Method merge

guava/src/com/google/common/cache/LocalCache.java:4248–4256

(
      K key, V newValue, BiFunction<? super V, ? super V, ? extends @Nullable V> function)

  }

  @Override
  public @Nullable V merge(
      K key, V newValue, BiFunction<? super V, ? super V, ? extends @Nullable V> function) {
    checkNotNull(key);
    checkNotNull(newValue);
    checkNotNull(function);
    return compute(
        key, (k, oldValue) -> (oldValue == null) ? newValue : function.apply(oldValue, newValue));
  }

  @Override
  public void putAll(Map<? extends K, ? extends V> m) {