如何对具有Collection返回值的Collection参数进行单元测试？

Question

I have a method that takes in a Collection of type T as a parameter and returns a Collection of type Integer. In this specific instance, I'm trying to return an ArrayList (am I wrong to do that? I figured since an ArrayList inherits from a Collection, that should be okay).

@Test public void test() {
  Collection<Integer> list = new ArrayList<Integer>();
  list.add(2);
  list.add(8);
  list.add(7);
  list.add(3);
  list.add(4);
  Comparison comp = new Comparison();
  int low = 1;
  int high = 5;
  ArrayList<Integer> actual = SampleClass.<Integer>range(list, low, high, comp);
  ArrayList<Integer> expected = new ArrayList<Integer>();
  expected.add(2);
  expected.add(3);
  expected.add(4);
  Assert.assertEquals(expected, actual);
}

What am I doing wrong here?

EDIT:

As requested, here is the discussed method:

public static <T> Collection<T> range(Collection<T> coll, T low, T high,
                                     Comparator<T> comp) {
  if (coll == null || comp == null) {
     throw new IllegalArgumentException("No Collection or Comparator.");
  }
  if (coll.size() == 0) {
     throw new NoSuchElementException("Collection is empty.");
  }
  ArrayList<T> al = new ArrayList<T>();
  for (T t : coll) {
     if (comp.compare(t, low) >= 0 && comp.compare(t, high) <= 0) {
        al.add(t);
     }
  }
  return al;
}

Answer 1

The comment from Andy Turner suggests copying the returned collection into an ArrayList , which can then be compared to another ArrayList using equals() . This makes the code compile and presumably lets the test pass.

The fact that the potential solutions include casting the result or copying it into a container of a different type raise some questions about what's actually going on here.

Copying the result Collection into a List makes some possibly unwarranted assumptions about the range() method. It's declared to return a Collection . If your test is modified to copy the result to a List and compares it to an expected result, also a List , the test constrains the range() implementation to produce its results in a particular order. If order is significant, perhaps range() should be modified to consume and return List instead of Collection .

But what if order doesn't matter? If the returned collection contained [4, 3, 2] instead of [2, 3, 4] , would that be acceptable? If so, then you'll need to compare the actual and expected results in an order-independent fashion. For techniques to do that, see this question:

Is there a way to check if two Collections contain the same elements, independent of order?