@Test
public void testKeysUnique() {

    final int firstKey = cache.getSpareKey(testActivity);
    final int secondKey = cache.getSpareKey(testActivity);
    final int thirdKey = cache.getSpareKey(testActivity);
    assertNotEquals(firstKey, secondKey);
    assertNotEquals(firstKey, thirdKey);
    assertNotEquals(secondKey, thirdKey);
}