code: @Override
      public V putIfAbsent(@NonNull K key, @NonNull V v) {
          Entry<K, V> current = get(key);
          if (current != null) {
              return current.mValue;
          }
          mHashMap.put(key, put(key, v));
          return null;
      }
docs: /* (non-Javadoc)
  @see android.arch.core.internal.SafeIterableMap#putIfAbsent(java.lang.Object, java.lang.Object)