There are times that you may want to use collections library other that the one provided by Java natively. Here are some alternative.
Commons-Collections seek to build upon the JDK classes by providing new interfaces, implementations and utilities. There are many features, including:
Bag interface for collections that have a number of copies of each object
Buffer interface for collections that have a well defined removal order, like FIFOs
BidiMap interface for maps that can be looked up from value to key as well and key to value
MapIterator interface to provide simple and quick iteration over maps
Type checking decorators to ensure that only instances of a certain type can be added