java collections

collection is the same as the intuitive, mathematical concept of a set. A set is just a group of unique items, meaning that the group contains no duplicates. The Collections Framework, in fact, includes a Set interface, and a number of concrete Set classes.

But the formal notion of a set predates Java technology by a century, when the British mathematician George Boole defined it in formal logic. Most people learned some set theory in elementary school when introduced to "set intersection" and "set union" through the familiar Venn Diagrams:

Some real-world examples of sets include the following:

* The set of uppercase letters 'A' through 'Z'
* The set of non-negative integers {0, 1, 2 ...}
* The set of reserved Java programming language keywords {'import', 'class', 'public','protected'...}
* A set of people (friends, employees, clients, ...)
* The set of records returned by a database query
* The set of Component objects in a Container
* The set of all pairs
* The empty set {} 

Sets have the following basic properties:

* They contains only one instance of each item
* They may be finite or infinite
* They can define abstract concepts

Sets are fundamental to logic, mathematics, and computer science, but also practical in everyday applications in business and systems. The idea of a "connection pool" is a set of open connections to a database server. Web servers have to manage sets of clients and connections. File descriptors provide another example of a set in the operating system. A map is a special kind of set. It is a set of pairs, each pair representing a one-directional mapping from one element to another.

Some examples of maps are:

* The map of IP addresses to domain names (DNS)
* A map from keys to database records.
* A dictionary (words mapped to meanings)
* The conversion from base 2 to base 10

Like sets, the idea behind a map is much older than the Java programming language, older even than computer science. Sets and maps are important tools in mathematics and their properties are well understood. People also long recognized the usefulness of solving programming problems with sets and maps. A language called SETL (Set Language) invented in 1969 included sets as one of its only primitive data types (SETL also included garbage collection -- not widely accepted until Java technology was developed in the 1990s).

Although sets and maps appear in many languages including C++, the Collections Framework is the best designed set and map package yet written for a popular language. (Users of C++ Standard Template Library (STL) and Smalltalk's collection hierarchy might argue that last point.)Also because they are sets, maps can be finite or infinite. An example of an infinite map is the conversion from base 2 to base 10. Unfortunately, the Collections Framework does not support infinite maps -- sometimes a mathematical function, formula, or algorithm is preferred. But when a problem can be solved with a finite map, the Collections Framework provides the Java programmer with a useful API.

Collection interfaces and classes


Now that you have some set theory under your belt, you should be able to understand the Collections Framework more easily. The Collections Framework is made up of a set of interfaces for working with groups of objects. The different interfaces describe the different types of groups. For the most part, once you understand the interfaces, you understand the framework. While you always need to create specific implementations of the interfaces, access to the actual collection should be restricted to the use of the interface methods, thus allowing you to change the underlying data structure, without altering the rest of your code. The following diagrams shows the framework interface hierarchy.