Java 8 Pocket Guide

Chapter 15. 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 Collection interface is at the root of the collection hierarchy. Subinterfaces of Collection include List, Queue, and Set. Table 15-1 shows these interfaces and whether they are ordered or allow duplicates. The Map interface is also included in the table, as it is part of the framework.

Table 15-1. 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 15-2 lists commonly used collection type implementations, their interfaces, and whether or not they are ordered, sorted, and/or contain duplicates.

Table 15-2. 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 access	No	No	Linked list/hash table
TreeMap	Map	Balanced	Yes	No	Red-black tree map
PriorityQueue	Queue	Priority	Yes	Yes	Heap implementation
HashSet	Set	No	No	No	Fast access ...