Java HashSet中的元素排序 [英] Ordering of elements in Java HashSet

问题描述

为什么第二个和第三个集保存顺序：

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

使用 HashSet ）只是巧合。从 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，它不保证订单会随时间保持不变 。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 ） 设计为：

Set接口的哈希表和链表实现，具有可预测的迭代顺序。此实现不同于HashSet，因为它维护一个贯穿其所有条目的双向链表。此链接列表定义迭代排序，它是将元素插入集合（插入顺序）的顺序。请注意，如果元素重新插入到集合中，则插入顺序不会受到影响。 （如果s.contains（e）在调用之前立即返回true，则调用s.add（e）时，元素e被重新插入到集合中。）

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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