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

Answer 1

Another collector approach:

Collectors:

public final class SingleCollector extends SingleCollectorBase {
    @Override
    public Function, T> finisher() {
        return a -> a.getItem();
    }
}

public final class SingleOrNullCollector extends SingleCollectorBase {
    @Override
    public Function, T> finisher() {
        return a -> a.getItemOrNull();
    }
}

SingleCollectorBase:

public abstract class SingleCollectorBase implements Collector, T> {
    @Override
    public Supplier> supplier() {
        return () -> new Single<>();
    }

    @Override
    public BiConsumer, T> accumulator() {
        return (list, item) -> list.set(item);
    }

    @Override
    public BinaryOperator> combiner() {
        return (s1, s2) -> {
            s1.set(s2);
            return s1;
        };
    }

    @Override
    public Set characteristics() {
        return EnumSet.of(Characteristics.UNORDERED);
    }
}

Single:

public final class Single {

    private T item;
    private boolean set;

    public void set(T item) {
        if (set) throw new SingleException("More than one item in collection");
        this.item = item;
        set = true;
    }

    public T getItem() {
        if (!set) throw new SingleException("No item in collection");
        return item;
    }

    public void set(Single other) {
        if (!other.set) return;
        set(other.item);
    }

    public T getItemOrNull() {
        return set ? item : null;
    }
}

public class SingleException extends RuntimeException {
    public SingleException(String message) {
        super(message);
    }
}

Tests and example usages, albeit lacking parallel tests.

public final class SingleTests {

    @Test
    public void collect_single() {
        ArrayList list = new ArrayList<>();
        list.add("ABC");

        String collect = list.stream().collect(new SingleCollector<>());
        assertEquals("ABC", collect);
    }

    @Test(expected = SingleException.class)
    public void collect_multiple_entries() {
        ArrayList list = new ArrayList<>();
        list.add("ABC");
        list.add("ABCD");

        list.stream().collect(new SingleCollector<>());
    }

    @Test(expected = SingleException.class)
    public void collect_no_entries() {
        ArrayList list = new ArrayList<>();

        list.stream().collect(new SingleCollector<>());
    }

    @Test
    public void collect_single_or_null() {
        ArrayList list = new ArrayList<>();
        list.add("ABC");

        String collect = list.stream().collect(new SingleOrNullCollector<>());
        assertEquals("ABC", collect);
    }

    @Test(expected = SingleException.class)
    public void collect_multiple_entries_or_null() {
        ArrayList list = new ArrayList<>();
        list.add("ABC");
        list.add("ABCD");

        list.stream().collect(new SingleOrNullCollector<>());
    }

    @Test
    public void collect_no_entries_or_null() {
        ArrayList list = new ArrayList<>();

        assertNull(list.stream().collect(new SingleOrNullCollector<>()));
    }

}