* implementations, except when the table is sparse.
 *
 * <p>Null row keys or column keys are not permitted.
 *
 * <p>This class provides methods involving the underlying array structure, where the array indices
 * correspond to the position of a row or column in the lists of allowed keys and values. See the
 * {@link #at}, {@link #set}, {@link #toArray}, {@link #rowKeyList}, and {@link #columnKeyList}