title: JUnit: Check that a collection only contains objects of a certain type
body: <p>Say I have a method that returns a collection of <code>Animal</code>s. In one case I want to check that this method contains exactly 7 <code>Dog</code>s. Checking the number isn't a problem, but how would you check that all the animals are instances of <code>Dog</code>?</p>
  
  <p>I suppose I could loop over the collection, but is there a more JUnity of saying "assert that this collection contains only objects which are dogs"?</p>