For a little while I've had this idea to create an open source library for utilities used for test driving. It seems that I'm not alone in this and recently the Boost library was made available. You can download the JAR (boost/boost) from the latest build.
For an even more eXtreme view, ContractualJ. Includes: all constructors are declared private and throw java.lang.UnsupportedOperationException, unless they are implicit constructors of anonymous classes and all contracts (interfaces) declare one (and only one) operation, unless they are composite interfaces, in which case, they make an effort to approach "Perfect Symbiosis of Contractual Operations" (to be defined). The only reference I found via Google was "How many bad ideas at once?" - an uninteresting post with some interesting discussion.