Avoid Active Waiting


/** thread safe */
class Counter {

  private final AtomicLong count = new AtomicLong();

  void increment(long amount) {
    if (amount < 1) {
      throw new IllegalArgumentException("Only increments!");
    }

    this.count.addAndGet(amount);
  }
}

AtomicLong is nonblocking (lock-free) but may result in lots of active waiting


/** thread safe */
class Counter {

  private final LongAdder count = new LongAdder();

  void increment(long amount) {
    if (amount < 1) {
      throw new IllegalArgumentException("Only increments!");
    }

    this.count.add(amount);
  }
}

LongAdder uses multiple atomatic longs (multiple stripes), one per thread. The sum of those stripes is the total.
Sum is not as exact as it is with the AtomicLong, but much higher performance if multiple threads increment simultaneously, eventually consistent