Collector Rosetta Stone

The interface Collector<T,A,R> serves the standard Java streams framework for MapReduce-like tasks with added in-memory processing capability a la Apache Spark.

You will use interface AccumulatorCombinerReducer<V,A,R> for our MapReduce assignments which is almost a one-to-one match with interface Collector<T,A,R> but de-ultra-uber-hyper-mega-super-lambdafied.

One To One

CSE 231s: AccumulatorCombinerReducer<V, A, R>		Java Streams: Collector<T, A, R>
public interface AccumulatorCombinerReducer<V, A, R> { A createMutableContainer(); void accumulate(A container, V item); void combine(A containerA, A containerB); R reduce(A container); }	<===> <===> <===> <===>	public interface Collector<T, A, R> { Supplier<A> supplier(); BiConsumer<A, T> accumulator(); BinaryOperator<A> combiner(); Function<A, R> finisher(); }

Differences

Each interface provides a way to customize 4 of 5 of the methods required for Spark-level MapReduce functionality. interface AccumulatorCombinerReducer<V,A,R> elects to perform the operations directly. interface Collector<T,A,R> elects to return single-abstract method interfaces which perform the operations.

interface Collector<T,A,R>

interface Supplier<T> returned by supplier()

interface BiConsumer<T,U> returned by accumulator()

interface BinaryOperator<T> returned by combiner()

interface Function<T,R> returned by finisher()

Methods

createMutableContainer a.k.a. supplier get

We use createMutableContainer() to create a new mutable container. For classic map reduce this would be a List<V>.

rosetta stone: container = collector.supplier().get() $\leftrightarrow$ container = reducer.createMutableContainer()

accumulate a.k.a. accumulator accept

We use accumulate(container,item) to accumulate a value. For classic map reduce this would add an item to a list.

rosetta stone: collector.accumulator().accept(container,item); $\leftrightarrow$ reducer.accumulate(container,item)

combine a.k.a. combiner apply

We use combine(containerA,containerB) to combine two accumulators. You may combine containerB into containerA or containerA into containerB. Just return whichever is the combined result.

rosetta stone: collector.combiner().apply(containerA,containerB) $\leftrightarrow$ reducer.combine(containerA,containerB)

reduce a.k.a. finisher apply

We use reduce(container) to reduce an accumulator.

rosetta stone: collector.finisher().apply(container) $\leftrightarrow$ r = reducer.reduce(container)

Converting Back And Forth

public class StreamUtils {
	public static <V, A, R> Collector<V, A, R> toCollector(
			AccumulatorCombinerReducer<V, A, R> accumulatorCombinerReducer) {
		return new Collector<V, A, R>() {
			@Override
			public Supplier<A> supplier() {
				return () -> accumulatorCombinerReducer.createMutableContainer();
			}

			@Override
			public BiConsumer<A, V> accumulator() {
				return (container, item) -> accumulatorCombinerReducer.accumulate(container, item);
			}

			@Override
			public BinaryOperator<A> combiner() {
				return (a, b) -> {
					accumulatorCombinerReducer.combine(a, b);
					return a;
				};
			}

			@Override
			public Function<A, R> finisher() {
				return (container) -> accumulatorCombinerReducer.reduce(container);
			}

			@Override
			public Set<Characteristics> characteristics() {
				return accumulatorCombinerReducer.collectorCharacteristics();
			}
		};
	}

	public static <V, A, R> AccumulatorCombinerReducer<V, A, R> toAccumulatorCombinerReducer(
			Collector<V, A, R> collector) {
		return new AccumulatorCombinerReducer<V, A, R>() {
			@Override
			public A createMutableContainer() {
				return collector.supplier().get();
			}

			@Override
			public void accumulate(A container, V item) {
				collector.accumulator().accept(container, item);
			}

			@Override
			public void combine(A containerA, A containerB) {
				A result = collector.combiner().apply(containerA, containerB);
				if (result != containerA) {
					throw new RuntimeException("collector must combine b into a and return a.");
				}
			}

			@Override
			public R reduce(A container) {
				return collector.finisher().apply(container);
			}

			@Override
			public Set<Characteristics> collectorCharacteristics() {
				return collector.characteristics();
			}
		};
	}

	public static AccumulatorCombinerReducer<Integer, ?, Integer> summingIntAccumulatorCombinerReducer() {
		return toAccumulatorCombinerReducer(Collectors.summingInt(Integer::intValue));
	}
}