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this is the first time I ask question here. I'm working on an rather small project that implement web chat using jsp and servlet. But I encounter a problem. I set an attribute in servletcontext using this 

if(sc.getAttribute("queue")==null)  //sc is servletcontext
    {  
        sc.setAttribute("queue", new       TreeMap<String,LinkedList<TreeMap<String,String>>>());
    }

then I create a map in the attribute by this

if(sn.getAttribute(sn.getId())==null) //sn is a HttpSession object
    {
        //store nickname
        sn.setAttribute(sn.getId(),request.getParameter("nickname"));

        //store the new entry in Context
        sc.setAttribute("queue",((TreeMap<String,LinkedList<TreeMap<String,String>>>)sc.getAttribute("queue")).put(sn.getId(),new LinkedList<TreeMap<String,String>>()));
    }

I know the code isn't so elegant. It's my first time to really work on a project. The important thing now is that it can work.

All above is in my servlet file. Next I want to call a function which has the format as this “public static TreeMap>> messagePut(ServletContext sc)”

public static TreeMap>> messagePut(ServletContext sc)

Sorry there is some problem in the dispaly of the TreeMap, please see the concrete code above.

The discreet implementation is 

public static TreeMap<String,LinkedList<TreeMap<String,String>>> messagePut(ServletContext sc)
{
    //obtain message queue
            TreeMap<String, LinkedList<TreeMap<String,String>>> temp=(TreeMap<String, LinkedList<TreeMap<String, String>>>)(sc.getAttribute("queue"));

            //iterate
            Set<String> keset=temp.keySet();
            Iterator<String> it=keset.iterator();
               TreeMap<String,LinkedList<TreeMap<String,String>>> mm=((TreeMap<String,LinkedList<TreeMap<String,String>>>)sc.getAttribute("queue"));

            //object store stores a reproduce of the whole message queue
            TreeMap<String,LinkedList<TreeMap<String,String>>> store=(TreeMap<String,LinkedList<TreeMap<String,String>>>)sc.getAttribute("queue");
            while(it.hasNext())
            {

                //put the message into queue
                //temp2 is user nickname
                String temp2=(String)it.next();
                if(temp2.equals(name))
                    continue;
                else
                {
                    TreeMap<String,String> c2=new TreeMap<String,String>();
                    c2.put(name,message);
                    LinkedList<TreeMap<String,String>> c3=store.get(temp2);
                    c3.add(c2);
                    store.put(temp2,c3);
                }

            }

            return store;
}

I got a NullPointer exception in the above, this statement:

Set<String> keset=temp.keySet();

throw a nullpointer exception, I don't know why. But I have some doubt about this statement :

sc.setAttribute("queue",((TreeMap<String,LinkedList<TreeMap<String,String>>>)sc.getAttribute("queue")).put(sn.getId(),new LinkedList<TreeMap<String,String>>()));

TreeMap>> is TreeMap<String,LinkedList<TreeMap<String,String>>>. I don't know whether this statement is correct.

Filburt
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blocks
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  • temp must be null then. Check if it's null prior to that line and debug through it. – ManoDestra Apr 12 '16 at 13:35
  • yes, it's null. But why. I set the attribute "queue" before. Or is there some problem with the parameter ServletContext delivery? thanks a lot. – blocks Apr 12 '16 at 13:52
  • I'd put some output statements in your code where you're setting that queue parameter. Put some output prior to entering your if statement and check its value after setting it. – ManoDestra Apr 12 '16 at 13:55
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    I got it. It was after the code inside the if(sn.getAttribuate(sn.getId)()), the queue attribute become null. I think I need to do some modification. thank you very much. – blocks Apr 12 '16 at 14:36

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