자바 스트림을 이용한 맵리듀스 예제.

MapReduce.java

import java.util.ArrayList;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class MapReduce {

	public static void main(String[] args) {
		
		ArrayList<String> strs = new ArrayList<>();
		strs.add("Deer Bear River");
		strs.add("Car Car River");
		strs.add("Deer Car Bear");
		
		// wordcount 1
		Map<String, Long> map = strs.stream().flatMap(x -> Stream.of(x.split(" ")))
				.collect(Collectors.groupingBy(x -> x, Collectors.counting()));
		
		map.forEach((x, y) -> System.out.printf("key: %s, value: %d\n", x, y));
		
		// wordcount 2
		Map<Object, Long> map2 = strs.stream().flatMap(x -> Stream.of(x.split(" ")))
				.map(x -> new WordCount(x, 1))
				.collect(Collectors.groupingBy(WordCount::getWord, Collectors.counting()));
		
		map2.forEach((x, y) -> System.out.printf("key: %s, value: %d\n", x, y));
	}
	
}

class WordCount {
	
	public String word;
	public int count;
	
	public WordCount(String word, int count) {
		this.word = word;
		this.count = count;
	}

	public String getWord() {
		return word;
	}

	public int getCount() {
		return count;
	}
}