Hash table and linked list implementation of the Map interface, with predictable iteration order. This implementation differs from HashMap in that it maintains a doubly-linked list running through all of its entries.
a) Because the entries are linked, not randomly accessible. The performance would be miserable,
O(N) if I'm not in error.
b) Because there is no interface to back up this functionality. So the choice would be to either introduce a dedicated interface just for this (badly performing) Implementation or require clients to program against implementation classes instead of interfaces
Btw with Guava there's a simple solution for you:
And for caching look at Guava's
MapMaker and it's expiration features.