code: public static <R, C, V> /* @Nullable */ V computeIfAbsent(final @NonNull Table<R, C, V> table, final @Nullable R rowKey, final @Nullable C columnKey, final @NonNull BiFunction<? super R, ? super C, ? extends V> valueFunction) {
      /* @Nullable */ V value = table.get(rowKey, columnKey);
      if(value == null) {
        value = valueFunction.apply(rowKey, columnKey);
        table.put(rowKey, columnKey, value);
      }
      return value;
    }
docs: Gets the value of {@code rowKey} and {@code columnKey} from {@code table}, or computes
  the value using {@code valueFunction} and inserts it into the table.
  
  @param table the table
  @param rowKey the row key
  @param columnKey the column key
  @param valueFunction the value function
  @param <R> the row type
  @param <C> the column type
  @param <V> the value type
  @return the value