HashSet 与HashMap底层实现
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HashSet 与HashMap底层实现
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1. HashSet底层通过包装HashMap来实现，HashSet在添加一个值的时候，实际上是将此值作为HashMap中的key来进行保存。
2. HashMap的底层实现是通过初始化化一个Entry数组来实现key、value的保存。
3. 在HashMap的Entry中有四个变量，key、value、hash、next，其中next用于在hash方法添加值冲突时候，所指向的下一个值。
4. 在HashMap中添加值步骤
第一步，对key的hashcode进行hash计算，获取应该保存到数组中的index。
第二步，判断index所指向的数组元素是否为空，如果为空则直接插入。
第三步，如果不为空，则依次查找entry中next所指定的元素，判读key是否相等，如果相等，则替换久的值，返回。
第四步，如果都不相等，则将此链表头元素赋值给待插入entry的next变量，让后将待插入元素插入到entry数组中去。
5. map遍历方法有两种，keyset 和Map.Entry
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