ETL.java

package etl;

import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

/**
 * Created by david.wursteisen on 02/07/2015.
 */
public class Etl {
    public static Map<String, Integer> transform(Map<Integer, List<String>> old) {
        return old.entrySet()
                .stream()
                .flatMap(e -> e.getValue().stream().flatMap(s -> {
                    HashMap<String, Integer> result = new HashMap<>();
                    result.put(s, e.getKey());
                    return result.entrySet().stream();
                }))
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
    }
    public static void main(String[] args) {
        HashMap<Integer, List<String>> map = new HashMap<>();
        map.put(1, Arrays.asList("1", "2", "3"));
        map.put(2, Arrays.asList("4", "5", "6"));
        transform(map).entrySet().stream().forEach(System.out::println);
    }
}