Builder pattern with a Java 8 Stream

Question

I am building an object with a simple loop:

WebTarget target = getClient().target(u);

for (Entry queryParam : queryParams.entrySet()) {
           


        

Answer 1

It's not very difficult to implement a correct foldLeft for Java 8 streams:

@SuppressWarnings("unchecked")
public static  U foldLeft(Stream stream, 
                                U identity, BiFunction accumulator) {
    Object[] result = new Object[] { identity };
    stream.forEachOrdered(t -> result[0] = accumulator.apply((U) result[0], t));
    return (U) result[0];
}

Or in type-safe manner:

public static  U foldLeft(Stream stream, 
                                U identity, BiFunction accumulator) {
    class Box {
        U value;
        Box(U value) { this.value = value; }
    }
    Box result = new Box(identity);
    stream.forEachOrdered(t -> result.value = accumulator.apply(result.value, t));
    return result.value;
}

This works correctly for sequential and parallel streams. You can even have a speed gain using parallel streams if your stream has some CPU-consuming stateless intermediate operations like map: in this case the next element can be processed by map in the parallel with the current element processed by foldLeft. I don't agree that such operation is not suitable for Stream API, because it can be correctly expressed via already existing forEachOrdered.

I have this operation in my StreamEx library, so you can use it like this:

WebTarget target = EntryStream.of(queryParams).foldLeft(getClient().target(u), 
        (t, entry) -> t.queryParam(entry.getKey(), entry.getValue()))