SetReducer implements Reducer<Set<T>> { @Override public boolean isZero(Set<T> ts) { return ts.isEmpty(); }  @Override Set<T> reduce(Set<T> v1, Set<T> v2); @Override Set<T> zero(); @Override boolean isZero(Set<T> ts);  }
@Test public void isZeroShouldReturnTrueOnEmptySet() { assertThat(reducer.isZero(new HashSet<Object>())).isTrue(); }