@GwtIncompatible("java.util.concurrent.ConcurrentHashMap concurrencyLevel") public MapMaker concurrencyLevel(int concurrencyLevel){
  builder.concurrencyLevel(concurrencyLevel);
  return this;
}

@ GwtIncompatible ( "java.util.concurrent.ConcurrentHashMap concurrencyLevel" ) public MapMaker concurrencyLevel ( int concurrencyLevel ) { builder . 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 placement in hash tables is essentially random , the actual concurrency will 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 is appropriate when it is known that only one thread will modify and all others will only read . defaults to 16 .
