Java HashSet 中元素的排序 [英] Ordering of elements in Java HashSet

问题描述

为什么第二组和第三组保持顺序:

Why do the second and third sets preserve order:

Integer[] j = new Integer[]{3,4,5,6,7,8,9};
LinkedHashSet<Integer> i = new LinkedHashSet<Integer>();
Collections.addAll(i,j);
System.out.println(i); 

HashSet<Integer> hi = new HashSet<Integer>(i);
System.out.println(hi); 

LinkedHashSet<Integer> o = new LinkedHashSet<Integer>(hi);
System.out.println(o); 

这是我得到的输出:

3,4,5,6,7,8,9
3,4,5,6,7,8,9
3,4,5,6,7,8,9

推荐答案

第二个(仅使用 HashSet)只是一个巧合.来自 JavaDocs:

The second one (just using HashSet) is only a coincidence. From the JavaDocs:

这个类实现了 Set 接口，由一个哈希表(实际上是一个 HashMap 实例)支持.它不保证集合的迭代顺序；特别是，它不保证订单会随着时间的推移保持不变.此类允许空元素.

This class implements the Set interface, backed by a hash table (actually a HashMap instance). It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time. This class permits the null element.

第三个 (LinkedHashSet) 是 设计就是这样:

The third one (LinkedHashSet) is designed to be like that:

Set 接口的哈希表和链表实现，具有可预测的迭代顺序.此实现与 HashSet 的不同之处在于它维护一个双向链表，贯穿其所有条目.这个链表定义了迭代顺序，也就是元素被插入到集合中的顺序(插入顺序).请注意，如果将元素重新插入到集合中，则插入顺序不会受到影响.(如果在调用 s.contains(e) 之前立即返回 true 时调用了 s.add(e)，则元素 e 将重新插入到集合 s 中.)

Hash table and linked list implementation of the Set interface, with predictable iteration order. This implementation differs from HashSet in that it maintains a doubly-linked list running through all of its entries. This linked list defines the iteration ordering, which is the order in which elements were inserted into the set (insertion-order). Note that insertion order is not affected if an element is re-inserted into the set. (An element e is reinserted into a set s if s.add(e) is invoked when s.contains(e) would return true immediately prior to the invocation.)

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