Guava - Google Core Libraries for Java
Guava is a set of core libraries that includes new collection types (such as multimap and multiset), immutable collections, a graph library, functional types, an in-memory cache, and APIs/utilities for concurrency, I/O, hashing, primitives, reflection, string processing, and much more.
It is memory efficient. Its most interesting class is MapMaker, which is a ConcurrentMap builder, providing any combination of these features: soft or weak keys, soft or weak values, timed expiration, and on-demand computation of values. Check out their video in Youtube.