MultiValueMap and Transformer

MultiValueMapConverter.java

import java.util.*;

public class MultiValueMapConverter {

    public static void main(String[] args) {
/*
        0 -> [0]
        1 -> [0]
        2 -> [1]
        3 -> [3]
        4 -> [5,6,7]
        5 -> [2]
 */
        MultiValueMap<Integer, Integer> engToEspMap = new MultiValueMap<>();
        engToEspMap.put(0, 0);
        engToEspMap.put(1, 0);
        engToEspMap.put(2, 1);
        engToEspMap.put(3, 3);
        engToEspMap.put(4, List.of(5, 6, 7));
        engToEspMap.put(5, 2);

/* should be
0 -> [0,1]
1 -> [2]
2 -> [5]
3 -> [3]
4 -> [] // not handled case, not implemented 
5 -> [4]
6 -> [4]
7 -> [4]
 */

        MultiValueMap<Integer, Integer> espToEngMap = transform(engToEspMap);
        System.out.println(espToEngMap);
    }

    private static MultiValueMap<Integer, Integer> transform(MultiValueMap<Integer, Integer> mapToTransform) {
        MultiValueMap<Integer, Integer> newMap = new MultiValueMap<>();
        for (Integer key : mapToTransform.keySet()) {
            for (Integer value : mapToTransform.get(key)) {
                newMap.put(value, key);
            }
        }
        return newMap;
    }

    public static class MultiValueMap<K, V> {

        private final Map<K, List<V>> mappings = new HashMap<>();

        public List<V> get(K key) {
            return mappings.get(key);
        }

        public Boolean put(K key, List<V> value) {
            List<V> target = mappings.get(key);
            if (target == null) {
                target = new ArrayList<>();
                mappings.put(key, target);
            }
            return target.addAll(value);
        }

        public Boolean put(K key, V value) {
            List<V> target = mappings.get(key);

            if (target == null) {
                target = new ArrayList<>();
                mappings.put(key, target);
            }
            return target.add(value);
        }

        public Set<K> keySet() {
            return mappings.keySet();
        }
    }

}