guava/src/com/google/common/util/concurrent/AtomicLongMap.java
@CanIgnoreReturnValue
  public long getAndAdd(K key, long delta) {
    return getAndAccumulate(key, delta, Long::sum);
  }
@CanIgnoreReturnValue
  public long getAndAdd(K key, long delta) {
    return getAndAccumulate(key, delta, Long::sum);
  }
Adds {@code delta} to the value currently associated with {@code key}, and returns the old
value.