Java 8 Collector that returns a value if there's only a single value [duplicate]

Answer 1

You can easily write your own Collector

public class AllOrNothing implements Collector, Optional>{



@Override
public Supplier> supplier() {
    return () -> new HashSet<>();
}



@Override
public BinaryOperator> combiner() {
    return (set1, set2)-> {
        set1.addAll(set2);
        return set1;
    };
}

@Override
public Function, Optional> finisher() {
    return (set) -> {
        if(set.size() ==1){
            return Optional.of(set.iterator().next());
        }
        return Optional.empty();
    };
}

@Override
public Set characteristics() {
    return Collections.emptySet();
}

@Override
public BiConsumer, T> accumulator() {
    return Set::add;
}

}

Which you can use like this:

   Optional result = myStream.collect( new AllOrNothing<>());

Here's your example test data

public static void main(String[] args) {
    System.out.println(run());

    System.out.println(run(1));
    System.out.println(run(1,1));
    System.out.println(run(2,2));
    System.out.println(run(1,2));
}

private static Optional run(Integer...ints){

    List asList = Arrays.asList(ints);
    System.out.println(asList);
    return asList
                .stream()
                .collect(new AllOrNothing<>());
}

which when run will print out

[]
Optional.empty
[1]
Optional[1]
[1, 1]
Optional[1]
[2, 2]
Optional[2]