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为什么第二组和第三组保留顺序：
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：
3,9
3,9
解决方法
第二个(只使用HashSet)只是巧合.从
JavaDocs：
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designed是这样的：
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.)
（编辑：李大同）
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