Brownies Collections complements the Java Collections Framework. GapList combines the strengths of both ArrayList and LinkedList. BigList is a list optimized for storing large number of elements. There are specialized List implementations for all primitive data types. The key collection classes offer support for keys and constraints for lists and collections.
Persistent (immutable) collection library for Java
A supplement to the Java Collections library.
Ponzu is a collections framework for Java. It has JDK-compatible List, Set and Map implementations with a rich API and set of utility classes that work with any JDK compatible Collections, Arrays, Maps or Strings.
Collections library of the Staccatissimo project, focused on providing new abstractions that mix object oriented and functional programming style for dealing with iterable objects.
Dynamic Collections provides a suite of tools to extend and enhance the Java Collections API.
Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more.
jin collections is a tool libraries that help working with standard Java Collections utility in a "Code by Intenction" way. It is inspired from Smalltalk Collections Utility.