### Question:
EmptyHashFileGenerator implements HashFileGenerator { @Override public Optional<String> hash(final Optional<URL> url) throws IOException { return Optional.absent(); }  @Override Optional<String> hash(final Optional<URL> url); @Override Optional<String> hashMulti(Collection<URL> urls);  }### Answer:
@Test public void defaultHashOneNotNull() throws Exception { Assert.assertNotNull("should not be null", emptyHashFileGenerator.hash(null)); }
@Test public void defaultHashOneAbsent() throws Exception { Assert.assertFalse("should be absent", emptyHashFileGenerator.hash(null).isPresent()); }
@Test public void defaultHashOneAbsentWithValue() throws Exception { Assert.assertFalse("should be absent", emptyHashFileGenerator.hash(Optional.of(new URL("file: }