How to add two HashSets into an ordered TreeSet respectively

Question

I am creating RDF statements in Java. I have two HashSets and would like to add both of them into a TreeSet for example. Here is my code:

Collection<Statement> model = new HashSet<Statement>();
Collection<Statement> model_1 = new HashSet<Statement>();
Collection<Statement> treeSet = new TreeSet<Statement>();

After storing some statements into these two Hashsets, I added both of them in the TreeSet: Here is the code:

treeSet.addAll(model);
treeSet.addAll(model_1);
for (Statement _state : treeSet)
     System.out.println(_state);

I thought that the first Hashset "model" would be added into the TreeSet and then the second Hashset "model_1" respectively. It seems that they are not ordered when I loop through the TreeSet. Could anyone please help me how to order the elements of the TreeSet in a way that I can add the first HashSet and then the second one as they are respectively.

Answer 1

The TreeSet will order on the elements you put in, not the order in which you put them in. A simple List may suffice here if you don't need the hashing capability, or perhaps a LinkedHashSet (which maintains insertion order whilst iterating)

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.)