String returning null value

[impact]

I am new to java, I just dont understand why this code always returns NULL as the answer. What ever I enter the answer is the same.

this code should basically convert "t" in the input string to "u".

public class Tool01Servlet extends HttpServlet {
   
    /** 
    * Processes requests for both HTTP <code>GET</code> and <code>POST</code> methods.
    * @param request servlet request
    * @param response servlet response
    */
    
    private String dnaText; // FORM dnaText
    
    
    /**
     * @param gets input from the FORM
     * Sends the data to tool()
     * Gets data from stringBean class
     * Puts data into HttpSession 
    */
    protected void processRequest(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        
        dnaText = request.getParameter("dnaText");
        tool(dnaText); 
        
        stringBean stringbean = new stringBean();  
               
        HttpSession session = request.getSession();
        session.setAttribute("answer", stringbean);
        
        String link = "answer.jsp";
        response.sendRedirect(link);
        }// End processRequest() 

        
        /**
         * Receives data from processRequest()
         * Sends data to sendToBean()
        */
        public void tool(String dnaText) throws IOException{
           
           try{
               if(Pattern.matches(".*[^atgc]+.*",dnaText)){
                   String mVal = "Input sequence does not match DNA sequence";
                   sendToBean(mVal);
                }else{
                    dnaText = dnaText.toLowerCase();   
                    dnaText = dnaText.replaceAll("t","u");
                    String mVal = dnaText;
                    sendToBean(mVal);
                }
           }catch(Exception e){
               sendToBean("Try catch error");
               e.printStackTrace();
           }
        } // End tool()
        
        
        /**
         * Receives data from tool()
         * sends data to stringBean()
        */
        public void sendToBean(String mVal) throws IOException{
             stringBean stringbean = new stringBean();
             stringbean.stringBean(mVal);
          } //End sendToBean()
        
        
        
    /** 
    * Handles the HTTP <code>GET</code> method.
    * @param request servlet request
    * @param response servlet response
    */
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {
        processRequest(request, response);
    } 

    /** 
    * Handles the HTTP <code>POST</code> method.
    * @param request servlet request
    * @param response servlet response
    */
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {
        processRequest(request, response);
    }

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */

package dna.tool1;

/**
 *
 * @author Owner
 */
public class stringBean {
    public String message;
    
    public void stringBean(String mVal) {
    setMessage(mVal);
    }

    public void setMessage(String mVal){
        message = mVal;
    }    
    
    public String returnMessage(){
        return message;
    }
}

Thank you,

[Eranga]

Simply debug and see what happened there in your code. You can easily find what happened there. Seems to me you are working on NetBeans. So add a break point at the line you collect the user information. Have a try and see.

[Norm]

If you can't figure out how to use interactive debug in your IDE, add a lot of System.out.println() statements to the code to see what is happening.

[impact]

Thanks for helping me.

[Nicholas Jordan]

change the template to be
if( String != null ) {
else {// error routine }

nulls are common in servlets if you try to do getParamater( String s )

doing a check for null string is better than programming by exceptions.

[Eranga]

doing a check for null string is better than programming by exceptions.

What's the different. I mean, if each exceptions handle in all right places no need to such a null pointer validations all the way. Depend on the implementations in some cases you have to do it, but not much. Isn't it?

[fishtoprecords]

You need to debug your code. In general, I first write code in a simple class that is easy to debug, and once its working, move it into the servlet world.

"NULL" is the standard default output of a string that is null. The runtime returns the four characters N, U, L and L when you have a null string. So clearly, its got a null string. So you are not passing what you think you are.

Use the IDE's debugger, or at least put in some PMD

PMD == Poor Man's Debugger,
i.e.
System.out.println("got something here");

[Eranga]

Yes, use of println() is not a good solution at all, if you don't know how to handle the IDE debugger. May be user can be confused with the println() output.