code: @CanIgnoreReturnValue
    public MapMaker concurrencyLevel(int concurrencyLevel) {
      checkState(
          this.concurrencyLevel == UNSET_INT,
          "concurrency level was already set to %s",
          this.concurrencyLevel);
      checkArgument(concurrencyLevel > 0);
      this.concurrencyLevel = concurrencyLevel;
      return this;
    }
docs: Guides the allowed concurrency among update operations. Used as a hint for internal sizing. The
  table is internally partitioned to try to permit the indicated number of concurrent updates
  without contention. Because assignment of entries to these partitions is not necessarily
  uniform, the actual concurrency observed may vary. Ideally, you should choose a value to
  accommodate as many threads as will ever concurrently modify the table. Using a significantly
  higher value than you need can waste space and time, and a significantly lower value can lead
  to thread contention. But overestimates and underestimates within an order of magnitude do not
  usually have much noticeable impact. A value of one permits only one thread to modify the map
  at a time, but since read operations can proceed concurrently, this still yields higher
  concurrency than full synchronization. Defaults to 4.
  
  <p><b>Note:</b> Prior to Guava release 9.0, the default was 16. It is possible the default will
  change again in the future. If you care about this value, you should always choose it
  explicitly.
  
  @throws IllegalArgumentException if {@code concurrencyLevel} is nonpositive
  @throws IllegalStateException if a concurrency level was already set