android/guava/src/com/google/common/collect/MapMaker.java
@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;
  }
@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;
  }
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