MapReduce Reducer Assignment

Motivation

interface Collector<T,A,R> is fundamental to the MapReduce Frameworks lab. Your frameworks will be general enough such that MapReduce is just a subset of what you will support.

In this studio we will build a ClassicReducer Collector which will implement the MapReduce style of accumulating all of the emitted values per key in a List.

We will also build a custom IntSumCollector to demonstrate the desired flexibility of the Collector interface.

Background

We debated between creating our own custom Reducer<V,A,R> interface versus adopting the standard interface Collector<T,A,R> from the standard Java streams framework as the basis for the MapReduce Frameworks Lab.

While It might have been slightly less confusing at the outset if we used something like this mythical non-existant interface below:

public interface Reducer<V, A, R> {
	A createMutableContainer();
	void accumulate(A container, V item);
	A combine(A containerA, A containerB);
	R reduce(A container);
}

we chose to go with the standard Collector<T,A,R> despite the extra level of indirection it requires (returning @Functional interfaces whose single abstract methods do the work).

We provide this mythical class CollectorReducerAdapter as a Rosetta Stone of sorts in an effort to reveal what each method is responsible for:

public class CollectorReducerAdapter<V, A, R> implements Reducer<V, A, R> {
	private final Collector<V,A,R> collector;
	public CollectorReducerAdapter(Collector<V,A,R> collector) {
		this.collector = collector;
	}
	@Override
	public A createMutableContainer() {
		return collector.supplier().get();
	}
	@Override
	public void accumulate(A container, V item) {
		collector.accumulator().accept(container, item);
	}
	@Override
	public A combine(A containerA, A containerB) {
		return collector.combiner().apply(containerA, containerB);
	}
	@Override
	public R reduce(A container) {
		return collector.finisher().apply(container);
	}
}

Code To Implement

ClassicReducer

class:	ClassicReducer.java
methods:	supplier accumulator combiner
package:	mapreduce.collector.studio
source folder:	student/src/main/java

IntSumCollector

class:	IntSumCollector.java
methods:	supplier accumulator combiner finisher
package:	mapreduce.collector.intsum.studio
source folder:	student/src/main/java

class MutableInt

Testing Your Solution

Correctness

class:	CollectorStudioTestSuite.java
package:	mapreduce
source folder:	testing/src/test/java