在我的arraylist.set（）上获取空值。 使用jsp

Question

My objective is to add words from a html form, store them in my array, display them in the page, and have two links one for delete the item and one for edit the item(the word for edit will be entered in another html form).

I am using an arrayList to store the input coming from an Input type. I am able to add and delete my item, but when I tried to change the value of an item in my arrayList (I am using arrayList.set()), my value returns null. If I hard code the set() I see my item change but for some reason I can't get the value from my input type. Wondering what I am missing.

I have two jsp pages. main.jsp and edit.jsp. One java class WordOperations with all the validations

Here is my code Sorry if it is messy and if doesn't display right in here. This is the first time I am posting on stackoverflow

Any help will be appreciated. Thanks

This is my code for the form where i display my items main.jsp

 <h3>Word List</h3>
        <form action="WordOperations" method="POST">
            Word: <input type="text" name="newWord">
            <input type="submit" value="Add">
        </form>
        <br/>

            <%
                ArrayList<String> words = (ArrayList<String>) session.getAttribute("words");

                if (words!=null)
                {
                    for (int i=0; i<words.size(); i++)
                    {
                         out.println("<p>" +  "<a href='http://www."+ words.get(i) +".com'>" + words.get(i) +"</a>"  + "&nbsp;&nbsp;<a href='WordOperations?delete=" + i + "'>Delete</a>&nbsp;&nbsp;<a href='WordOperations?edit=" + i + "'>Edit</a></p>");

                    }
                }

            %>

This is my code for my for where I enter the new information edit.jsp

  <form action="WordOperations?" method="POST">
        New Link:<input type="text" name="userChange">
        <input type="submit" name="userChangePassword" value="Change link">
        </form>

And this is the code for validation of my forms WordOperations.java

 HttpSession session = request.getSession();

        String newWord = request.getParameter("newWord");
        String delete = request.getParameter("delete");
        String edit = request.getParameter("edit");
        String newLink = request.getParameter("userChange");

        if (delete!=null) //delete a word
        {
            int delIndex = Integer.parseInt(delete);

            ArrayList<String> words = (ArrayList<String>) session.getAttribute("words");
            words.remove(delIndex);

            RequestDispatcher rd = getServletContext().getRequestDispatcher("/main.jsp");
            request.setAttribute("message", "Word deleted");
            rd.forward(request, response);  
        }
        else if(edit !=null)
        {   


            int edIndex = Integer.parseInt(edit);
            String link = newLink;
            ArrayList<String> words = (ArrayList<String>) session.getAttribute("words");
            words.set(edIndex,"google");
            words.add(newLink);
                         RequestDispatcher rd = getServletContext().getRequestDispatcher("/edit.jsp");


            request.setAttribute("message", "Word edited");
            rd.forward(request, response);

        }
        else //logging in
        {
            if (newWord!=null && !newWord.equals(""))
            {
                ArrayList<String> words = (ArrayList<String>) session.getAttribute("words");

                words.add(newWord);              

                RequestDispatcher rd = getServletContext().getRequestDispatcher("/main.jsp");
                request.setAttribute("message", "Word added");
                rd.forward(request, response);

            }
            else
            {
                RequestDispatcher rd = getServletContext().getRequestDispatcher("/main.jsp");
                request.setAttribute("message", "Word is required");
                rd.forward(request, response);
            }
        }

Answer 1

It's a logical mistake ..

You are running servlet WordOperations.java twice in editing the word and passing edit parameter once and passing userChange parameter in another and NOT together which would solve the problem.

Here is the mistake.

But first set the Arraylist words in session on adding of first word. You can do like this:

else //logging in
            {
                if (newWord != null && !newWord.equals("")) {
                    ArrayList<String> words = (ArrayList<String>) s.getAttribute("words");
                    if (words == null) {
                        words = new ArrayList<>();
                        words.add(newWord);
                        s.setAttribute("words", words);
                    } else {
                        words.add(newWord);
                    }

                    RequestDispatcher rd = getServletContext().getRequestDispatcher("/main.jsp");
                    request.setAttribute("message", "Word added");
                    rd.forward(request, response);

                } 

Now Here is the mistake :

When the Arraylist words is listed down in main.jsp . It contains

word Delete_link Edit_link

Now Delete_link > <a href='WordOperations?delete=" + i + "'>Delete</a> is working fine because you are just redirecting it to WordsOperations.java with parameter delete = i and deleting the word from ArrayList.

WordsOperations.java

String delete = request.getParameter("delete");

 if (delete!=null) //delete a word
        {
            int delIndex = Integer.parseInt(delete);

            ArrayList<String> words = (ArrayList<String>) session.getAttribute("words");
            words.remove(delIndex);

            RequestDispatcher rd = getServletContext().getRequestDispatcher("/main.jsp");
            request.setAttribute("message", "Word deleted");
            rd.forward(request, response);  
        }

BUT

In Edit_link > <a href='WordOperations?edit=" + i + "'>Edit</a> You are calling servlet WordsOperations.java with just parameter edit = i and NOT passing userChange parameter with it.

Thus newLink is NULL on

String newLink = request.getParameter("userChange");

SO the code in WordsOperation.java ie

else if(edit !=null)
        {   


            int edIndex = Integer.parseInt(edit);
            String link = newLink;
            ArrayList<String> words = (ArrayList<String>) session.getAttribute("words");
            words.set(edIndex,"google");
            words.add(newLink);
                         RequestDispatcher rd = getServletContext().getRequestDispatcher("/edit.jsp");


            request.setAttribute("message", "Word edited");
            rd.forward(request, response);

        }

Will get executed as edit!=null is true . But it will add newLink in Arraylist words which happens to be NULL. Hence null is getting added to your list.

There is no problem with Arraylist.set() method.

Now in the above code you are calling edit.jsp after adding newLink . But when WordsOperation.java gets called from edit.jsp form, it is just passing userChange parameter and NOT edit parameter which is needed to be there to execute the else if(edit!=null) block of servlet WordsOperations.java .

<form action="WordOperations?" method="POST">
            New Link:<input type="text" name="userChange">
            <input type="submit" name="userChangePassword" value="Change link">
        </form>

Rest is on you. Just take care you pass edit and UserChange parameter both to Servlet WordsOperations.java to get your work done.

Suggestion: Pass edit parameter to edit.jsp and then pass edit and UserChange parameter to WordsOperation.java .

Hope it helps.