See SBQL4J. It's type-safe strong query language integrated with Java. Allows to write complicated and multiply nested queries. There is a lot of operators, Java methods can be invoked inside queries so as constructors. Queries are translated to pure Java code (there is no reflection at runtime) so execution is very fast.
EDIT: Well, so far SBQL4J it's the ONLY extension to Java language which gives query capabilities similar to LINQ. There are some interesting project like Quaere and JaQue but they are only API's, not syntax / semantics extension with strong type safety in compile time.
