JDBC - how to avoid null pointer exceptions on NULL fields

I am using the following method in a database class which I pass an SQL string into. It in turn fills a Vector of rows each element of which is a vector containing individual field values. This works fine until I encounter NULL fields in the database when null pointer exceptions occur. I have tried catching them but it still doesn't recover and continue.
How should I alter this code (or if someone had code to do what I want that would be great)
Thanks
gironszeg

      public void executeQuery(String sQuery){//For result returning SQL statements      
            this.theQuery = sQuery;
            ResultRows = new Vector();
            try
            {
              Statement queryStatement = databaseConnection.createStatement();
              ResultSet theResults = queryStatement.executeQuery(theQuery);
              ResultSetMetaData rsmeta = theResults.getMetaData();

              int numColumns = rsmeta.getColumnCount();
              String resultsText = new String("");
              while(theResults.next())
              {
                numRows++;
                  Cols = new Vector();
                  for(thisCol = 1; thisCol <= numColumns; thisCol++){
                        resultsText = theResults.getString(thisCol).trim();
                        Cols.addElement(resultsText);
                  }
                  ResultRows.addElement(Cols);
              }
            }
            catch(SQLException sqle)
            {
              System.out.println(sqle.toString());
            }      
      }