Java Stream : How to filter conditionally?

I have the following stream statement with Java Stream and I filter records based on the typeId values as types.getIds().contains(x.getType().getId()):

return petRepository.findAll().stream()
                .filter(x -> types.getIds().contains(x.getType().getId()))
                .collect(Collectors.groupingBy(x -> x.getType().getName(), Collectors.counting()));

However, I want to fetch all values when types.getIds().size() == 0 and tried something as shpon below:

.filter(types.getIds().size() > 0 ? x -> types.getIds().contains(x.getType().getId()) : <NO FILTERING>)

So, how can I skip filtering when types.getIds().size() == 0 ?

If you don't want to check for empty on every x, you can put the whole lambda in a ternary expression:

return petRepository.findAll().stream()
            .filter(types.getIds().isEmpty() ? x -> true 
                    : x -> types.getIds().contains(x.getType().getId()))
            .collect(Collectors.groupingBy(x -> x.getType().getName(), Collectors.counting()));

Or pop the stream in a temporary variable and apply the entire filter() conditionally:

Stream<X> stream = petRepository.findAll().stream();
if (!types.getIds().isEmpty()) {
    stream = stream.filter(x -> types.getIds().contains(x.getType().getId()));
}
return stream.collect(Collectors.groupingBy(x -> x.getType().getName(), Collectors.counting()));

You just need to remember that the predicate in the filter has to return true whenever you want a result to be included. So in your case the filter condition must be such that, if the ids are NOT empty, filter will only return true for the condition you want. If the ids are empty, then filter must always return true.

You can play with this by writing a minimal test example (or better yet, mocking your petRepository with junit or another testing framework).

Consider:

Set<Integer> ids = Set.of(2);
List<Integer> elements = List.of(1, 2, 3);
List<Integer> list = elements.stream()
        .filter(u -> ids.size() > 0 ? ids.contains(u) : true)
        .toList();
System.out.println(list);

// output here will be [2]
// but if ids is empty it will be [1, 2, 3]

Look at what's inside filter: ids.size() > 0 ? ids.contains(u) : true

If there are ids to check, see if the streamed element of elements is contained in them. If there are no ids to check, return true regardless (keep everything in the elements stream for the next step in the pipeline).

The ternary expression in this case could be further simplified to u -> ids.size() <= 0 || ids.contains(u).

How about

return petRepository.findAll().stream()
            .filter(x -> types.getIds().isEmpty() ||
                         types.getIds().contains(x.getType().getId()))
            .collect(Collectors.groupingBy(x -> x.getType().getName(), Collectors.counting()));