Java HashMap构造函数：一个参数搞不定，两个参数要你命

一种在Java里常用的Map实现方式, 它的底层是依靠哈希表来达成的。接下来要阐述的是, 这种实现方式的原理:

数据结构

底层存在着一个数组, 数组的每个元素都是一个链表, 也就是所谓的一个桶, 桶里面存放的是一个个 key-value 对。当桶中的链表长度超出阈值, 该阈值默认是8的时候, 链表就会转变为红黑树。

哈希函数

将 key 映射为数组下标的是哈希函数, 在其中, 所使用的为类中的 () 方法, 因为 () 方法返回值的范围属于整型, 所以要对 () 值开展转换。

解决哈希冲突

当各异的 key 通过哈希函数映射至同一桶里时, 便会出现哈希冲突, 解决此哈希冲突的方式是运用链式存储法,也就是于桶中的链表内增添新的 key - value 对。

链表长度达到或者超过阈值（默认是8）之际, 链表会自行转变为红黑树。红黑树里节点数量小于或者等于6之时, 红黑树会变回链表。

扩容机制

一旦桶里头元素的数量超出了负载因子（这个负载因子默认是0.75）, 那就会开展扩容行动。扩容这项操作会把桶的数量扩充成原先的两倍, 并且会把原来的全部key-value对进行再次哈希, 然后重新分配到扩容后的桶当中。

线程安全问题

它不是线程安全的, 于多线程环境当中, 是需要开展额外的同步处理操作的。在Java 1.5之后, 是提供了解决此问题的办法的。

总而言之, 借助哈希函数把 key 映射成数组下标, 运用链式存储法去解决哈希冲突, 利用扩容机制来提升性能。要留意的是, 并非线程安全的。Csdn.tphep.cN/Article/details/81536.shtml
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