Question

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); 

Here’s the output I get:

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

Answer 1

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

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

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.)