JDK1.8 的 HashMap 源码之注意事项

￡神魔★判官ぃ 2022-02-15 13:47 327阅读 0赞

### 文章目录 ###

*   *  链表变树
     *  树形结构与Comparable，性能极致与降低
     *  链表与树之间转换的阈值

> 英语渣靠着翻译插件，大概翻译的，难免有错误之处，注意甄别；

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## 链表变树 ##

This map usually acts as a binned (bucketed) hash table, but when bins get too large, they are transformed into bins of TreeNodes, each structured similarly to those in java.util.TreeMap. Most methods try to use normal bins, but relay to TreeNode methods when applicable (simply by checking instanceof a node). Bins of TreeNodes may be traversed and used like any others, but additionally support faster lookup when overpopulated. However, since the vast majority of bins in normal use are not overpopulated, checking for existence of tree bins may be delayed in the course of table methods.

`HashMap` 通常表现为一张哈希表，每个桶里面放一个元素，当有多个元素的时候，变为链表，这时候每个桶里面都是放链表；但是当链表的长度达到一个临界的时候，链表转换为树，每个树的结构就像 `TreeMap` 一样，这时候，每个桶里面就是一个树形结构；

大多数方法只是用普通的桶，即里面只是链表；但是在合适的时候，链表会被转为树，比如检查每个节点的时候；因为这时候转换为树，不但支持原来链表的遍历和使用，同时还能获得更快的查找；

但是由于大多数时候，每个桶都没有被过度填充，即里面都是链表，还达不到转换为树的条件，因此 `HashMap` 的方法可能会延迟检查桶里面到底是链表还是树形结构 ；

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## 树形结构与Comparable，性能极致与降低 ##

Tree bins (i.e., bins whose elements are all TreeNodes) are ordered primarily by hashCode, but in the case of ties, if two elements are of the same "class C implements Comparable", type then their compareTo method is used for ordering. (We conservatively check generic types via reflection to validate this -- see method comparableClassFor). The added complexity of tree bins is worthwhile in providing worst-case O(log n) operations when keys either have distinct hashes or are orderable, Thus, performance degrades gracefully under accidental or malicious usages in which hashCode() methods return values that are poorly distributed, as well as those in which many keys share a hashCode, so long as they are also Comparable. (If neither of these apply, we may waste about a factor of two in time and space compared to taking no precautions. But the only known cases stem from poor user programming practices that are already so slow that this makes little difference.)

当桶里面的结构是树形结构的时候，通常情况下是按照 `HashCode` 来算下标位置的；但是如果要插入的元素实现了 `Comparable` 接口，则使用接口的 `compareTo` 方法，计算排序的位置 ；

树形结构（`HashMap`中的树是红黑树）保证了在元素具有 **不同的哈希码或者可以排序** 的情况下，插入元素复杂度在最坏的情况下是 `O log(n)` ，因此，将桶中的链表在一定情况下转成树是值得的;

因此，如果恶意的将 `hashCode` 方法的返回值故意分布在一起，比如返回同一个哈希码，或者实现了 `Comparable` 接口，但是 `compareTo` 永远返回 `0`，这时候 `HashCdoe` 的性能会下降 ；

如果这两种方法都不适用（**不同的哈希码或者可以排序**），与不采取预防措施相比，我们可能会浪费大约两倍的时间和空间。但目前所知的唯一案例源于糟糕的用户编程实践，这些实践已经非常缓慢，以至于没有什么区别；

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## 链表与树之间转换的阈值 ##

Because TreeNodes are about twice the size of regular nodes, we use them only when bins contain enough nodes to warrant use (see TREEIFY\_THRESHOLD). And when they become too small (due to removal or resizing) they are converted back to plain bins. In usages with well-distributed user hashCodes, tree bins are rarely used. Ideally, under random hashCodes, the frequency of nodes in bins follows a Poisson distribution (http://en.wikipedia.org/wiki/Poisson\_distribution) with a parameter of about 0.5 on average for the default resizing threshold of 0.75, although with a large variance because of resizing granularity. Ignoring variance, the expected occurrences of list size k are (exp(-0.5) \* pow(0.5, k) / factorial(k)).

因为树节点的大小大约是普通节点的两倍，所以我们只在桶中包含足够的节点（`TREEIFY_THRESHOLD = 8`）时才进行链表转换成树的操作；

当树因为删除节点变得很小的时候，会再次转换回链表；

如果 `HashCode` 方法设计的很好的话，哈希冲突降低，链表的长度基本就不会很长，树是很少用到的；

理想情况下，随机的哈希码，遵循 `Poisson` （泊松）分布；

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下面还有一些，粗略看了下，没有什么震撼的信息，就不在翻译了；

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看源码，英文注释，也很费时间啊

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