Partial ordered Comparator

Question

How to implement java.util.Comparator that orders its elements according to a partial order relation?

For example given a partial order relation a

Answer 1

The problem is that, when you have incomparable elements, you need to fall back to something cleverer than comparing hash codes. For example, given a partial order {a < b, c < d}, the hash codes could satisfy h(d) < h(b) < h(c) < h(a), which means that a < b < c < d < a (bold denotes tie broken by hash code), which will cause problems with a TreeMap.

In general, there's probably nothing for you to do except topologically sort the keys beforehand, so some details about the partial orders of interest to you would be welcome.