The Graph class uses Hash Tables (as implemented by java.util.Hashtable) to efficiently access a vertex's adjacency list.

The Hashtable type is parameterized by its keys and values:

• The graph's hash table keys are of type Vertex
• The graph's hash table values are of type List<Vertex>

The instance field declaration for the hash table:

     private Hashtable<Vertex, List<Vertex>> adj_hash;