Map implementation with duplicate keys

Question

I want to have a map with duplicate keys.

I know there are many map implementations (Eclipse shows me about 50), so I bet there must be one that allows this. I know

Answer 1

what about such a MultiMap impl?

public class MultiMap extends HashMap> {
  private static final long serialVersionUID = 1L;
  private Map> innerMap = new HashMap<>();

  public Set put(K key, V value) {
    Set valuesOld = this.innerMap.get(key);
    HashSet valuesNewTotal = new HashSet<>();
    if (valuesOld != null) {
      valuesNewTotal.addAll(valuesOld);
    }
    valuesNewTotal.add(value);
    this.innerMap.put(key, valuesNewTotal);
    return valuesOld;
  }

  public void putAll(K key, Set values) {
    for (V value : values) {
      put(key, value);
    }
  }

  @Override
  public Set put(K key, Set value) {
    Set valuesOld = this.innerMap.get(key);
    putAll(key, value);
    return valuesOld;
  }

  @Override
  public void putAll(Map> mapOfValues) {
    for (Map.Entry> valueEntry : mapOfValues.entrySet()) {
      K key = valueEntry.getKey();
      Set value = valueEntry.getValue();
      putAll(key, value);
    }
  }

  @Override
  public Set putIfAbsent(K key, Set value) {
    Set valueOld = this.innerMap.get(key);
    if (valueOld == null) {
      putAll(key, value);
    }
    return valueOld;
  }

  @Override
  public Set get(Object key) {
    return this.innerMap.get(key);
  }

  @Override
  etc. etc. override all public methods size(), clear() .....

}