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A collection — sometimes called a container — is simply an object that groups multiple elements into a single unit. Collections are used to store, retrieve, manipulate, and communicate aggregate data. Typically, they represent data items that form a natural group, such as a poker hand (a collection of cards), a mail folder (a collection of letters), or a telephone directory (a mapping of names to phone numbers).
If you've used the Java programming language — or just about any other programming language — you're already familiar with collections. Collection implementations in earlier (pre-1.2) versions of the Java platform included Vector, Hashtable, and array. However, those earlier versions did not contain a collections framework.
A collections framework is a unified architecture for representing and manipulating collections. All collections frameworks contain the following:
Apart from the Java Collections Framework, the best-known examples of collections frameworks are the C++ Standard Template Library (STL) and Smalltalk's collection hierarchy. Historically, collections frameworks have been quite complex, which gave them a reputation for having a steep learning curve. We believe that the Java Collections Framework breaks with this tradition, as you will learn for yourself in this chapter.
The Java Collections Framework provides the following benefits: