java集合

前言

本文档对java 集合的相关类和接口及其关系进行简单介绍。

1 Collection

Collection是集合的顶层接口，其相关类和接口的关系如下图所示。

Collection： A collection represents a group of objects, known as its elements.  Some collections allow duplicate elements and others do not. Some are ordered and others unordered.

List：An ordered collection.

Set：A collection that contains no duplicate elements.

Queue：A collection designed for holding elements prior to processing.  Queues typically, but do not necessarily, order elements in a FIFO (first-in-first-out) manner.

2 List

Deque：A linear collection that supports element insertion and removal at both ends. Deques can also be used as LIFO (Last-In-First-Out) stacks.

ArrayList：Resizable-array implementation of the List interface. Note that this implementation is not synchronized.

集合的元素存储在一个数组中，可以指定数组的大小，如果不指定，则数组的初始大小是10，当数组不够用时，数组长度会自动扩大2倍。

LinkedList：Doubly-linked list implementation of the  List and Deque interfaces. Note that this implementation is not synchronized.

Vector：thread-safe

Stack：The Stack class represents a last-in-first-out (LIFO) stack of objects.

thread-safe

3 Set

HashSet：This class implements the Set interface, backed by a hash table (actually a HashMap instance).  It makes no guarantees as to the iteration order of the set。

This class offers constant time performance for the basic operations (<tt>add</tt>, <tt>remove</tt>, <tt>contains</tt> and <tt>size</tt>), assuming the hash function disperses the elements properly among the buckets.

Note that this implementation is not synchronized.

TreeSet：A NavigableSet implementation based on a TreeMap.

The elements are ordered using their {@linkplain Comparable natural ordering}, or by a {@link Comparator} provided at set creation time, depending on which constructor is used.

Note that this implementation is not synchronized.

This implementation provides guaranteed log(n) time cost for the basic operations ({@code add}, {@code remove} and {@code contains}).

4 Map

TreeMap：A Red-Black tree based {@link NavigableMap} implementation.

This implementation provides guaranteed log(n) time cost for the{@code containsKey}, {@code get}, {@code put} and {@code remove} operations.

Note that this implementation is not synchronized.

存储的Key需要实现Comparable接口，或者在TreeMap初始化的时候指定Comparator的实现。

HashMap：Hash table based implementation of the Map interface.

This implementation provides constant-time performance for the basic operations (<tt>get</tt> and <tt>put</tt>), assuming the hash function disperses the elements properly among the buckets.

与HashTable类似，唯一的区别是HashTable是thread-safe，HashMap不是。

散列表用链表数组实现，每个列表叫做一个bucket。

存储的Key需要实现hashCode()方法，同时也要注意与equals()兼容。