### Question:
EmptyHashFileGenerator implements HashFileGenerator { @Override public Optional<String> hashMulti(Collection<URL> urls) throws IOException { return Optional.absent(); }  @Override Optional<String> hash(final Optional<URL> url); @Override Optional<String> hashMulti(Collection<URL> urls);  }### Answer:
@Test public void defaultHashMultiNotNull() throws Exception { Assert.assertNotNull("should not be null", emptyHashFileGenerator.hashMulti(null)); }
@Test public void defaultHashMultiAbsent() throws Exception { Assert.assertFalse("should be absent", emptyHashFileGenerator.hashMulti(null).isPresent()); }
@Test public void defaultHashMultiEmptyNotNull() throws Exception { Assert.assertNotNull("should not be null", emptyHashFileGenerator.hashMulti(Collections.EMPTY_LIST)); }
@Test public void defaultHashMultiEmptyNotNullAbsentWithValue() throws Exception { Assert.assertNotNull("should not be null", emptyHashFileGenerator.hashMulti(Lists.newArrayList(new URL("file: }
@Test public void defaultHashMultiEmptyAbsent() throws Exception { Assert.assertFalse("should be absent", emptyHashFileGenerator.hashMulti(Collections.EMPTY_LIST).isPresent()); }