Java Pocket Guide

Chapter 12. Java Collections Framework

The Java Collections Framework is designed to support numerous collections in a hierarchical fashion. It is essentially made up of interfaces, implementations, and algorithms.

The Collection Interface

Collections are objects that group multiple elements and store, retrieve, and manipulate those elements. The interface Collection is at the root of the collection hierarchy. Subinterfaces of Collection include List, Queue, and Set. Table 12-1 shows these interfaces and whether they are ordered or allow duplicates. The interface Map is also included in the table, as it is part of the framework.

Table 12-1. Common collections

Interface	Ordered	Dupes	Notes
List	Yes	Yes	Positional access. Element insertion control.
Map	Can be	No (Keys)	Unique keys. One value mapping max per key.
Queue	Yes	Yes	Holds elements. Usually FIFO.
Set	Can be	No	Uniqueness matters.

Implementations

Table 12-2 lists commonly used collection type implementations, their interfaces, and whether or not they are ordered, sorted, and/or contain duplicates.

Table 12-2. Collection type implementations

Implementations	Interface	Ordered	Sorted	Dupes	Notes
ArrayList	List	Index	No	Yes	Fast resizable array
LinkedList	List	Index	No	Yes	Doubly linked list
Vector	List	Index	No	Yes	Legacy, synchronized
HashMap	Map	No	No	No	Key/value pairs
Hashtable	Map	No	No	No	Legacy, synchronized
LinkedHashMap	Map	Insertion,last ...