MapReduce.java

import org.junit.Assert;

import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;
import java.util.Map;

public class MapReduce<V, T> {

    private Collection<V> collection;
    private T param;

    public static <V, T> MapReduce<V, T> reduce(Collection<V> coll, T arg) {
        return new MapReduce<V, T>(coll, arg);
    }

    private MapReduce(Collection<V> coll, T param) {
        collection = coll;
        this.param = param;
    }

    public T runBlock(Transformer<V, T> transformer) {
        for (V obj : collection) {
            param = transformer.execute(param, obj);
        }
        return param;
    }

    public static interface Transformer<V, T> {
        T execute(T singleEntry, V result);
    }

    public static void main(String[] args) {
        ArrayList<String> list = new ArrayList<String>();
        list.add("abc");
        list.add("www");

        Integer result = MapReduce.reduce(list, 0).runBlock(new Transformer<String, Integer>() {
            public Integer execute(Integer total, String str) {
                return total + str.length();
            }
        });

        Assert.assertEquals(6, result.intValue());

        Map mappings = MapReduce.reduce(list, new HashMap()).runBlock(new Transformer<String, HashMap>() {
            public HashMap execute(HashMap total, String str) {
                if (str.equals("abc")) {
                    total.put("a", str);
                }
                return total;
            }
        });

        YamlDumper.dump(mappings);
    }

}