@Test
public void testContract() {

  Comparator<File> comparator = new DensityAwareFileComparator(Collections.emptySet());
  List<File> files = Arrays.asList(
      new File("/res/mipmap-mdpi"),
      new File("/res/drawable-hdpi"),
      new File("/res/mipmap")
  );
  files.sort(comparator);
  assertThat(comparator.compare(files.get(0), files.get(1))).isLessThan(0);
  assertThat(comparator.compare(files.get(1), files.get(2))).isLessThan(0);
  assertThat(comparator.compare(files.get(0), files.get(2))).isLessThan(0);
}