MapUsingReduce.java

package com.insightfullogic.java8.answers.chapter3;

import java.util.ArrayList;
import java.util.List;
import java.util.function.Function;
import java.util.stream.Stream;

/**
 * Advanced Exercises Question 2
 */
public class MapUsingReduce {

    public static <I, O> List<O> map(Stream<I> stream, Function<I, O> mapper) {
        return stream.reduce(new ArrayList<O>(), (acc, x) -> {
        	// We are copying data from acc to new list instance. It is very inefficient,
        	// but contract of Stream.reduce method requires that accumulator function does
        	// not mutate its arguments.
        	// Stream.collect method could be used to implement more efficient mutable reduction,
        	// but this exercise asks to use reduce method.
        	List<O> newAcc = new ArrayList<>(acc);
        	newAcc.add(mapper.apply(x));
            return newAcc;
        }, (List<O> left, List<O> right) -> {
        	// We are copying left to new list to avoid mutating it. 
        	List<O> newLeft = new ArrayList<>(left);
        	newLeft.addAll(right);
            return newLeft;
        });
    }

}