The very first interface listed in java.util.concurrent is BlockingQueue, a java.util.Queue subclass. When clients attempt to retrieve elements but none are available, a BlockingQueue encapsulates code that waits until an element becomes available. The Queue class, also new to Java 5, is a new collection class that supports the classic concept of a FIFO (first-in, first out) data structure. Other Java 5 queue implementations include support for removal based on priority, and a collection that acts like a stack.