An anonymous inner class is also ephemeral, declarative, inline, capable of extending as well as implementing, and readily readable. What it isn't is terse.

Mocking's killer feature is the ability to partially implement/extend by having some default that makes some sense in a testing situation and is easily instantiable without calling a super constructor.

Magicmock in python is the single best mocking library though, too many times have I really wanted mockito to also default to returning a mock instead of null.