HashMap源码-描述部分
/**
* Hash table based implementation of the <tt>Map</tt> interface. This
* implementation provides all of the optional map operations, and permits
* <tt>null</tt> values and the <tt>null</tt> key. (The <tt>HashMap</tt>
* class is roughly equivalent to <tt>Hashtable</tt>, except that it is
* unsynchronized and permits nulls.) This class makes no guarantees as to
* the order of the map; in particular, it does not guarantee that the order
* will remain constant over time.
*
* Hash table实现了Map接口,支持map的所有操作。
* HashMap的key和value都支持 null值。
* HashMap相比HashTable,除了(1):key和value都支持null;(2):线程不安全。两点以外其他大体相同。
* HashMap是无序的,同一个HashMap对象不同的迭代也不保证顺序一下。
* <p>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. Iteration over
* collection views requires time proportional to the "capacity" of the
* <tt>HashMap</tt> instance (the number of buckets) plus its size (the number
* of key-value mappings). Thus, it's very important not to set the initial
* capacity too high (or the load factor too low) if iteration performance is
* important.
*
* <p>An instance of <tt>HashMap</tt> has two parameters that affect its
* performance: <i>initial capacity</i> and <i>load factor</i>. The
* <i>capacity</i> is the number of buckets in the hash table, and the initial
* capacity is simply the capacity at the time the hash table is created. The
* <i>load factor</i> is a measure of how full the hash table is allowed to
* get before its capacity is automatically increased. When the number of
* entries in the hash table exceeds the product of the load factor and the
* current capacity, the hash table is <i>rehashed</i> (that is, internal data
* structures are rebuilt) so that the hash table has approximately twice the
* number of buckets.
*
* <p>As a general rule, the default load factor (.75) offers a good
* tradeoff between time and space costs. Higher values decrease the
* space overhead but increase the lookup cost (reflected in most of
* the operations of the <tt>HashMap</tt> class, including
* <tt>get</tt> and <tt>put</tt>). The expected number of entries in
* the map and its load factor should be taken into account when
* setting its initial capacity, so as to minimize the number of
* rehash operations. If the initial capacity is greater than the
* maximum number of entries divided by the load factor, no rehash
* operations will ever occur.
*
* 在元素离散的情况下,HashMap在执行get和put操作时效率稳定。
* capacity配置不要太大,负载因子不要太小
* 容量capacity和负载因子load factor是影响HashTable两个重要参数。
capacity是hash table上的bucket个数
load factor是hash table的容量达到多少的时候进行rehashed,之后内部数据会重新调整,容量翻倍
* 默认负载因子为0.75,这个值是在性能和空间最好的折衷
*
* <p>If many mappings are to be stored in a <tt>HashMap</tt>
* instance, creating it with a sufficiently large capacity will allow
* the mappings to be stored more efficiently than letting it perform
* automatic rehashing as needed to grow the table. Note that using
* many keys with the same {@code hashCode()} is a sure way to slow
* down performance of any hash table. To ameliorate impact, when keys
* are {@link Comparable}, this class may use comparison order among
* keys to help break ties.
*
*当有很多数据需要存到HashMap中的时候初始化一个大容量的实例比HashMap实例自动增长造成rehash的效率高。
*Hash碰撞严重影响效率。如果Key值是Comparable的,可以缓解Hash碰撞
*
* <p><strong>Note that this implementation is not synchronized.</strong>
* If multiple threads access a hash map concurrently, and at least one of
* the threads modifies the map structurally, it <i>must</i> be
* synchronized externally. (A structural modification is any operation
* that adds or deletes one or more mappings; merely changing the value
* associated with a key that an instance already contains is not a
* structural modification.) This is typically accomplished by
* synchronizing on some object that naturally encapsulates the map.
*
* If no such object exists, the map should be "wrapped" using the
* {@link Collections#synchronizedMap Collections.synchronizedMap}
* method. This is best done at creation time, to prevent accidental
* unsynchronized access to the map:<pre>
* Map m = Collections.synchronizedMap(new HashMap(...));</pre>
*
* HashMap不是线程安全的,添加、删除、修改数据都会造成多线程场景下的数据问题
* 为了线程安全应该用Map m = Collections.synchronizedMap(new HashMap(...)) 这种方式创建
*
* <p>The iterators returned by all of this class's "collection view methods"
* are <i>fail-fast</i>: if the map is structurally modified at any time after
* the iterator is created, in any way except through the iterator's own
* <tt>remove</tt> method, the iterator will throw a
* {@link ConcurrentModificationException}. Thus, in the face of concurrent
* modification, the iterator fails quickly and cleanly, rather than risking
* arbitrary, non-deterministic behavior at an undetermined time in the
* future.
