Introduction

A collectionsometimes called a containeris simply an object that groups multiple elements into a single unit. Collections are used to store, to retrieve, and to manipulate data and to transmit data from one method to another. Collections typically 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 languageor just about any other programming languageyou're already familiar with collections. Collection implementations in earlier versions of the Java platform included Vector, Hashtable, and array. Those earlier versions, however, did not contain a Collections Framework.

What Is a Collections Framework?

A Collections Framework is a unified architecture for representing and manipulating collections. All collections frameworks contain

• Interfaces: abstract data types representing collections. Interfaces allow collections to be manipulated independently of the details of their representation. In object-oriented languages, these interfaces generally form a hierarchy.
• Implementations: concrete implementations of the collection interfaces. In essence, these are reusable data structures.
• Algorithms: methods that perform useful computations, such as searching and sorting, on objects that implement collection interfaces. These algorithms are said to be polymorphic; the same method can be used on many different implementations of the appropriate collection interface. In essence, algorithms are reusable functionality.

The best-known examples of collections frameworks are the C++ Standard Template Library (STL) and Smalltalk's collection hierarchy.

Benefits

The Collections Framework provides the following benefits.

• Reduces programming effort: By providing useful data structures and algorithms, the Collections Framework frees you to concentrate on the important parts of your program rather than on the low-level plumbing required to make it work. By facilitating interoperability among unrelated APIs, the Collections Framework frees you from writing oodles of adapter objects or conversion code to connect APIs.
• Increases program speed and quality: The Collections Framework does this primarily by providing high-performance, high-quality implementations of useful data structures and algorithms. Also, because the various implementations of each interface are interchangeable, programs can be easily tuned by switching collection implementations. Finally, because you're freed from the drudgery of writing your own data structures, you'll have more time to devote to improving the rest of your program's quality and performance.
• Allows interoperability among unrelated APIs: The collection interfaces will become the vernacular by which APIs pass collections back and forth. If my network administration API furnishes a Collection of node names and if your GUI toolkit expects a Collection of column headings, our APIs will interoperate seamlessly, even though they were written independently.
• Reduces effort to learn and to use new APIs: Many APIs naturally take collections on input and output. In the past, each such API had a little "sub-API" devoted to manipulating its collections. There was little consistency among these ad hoc collections sub-APIs, so you had to learn each one from scratch, and it was easy to make mistakes when using them. With the advent of standard collection interfaces, the problem goes away.
• Reduces effort to design new APIs: This is the flip side of the previous advantage. Designers and implementers don't have to reinvent the wheel each time they create an API that relies on collections but instead just use the standard collection interfaces.
• Fosters software reuse: New data structures that conform to the standard collection interfaces are by nature reusable. The same goes for new algorithms that operate on objects that implement these interfaces.

Drawbacks of the Collections Framework

Historically, collections frameworks have been quite complex, which gave them a reputation for having a steep learning curve. We believe that Java's new Collections Framework breaks with this tradition, as you will learn for yourself in this appendix.