How to filter using streams when we have multiple for loop and collect list out of filtered items from second list stream

Question

I am trying to change the set of code for normal forEach loops to stream and filters. But finding errors in the way it's getting implemented.

Current Code:

List<Long> validFromPersonIds;
List<Person> existingPersonIds;

Person class has two variables:
Long fromPersonId
Long toPersonId;

List<Long> insertionAvailableValidFromPersonIds = new ArrayList<>(validFromPersonIds);
validFromPersonIds.forEach(fromPersonId -> {
    existingPersonIds.forEach(existingPersonId -> {
        if(existingPersonId.getToPersonId() == toPersonId && existingPersonId.getFromPersonId() == fromPersonId)
        {
            insertionAvailableValidFromPersonIds.remove(fromPersonId);
        }
    });
});

In above code I have validFromPersonIds and existingPersonIds list. If any fromPersonId is existing in the table, I dont want to insert to the table. So, i am trying to remove fromPersonId if its already existing in the table.

This is working properly.

Using Streams and Filters, tried code:

List<Long> insertionAvailableValidFromPersonIds = new ArrayList<>();
insertionAvailableValidFromPersonIds = validFromPersonIds.stream()
    .filter(fromPersonId -> existingPersonIds.stream()
            .filter(existingPersonId-> existingPersonId.getToPersonId() == toPersonId && existingPersonId.getFromPersonId() == fromPersonId)
            .collect(Collectors.toList()).contains(fromPersonId).collect(Collectors.toList());

But instead of using normal forEach i am trying to use streams and filters, where I get filtered record and collect it as list. But I am not able to come up with correct syntax for that. Could anyone check where I am going wrong here.

Answer 1

looks like you are filtering by fromPersonId twice.. the first time in the inner filter and then again via this contains. Instead, you can use noneMatch:

private List<Long> newFilter(List<Long> validFromPersonIds, List<Person> existingPersonIds, long toPersonId) {
   return List<Long> filtered = validFromPersonIds.stream()
      .filter(fromPersonId -> existingPersonIds.stream()
          .noneMatch(eid -> eid.getToPersonId() == toPersonId && eid.getFromPersonId() == fromPersonId)
      )
      .toList();
}

Updated the answer with a test:

@Test
public void test() {
    List<Long> validFromPersonIds = List.of(1l, 2l, 3l, 4l);
    List<Person> existingPersonIds = List.of(
            new Person(1l, 2l)
    );
    long toPersonId = 2l;

    assertThat(newFilter(validFromPersonIds, existingPersonIds, toPersonId))
            .containsExactlyElementsOf(currentFilter(validFromPersonIds, existingPersonIds, toPersonId));
}

// current behavior described in the question
private List<Long> currentFilter(List<Long> validFromPersonIds, List<Person> existingPersonIds, long toPersonId) {
    List<Long> insertionAvailableValidFromPersonIds = new ArrayList<>(validFromPersonIds);
    validFromPersonIds.forEach(fromPersonId -> {
        existingPersonIds.forEach(existingPersonId -> {
            if (existingPersonId.getToPersonId() == toPersonId
                    && existingPersonId.getFromPersonId() == fromPersonId) {
                insertionAvailableValidFromPersonIds.remove(fromPersonId);
            }
        });
    });
    return insertionAvailableValidFromPersonIds;
}