How to check for NULL and write a blank?

How can I check if a field has a NULL value, and if it does make it blank in the output?

Here is my code that iterates thru each record and field.
How can I change?

        while (result.next()) {
           out.write("<tr>");
           for (int i=1; i<=columns; i++) {
             out.write("<td>" + result.getString(i) + "</td>");
           }
           out.write("</tr>");
        }