code: @CanIgnoreReturnValue
    public final double getAndAdd(int i, double delta) {
      while (true) {
        long current = longs.get(i);
        double currentVal = longBitsToDouble(current);
        double nextVal = currentVal + delta;
        long next = doubleToRawLongBits(nextVal);
        if (longs.compareAndSet(i, current, next)) {
          return currentVal;
        }
      }
    }
docs: Atomically adds the given value to the element at index {@code i}.
  
  @param i the index
  @param delta the value to add
  @return the previous value