java从Set中检索对象

Question

在 Java 5 中，如果我有一个 Set，并且我将两个对象添加到 Set 中。当我检索对象时，它会按照我添加的顺序返回给我吗？我无法找到这个问题的答案。这取决于我使用的 Set 实现吗？

Answer 1

是的，这取决于您使用的实现。例如，LinkedHashSet将保留插入顺序:

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.)

...但是HashSet不会:

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.