How to JUnit test a iterator

Question

For example, how do I test:

ArrayList<String> list = new ArrayList<String>();
list.iterator();

How to test this "iterator()" method?

Thanks.

Answer 1

The few tests I can think of are:

• test hasNext on an empty collection (returns false)
• test next() on an empty collection (throws exception)
• test hasNext on a collection with one item (returns true, several times)
• test hasNext/next on a collection with one item: hasNext returns true, next returns the item, hasNext returns false, twice
• test remove on that collection: check size is 0 after
• test remove again: exception
• final test with a collection with several items, make sure the iterator goes through each item, in the correct order (if there is one)
• remove all elements from the collection: collection is now empty

You can also have a look at the tests used in openJDK.

Answer 2

You don't.

Guys from oracle and sun have already done that.

If you create your own implementation of iterator then you have to implement AFAIR 2 methods and you have to check if they obey to the contract.

Now that means: returning next element of underlying collection or throwing an exception and telling if there are subsequent elements. Just call those methods on your iterator and assert the result.