[magicalhippo]

GP was talking about modelling your problem. A hashmap is seldom a good model for a given problem. Typically they're "just" an implementation detail.

For example, in some cases it can be useful to consider triangles in a 3D model as cyclic graphs of vertices. The edges of the triangle correspond to the edges in the graph.

However I can't think of any case where it's useful to think of a triangle as a hashmap.