*
   * @param i the index
   * @param delta the value to add
   * @return the updated value
   */
  @CanIgnoreReturnValue
  public double addAndGet(int i, double delta) {
    return accumulateAndGet(i, delta, Double::sum);
  }

  /**
   * Atomically updates the element at index {@code i} with the results of applying the given
   * function to the current and given values.
   *

  /**
   * Adds {@code delta} to the value currently associated with {@code key}, and returns the new
   * value.
   */
  @CanIgnoreReturnValue
  public long addAndGet(K key, long delta) {
    return accumulateAndGet(key, delta, Long::sum);
  }

  /**
   * Increments by one the value currently associated with {@code key}, and returns the old value.
   */
  @CanIgnoreReturnValue
  public long getAndIncrement(K key) {