Question

I have a JSP file in which jQuery is supposed to send a POST request containing the elements (categories) I have ticked in a multi select box to the server. The server will analyse the string containing all ticked categories and return a list of subjects, for the jQuery to analyse it and display the subjects in another multi select.

The only problem is, when I tick no category, the jQuery does send an empty parameter to the server, but the server never detects it as null!

I tried an alert that does display “”, but it just won’t work! The server crashes pretexting a NullPointerException related to the code in the if condition obviously.

Here’s my code:

 === JSP ===

<script>
                $(document).ready(function ()  
                    {
                        $('#subjectCategories').change(function()
                            {
                                var selectedOptions = $('#subjectCategories option:selected');
                                var selectedValues = $.map(selectedOptions ,function(option) 
                                {
                                    return option.value;
                                }).join(',');

                                $.ajax({
                                       type: "POST",
                                       url: "creation",
                                       data: 'selectedSubjectCategories='+selectedValues,
                                       success: function(data)
                                       {
                                           if (data!=null)
                                            {
                                                   //alert("\""+data+"\"");
                                                   var subjects = data.split(',');
                                                   var select = $('#subjectslist');
                                                    $('#subjectslist').children().remove();
                                                   $.each(subjects, function(key, subject) 
                                                   {
                                                       if (select.find('option[value="' + subject + '"]').length === 0 && subject!="") 
                                                       {
                                                             //Ajouter la nouvelle catégorie dans la liste
                                                               $('<option>', {
                                                                 value: subject,
                                                                 text: subject
                                                                 }).appendTo(select);
                                                       }
                                                   });
                                            }
                                           else
                                            {
                                                $('#subjectslist').children().remove();
                                            }
                                       }
                                     });
                            }
                        );
                    }).change();
              </script>

=== SERVLET ===

            if (request.getParameter("selectedSubjectCategories")!=null)
        {
            String[] categoryList = request.getParameter("selectedSubjectCategories").split(",");

            ArrayList<String> pourDoublons = new ArrayList<String>();
            String subjectsToShow = new String();

            for (int i=0; i<categoryList.length; i++)
            {
                if (categorySubjectsHashMap.containsKey(categoryList[i]))
                {
                    ArrayList<String> subjectsOfCategory = categorySubjectsHashMap.get(categoryList[i]);
                    for (int j=0; j<subjectsOfCategory.size(); j++)
                    {
                        if (!pourDoublons.contains(subjectsOfCategory.get(j)))
                        {
                            subjectsToShow += subjectsOfCategory.get(j)+",";
                            pourDoublons.add(subjectsOfCategory.get(j));
                        }
                    }
                }
            }
            categoryList = null;
            pourDoublons = null;
            response.setContentType("text/plain");  
            response.setCharacterEncoding("UTF-8");
            response.getWriter().write(subjectsToShow.substring(0,subjectsToShow.length()-1));
        }
        else
        {
            response.setContentType("text/plain");  
            response.setCharacterEncoding("UTF-8");
            response.getWriter().write("");
        }

Why can’t my parameter be detected as null? I also tried .equals(null) and a lot of stuff like that, but nothing worked! 🙁

Thanks in advance.

Answer 1

if (request.getParameter("selectedSubjectCategories")!='')

worked fine. 🙂