docs: @Override
    public V putIfAbsent(K key, final V value) {
      return cache.invoke(key, new EntryProcessor<K, V, V>() {
        @Override
        public V process(MutableCacheEntry<K, V> e) {
          if (!e.exists()) {
            e.setValue(value);
            return null;
          }
          return e.getValue();
        }
      });
    }