The concurrent java library provides standardized, efficient versions of
 utility classes commonly encountered in concurrent Java programming.
 Discussions of the rationale and applications of several of these classes
 can be found in the second edition of Concurrent Programming in Java.
 .
 The package mainly consists of implementations of a few interfaces:
 .
  * Sync -- locks, conditions
  * Channel -- queues, buffers
  * Barrier -- multi-party synchronization
  * SynchronizedVariable -- atomic ints, refs etc
  * java.util.Collection -- collections
  * Executor -- replacements for direct use of Thread