*
* <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
* as it is, generally speaking, impossible to make any hard guarantees in the
* presence of unsynchronized concurrent modification. Fail-fast iterators
* throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
* Therefore, it would be wrong to write a program that depended on this
* exception for its correctness: <i>the fail-fast behavior of iterators
* should be used only to detect bugs.</i>
*
* HashMap的迭代是很容易过期的,容易造成ConcurrentModificationException异常
*
* <p>This class is a member of the
* <a href="{@docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
* @param <K> the type of keys maintained by this map
* @param <V> the type of mapped values
*
* @author Doug Lea
* @author Josh Bloch
* @author Arthur van Hoff
* @author Neal Gafter
* @see Object#hashCode()
* @see Collection
* @see Map
* @see TreeMap
* @see Hashtable
* @since 1.2
*/
HashMap源码-描述部分的更多相关文章
- [java源码解析]对HashMap源码的分析(二)
上文我们讲了HashMap那骚骚的逻辑结构,这一篇我们来吹吹它的实现思想,也就是算法层面.有兴趣看下或者回顾上一篇HashMap逻辑层面的,可以看下HashMap源码解析(一).使用了哈希表得“拉链法 ...
- HashMap 源码解析
HashMap简介: HashMap在日常的开发中应用的非常之广泛,它是基于Hash表,实现了Map接口,以键值对(key-value)形式进行数据存储,HashMap在数据结构上使用的是数组+链表. ...
- HashMap源码分析
最近一直特别忙,好不容易闲下来了.准备把HashMap的知识总结一下,很久以前看过HashMap源码.一直想把集合类的知识都总结一下,加深自己的基础.我觉的java的集合类特别重要,能够深刻理解和应用 ...
- JAVA源码分析-HashMap源码分析(一)
一直以来,HashMap就是Java面试过程中的常客,不管是刚毕业的,还是工作了好多年的同学,在Java面试过程中,经常会被问到HashMap相关的一些问题,而且每次面试都被问到一些自己平时没有注意的 ...
- Java集合---HashMap源码剖析
一.HashMap概述二.HashMap的数据结构三.HashMap源码分析 1.关键属性 2.构造方法 3.存储数据 4.调整大小 5.数据读取 ...
- 【转】Java HashMap 源码解析(好文章)
.fluid-width-video-wrapper { width: 100%; position: relative; padding: 0; } .fluid-width-video-wra ...
- 【JAVA集合】HashMap源码分析(转载)
原文出处:http://www.cnblogs.com/chenpi/p/5280304.html 以下内容基于jdk1.7.0_79源码: 什么是HashMap 基于哈希表的一个Map接口实现,存储 ...
- HashMap源码解读(转)
http://www.360doc.com/content/10/1214/22/573136_78188909.shtml 最近朋友推荐的一个很好的工作,又是面了2轮没通过,已经是好几次朋友内推没过 ...
- HashMap源码剖析
HashMap源码剖析 无论是在平时的练习还是项目当中,HashMap用的是非常的广,真可谓无处不在.平时用的时候只知道HashMap是用来存储键值对的,却不知道它的底层是如何实现的. 一.HashM ...
随机推荐
- 聊聊、Java SPI
SPI,Service Provider Interface,服务提供者接口. Animal 接口 package com.rockcode.www.spi; public interface Ani ...
- php 数据库内容增删改查----增
首先,建立一个主页面(crud.php) <!DOCTYPE html> <html lang="en"> <head> <meta ch ...
- Spring 学习笔记(六)—— AOP的简单理解
系统中的业务可以分为核心关注点和横切关注点. 核心关注点时业务处理的主要流程,而横切关注点是与核心业务无关但更为通用的业务. 各个横切关注点离散地穿插于核心业务之中,导致系统地每一个模块都与这些业务具 ...
- canvas 基础
1.<canvas>元素 <canvas id="tutorial" width="150" height="150"&g ...
- 【bzoj4940】[Ynoi2016]这是我自己的发明 DFS序+树上倍增+莫队算法
题目描述 给一个树,n 个点,有点权,初始根是 1. m 个操作,每次操作: 1. 将树根换为 x. 2. 给出两个点 x,y,从 x 的子树中选每一个点,y 的子树中选每一个点,如果两个点点权相等, ...
- hihoCoder 1403 后缀数组一·重复旋律(后缀数组+单调队列)
#1403 : 后缀数组一·重复旋律 时间限制:5000ms 单点时限:1000ms 内存限制:256MB 描述 小Hi平时的一大兴趣爱好就是演奏钢琴.我们知道一个音乐旋律被表示为长度为 N 的数构成 ...
- POJ 2115 C-Looooops | exgcd
题目 给出一个循环for(int i=A;i!=B;i+=C) 在mod (1<<k) 下是否可以退出循环 是,输出时间,否输出FORVEER 题解: 题意可以变换成 A+Cx=B (mo ...
- 设置(settings)
设置(settings) 题目描述 如题所示,这将是一个关于设置的问题. 你需要通过对一个控制台进行设置,来得到不同的效果. 这个控制台由n个控制元件组成,每个元件有m种设置,其中i号元件的第j种设置 ...
- codeforces ~ 1009 B Minimum Ternary String(超级恶心的思维题
http://codeforces.com/problemset/problem/1009/B B. Minimum Ternary String time limit per test 1 seco ...
- 无法定位程序输入点GetTickCount64 在动态链接库kernel32.dll上
winxp系统,在使用boost中的thread中的sleep的时候出现“无法定位程序输入点GetTickCount64 在动态链接库kernel32.dll上”的错误, 1.在引用boost库之前( ...