial set of edges in the graph
   */
  AbstractGraph(Collection vertices, Collection edges) {
    this.vertices = vertices;
    this.edges = edges;
  }
  
  /**
   * Return the vertices in this graph
   * @return the set of vertices in this graph
   */
  public Collection vertices() {
    return vertices;
  }
  
  /**
   * Return the edges in this graph
   * @return the set of edges in this graph
   */
  public Collection edges() {
    return edges;
  }
  
}