JoinRowSet should also "join" metadata, otherwise the class is absolutely useless.
See attached code
Also method columnUpdated(String) throws NullPointerException probably because of this issue. Please see example below:

import java.sql.Types;
import java.util.Arrays;
import com.sun.rowset.JoinRowSetImpl;
import javax.sql.rowset.JoinRowSet;
import javax.sql.rowset.RowSetMetaDataImpl;

/**
 *
 * @author aag
 */
public class B {
    
    public static void main(String[] argv) throws Exception {
        
        JoinRowSet jrs = new JoinRowSetImpl();
        RowSetMetaDataImpl rsmd = new RowSetMetaDataImpl();
        rsmd.setColumnCount(1);
        rsmd.setColumnName(1, "FIELD1");
        rsmd.setColumnType(1, Types.VARCHAR);
        jrs.setMetaData(rsmd);
        jrs.moveToInsertRow();
        jrs.updateString(1, "VALUE1");
        jrs.insertRow();
        jrs.moveToInsertRow();
        jrs.updateString(1, "VALUE2");
        jrs.insertRow();
        jrs.moveToCurrentRow();
        jrs.absolute(1);
        
        jrs.columnUpdated(1);
        System.out.println("columnUpdated(int) - OK");
        
        try {
            jrs.columnUpdated("FIELD1");
        } catch (NullPointerException e) {
            System.out.println("columnUpdated(String) - failed: throws NPE");
        }
        
    }
    
    
}

run:
columnUpdated(int) - OK
columnUpdated(String) - throws NPE