* @throws IllegalStateException if a concurrency level was already set
   */
  @CanIgnoreReturnValue
  public CacheBuilder<K, V> concurrencyLevel(int concurrencyLevel) {
    checkState(
        this.concurrencyLevel == UNSET_INT,
        "concurrency level was already set to %s",
        this.concurrencyLevel);
    checkArgument(concurrencyLevel > 0);