Question

If I implement a custom comparator is it considered good practice to overide equals besides compare?
Additionally is there a defined contract for a Comparator?

Answer 1

The contract of Comparator is defined in its javadoc. In particular:

Caution should be exercised when using a comparator capable of imposing an ordering inconsistent with equals to order a sorted set (or sorted map). Suppose a sorted set (or sorted map) with an explicit comparator c is used with elements (or keys) drawn from a set S. If the ordering imposed by c on S is inconsistent with equals, the sorted set (or sorted map) will behave “strangely.” In particular the sorted set (or sorted map) will violate the general contract for set (or map), which is defined in terms of equals.

Typically, if 2 objects are equal from an equals perspective but not from a compareTo perspective, you can store both objects as keys in a TreeMap. This can lead to un-intuitive behaviour. It can also be done on purpose in specific situations.

For example, this answer shows an example where it is desirable that equals and compareTo be inconsistent.