Intersection of stream of sets into new set

Question

Is there a better, simpler approach to this problem?

@Test
public void testReduce() {
    Set<Integer> foo = ImmutableSet.of(1,2,3,4,8,9);
    Set<Integer> bar = ImmutableSet.of(1,3,8,5,11);

    //DO think about solution for 1..n sets, and not only two.
    Set<Integer> intersection = ImmutableList.of(foo,bar)
            .stream()
            .reduce( null, (a, b) -> {
                if ( a == null ) {
                    a = new HashSet<Integer>(b);
                }
                else {
                    a.retainAll(b);
                }
                return a;
            });
    assertThat( intersection, is( ImmutableSet.of( 1,3,8) ) );
}

Answer 1

reduce is the wrong method for this, as you are not allowed to modify the function’s argument this way. This is a mutable reduction, also known as collect:

List<Set<Integer>> listOfSets=…;

if (listOfSets.isEmpty()) {
  return new HashSet<>();
}

Set<Integer> intersection = listOfSets.stream().skip(1)
    .collect(()->new HashSet<>(listOfSets.get(0)), Set::retainAll, Set::retainAll);

Having to peek for the first set is a drawback here, but using null as identity value isn’t clean either (and wouldn’t work with collect as the accumulator can’t return a new set).