*
   * @throws IllegalArgumentException if {@code rowKey} is not in {@link #rowKeySet()} or {@code
   *     columnKey} is not in {@link #columnKeySet()}.
   */
  @CanIgnoreReturnValue
  @Override
  @CheckForNull
  public V put(R rowKey, C columnKey, @CheckForNull V value) {
    checkNotNull(rowKey);
    checkNotNull(columnKey);
    Integer rowIndex = rowKeyToIndex.get(rowKey);

   *
   * @throws IllegalArgumentException if {@code rowKey} is not in {@link #rowKeySet()} or {@code
   *     columnKey} is not in {@link #columnKeySet()}.
   */
  @CanIgnoreReturnValue
  @Override
  @CheckForNull
  public V put(R rowKey, C columnKey, @CheckForNull V value) {
    checkNotNull(rowKey);
    checkNotNull(columnKey);
    Integer rowIndex = rowKeyToIndex.get(rowKey);