本人大四,以前也开发过几个项目,Map相关集合也总用。但是从来没有研究过底层的实现,只知道杂用。结果在最开始的几次面试中一脸懵逼。认识到不足后,浅显的学习了一下,总结成一下几点。(如果写错了还望指正)

1.HashMap基于Map接口实现,元素以键值对的方式存储,并且允许使用null 建和null值,因为key不能重复,所以只能有一个键为null,另外HashMap不能保证放入元素的顺序,它是无序的,HashMap是线程不安全的。需要同步可以用ConcurrentHashMap和 Collections.synchronizedMap(Map m),推荐使用ConcurrentHashMap,下次会提到它两

2.默认初始化大小16,负载因子0.75

容量是哈希表中桶(Entry数组)的数量,初始容量知识哈希表在创建时的容量。加载因子是哈希表在其容量自动增加之前可以达到多满的一种尺度。当哈希表中的条目数超出了加载因子与当前你容量的乘积时,通过rehash方法将容量翻倍。

3

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" alt="" />

HashMap采用Entry数组来存储key-value对,每一个键值对组成了一个Entry实体,Entry类实际上是一个单向的链表结构,它具有Next指针,可以连接下一个Entry实体,依次来解决Hash冲突的问题,因为HashMap是按照Key的hash值来计算Entry在HashMap中存储的位置的,如果hash值相同,而key内容不相等,那么就用链表来解决这种hash冲突
(注:JDK1.8 当链表长度大于8,转化为红黑树存储)
4.hash碰撞:两个对象的key的hashcode是一样的
(1)if碰撞 如何get value?
通过equals遍历table那个位置上面的Entry链表
(2)我当初理解时候就是在网上看见了这张图片(时间有点长,当初就保存了,都忘在哪篇博客看到的了,勿怪),分享给大家


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