Learning Java

Collections

Collections are a fundamental idea in programming. Applications frequently need to keep track of many related things, like a group of employees or a set of images. To support the concept of many at a fundamental level, of course, Java includes the concept of arrays. Since a one-dimensional array has a fixed length, arrays are awkward for sets of things that grow and shrink over the lifetime of an application. Ever since SDK 1.0, the Java platform has had two handy classes for keeping track of sets. The java.util.Vector class represents a dynamic list of objects, and the java.util.Hashtable class is a set of key/value pairs. The Java 2 platform introduces a more comprehensive approach to collections called the Collections Framework. The Vector and Hashtable classes still exist, but they are now a part of the framework.

If you work with dictionaries or associative arrays in other languages, you should understand how useful these classes are. If you are someone who has worked in C or another static language, you should find collections to be truly magical. They are part of what makes Java powerful and dynamic. Being able to work with lists of objects and make associations between them is an abstraction from the details of the types. It lets you think about the problems at a higher level and saves you from having to reproduce common structures every time you need them.

The Collections Framework is based around a handful of interfaces in the java.util package. These ...