java：不兼容的类型：推断变量 RR 的边界不兼容

Question

jdk 1.8

Set and Collection.

I want to get count of intersection that > 0.

I try this:

    SortedSet<VsatVlan> vsatVlans;
  Collection<Integer> mobilityNsVlansList = mobilityNS.getVlanIds();
    ....
      long intersectionCount = vsatVlans.stream()
                        .filter(vsatVlan -> mobilityNsVlansList.contains(vsatVlan.getVlanId()))
                        .collect(Collectors.collectingAndThen(
                                Collectors.toSet(),
                                set -> { // error here
                                    if (set.size() != 1) {
                                        throw new IllegalStateException();
                                    }
                                    return set.size();
                                }
                        ));

But I get error:

incompatible parameter types in lambda expression: expected Set<Object> but found Set<VsatVlan>

java: incompatible types: inference variable RR has incompatible bounds
    lower bounds: java.lang.Long,java.lang.Object
    lower bounds: java.lang.Integer

Answer 1

If I get it right you want to return count of intersection only if not zero else you want to throw an exception. You need to explicitly supply the type of the set to get rid of the error you get:

return vsatVlans.stream()
                .filter(vsatVlan -> mobilityNsVlansList.contains(vsatVlan.getVlanId()))
                .collect(Collectors.collectingAndThen(Collectors.toSet(),
                    (Set<VsatVlan> set) -> {
                        if (set.size() == 0) {
                            throw new IllegalStateException("");
                        }
                        return set.size();
                    }
               ));

You also could use Optionals here and and avoid the if block:

return vsatVlans.stream()
               .filter(vsatVlan -> mobilityNsVlansList.contains(vsatVlan.getVlanId()))
               .collect(Collectors.collectingAndThen(Collectors.counting(), Optional::of))
               .filter(x -> x > 0)
               .orElseThrow(() -> new IllegalStateException(""));

but IMO counting and then checking in a separate if block, is way more readable than the above two:

long count = vsatVlans.stream()
                      .filter(vsatVlan -> mobilityNsVlansList.contains(vsatVlan.getVlanId()))
                      .count();
if(count == 0){
    new IllegalStateException("")
}
return count;