### Question:
MutableDataSet implements DataSet { @Override public int numberOfIndividuals() { return this.individuals.size(); } MutableDataSet(Name<?> name); @Override Name<?> name(); @Override int numberOfIndividuals(); @Override boolean hasIndividuals(); @Override Collection<Individual<?, ?>> individuals(); @Override Set<Serializable> individualIds(); @Override boolean hasIndividual(Object id); @SuppressWarnings("unchecked") @Override S individualOfId(T id); @Override S individual(T id, Class<? extends S> clazz); @Override Iterator<Individual<?, ?>> iterator(); @Override boolean isEmpty(); @Override void remove(final Individual<?, ?> src); @Override String toString();  }### Answer:
@Test public void testNumberOfIndividuals$noIndividuals() throws Exception { assertThat(sut.numberOfIndividuals(),equalTo(0)); }
@Test public void testNumberOfIndividuals$anEmptyIndividual() throws Exception { sut.individual(URI.create("urn:individual"), ExternalIndividual.class); assertThat(sut.numberOfIndividuals(),equalTo(1)); }