@Test
public void execute_withEmptyDiskCache_shouldReturnNullsInMap() throws Exception {

    new ImageDiskTaskManager(list, imageTaskManagerListener, TEST_WIDTH).execute();
    semaphore.acquire();
    assertThat(bitmaps.size()).isEqualTo(2);
    assertThat(bitmaps.containsKey(url1)).isTrue();
    assertThat(bitmaps.containsKey(url2)).isTrue();
    assertThat(bitmaps.get(url1)).isNull();
    assertThat(bitmaps.get(url2)).isNull();
}