Java 2 Standard Edition (J2SE) introduced the Collections API—a dramatic improvement over Vector and Hashtable in Java 1.1. This new API provides Set, List, Map, and Iterator interfaces with various implementations addressing different needs for different applications. Despite the additions and improvements, there are still gaps in the Java's Collections API—functionality that is addressed by a supplemental library, Commons Collections. Most of the features introduced in Commons Collections are easily anticipated extensions of the Java 2 platform: a reversible Comparator or a Bag interface, for example. Other concepts in Commons Collections are innovative additions to the J2SE—predicated collections, self-validating collections, set operations, and lazy collections using transformers. Commons Collections 3.0 also introduces the concept of functors (see Chapter 4).

Java 5.0 (a.k.a. Tiger) has introduced a number of new language features and improvements to the Collections API. A Queue class and type-safe generics are two major additions that will become available to most programmers with the production-ready release of 5.0. Some recipes in this chapter introduce utilities that overlap the functionality of 5.0, and, if a recipe introduces a utility with an analog in 5.0, an effort has been made to identify any opportunity to achieve the same result with new 5.0 classes and features.