A simple adjacency list implementation in OOP and not-so-OOP

graph.java

/**I actually needed a typed language to make this clear, but fret not, there's a ruby example below!*/
public class Graph{
  static class Vertex{
    /** Every vertex is aware of its immediate adjacent neighbors*/
    private List<Vertex> adjacent_vertices;
    public void add_adjacent(Vertex neighbor){
       adjacent_vertices.append(neighbor);
    }
  }
  
  /** A graph contains its own list of vertices */
  private List<Vertex> vertices;
  public Graph(Vertex... initial_vertices){
    //...
  }
}

graph.rb

class Vertex
  attr_accessor :neighbors, :datum
  def initialize(datum, *neighbors)
    @neighbors = neighbors
    @datum = datum
  end     
  def inspect
    "#{datum} -> [#{neighbors.map(&:datum).join(", ")}]"
  end
end

#we get all the cool methods from Array and Enumerable!
class Graph < Array; end


vertex_a = Vertex.new(:hello)
vertex_b = Vertex.new(:world, vertex_a)
vertex_c = Vertex.new(:im_out_of_strings, vertex_b)
vertex_a.neighbors << vertex_c

a_graph = Graph.new([
  vertex_a,
  vertex_b,
  vertex_c
])