public void updateNull(int columnIndex) throws SQLException {
        // sanity check.
        checkIndex(columnIndex);
        // make sure the cursor is on a valid row
        checkCursor();

        BaseRow row = getCurrentRow();
        row.setColumnObject(columnIndex, null);

    }
Sets the designated nullable column in the current row or the
insert row of this <code>CachedRowSetImpl</code> object with
<code>null</code> value.
<P>
This method updates a column value in the current row or the insert
row of this rowset; however, another method must be called to complete
the update process. If the cursor is on a row in the rowset, the
method {@link #updateRow} must be called to mark the row as updated
and to notify listeners that the row has changed.
If the cursor is on the insert row, the method {@link #insertRow}
must be called to insert the new row into this rowset and to notify
listeners that a row has changed.
<P>
In order to propagate updates in this rowset to the underlying
data source, an application must call the method {@link #acceptChanges}
after it calls either <code>updateRow</code> or <code>insertRow</code>.

@param columnIndex the first column is <code>1</code>, the second
       is <code>2</code>, and so on; must be <code>1</code> or larger
       and equal to or less than the number of columns in this rowset
@throws SQLException if (1) the given column index is out of bounds,
           (2) the cursor is not on one of this rowset's rows or its
           insert row, or (3) this rowset is
           <code>ResultSet.CONCUR_READ_ONLY</code>
src/main/java/org/replicadb/rowset/sun/rowset/CachedRowSetImpl.java