Java HashTable null pointer exception

Question

i am building a server to sends questions to clients and then receive answer from them, but when i send a question to a client i want to make it as unaviable for sending again for one hour, i mean after i send a question to client , i want for one hour that question will never sends to any other clients,

static Hashtable<Integer, List<Integer>> unavialbeQuestions =
new Hashtable<Integer, List<Integer>>();

the key is the clientID, the value is list of the IDs of questions that will never send to that client in one hour, this function i used to make question as unaivalble

public void setUnavialbeQuestion(final String cellName, final int questionID) {
        List<Integer> cells = getCellsIntersectedWithCell(cellName);
        int cellID = getCellID(cellName);
        cells.add(cellID);
        for (int i = 0; i < cells.size(); i++) {
            if (unavialbeQuestions.containsKey(cellID)) {
                unavialbeQuestions.get(cells.get(i)).add(questionID);
            } else {
                List<Integer> lll = new LinkedList<Integer>();
                lll.add(questionID);
                unavialbeQuestions.put(cellID, lll);
            }
            Timer timer = new Timer();
            timer.schedule(new TimerTask() {
                @Override
                public void run() {
                    // TODO Auto-generated method stub
                    f(cellName, questionID);
                }
            }, 1000 * 120);
        }
    }

see the function f() that will fired after one hour (in my demo just 2mins), here is the problem, i got null pointer exception when executing the f() function:

protected void f(String cellName, int questionDI) {
        // TODO Auto-generated method stub
        List<Integer> cells = getCellsIntersectedWithCell(cellName);
        int cellID = getCellID(cellName);
        cells.add(cellID);

        for(int i=0;i<cells.size();i++){
            int index = unavialbeQuestions.get(cells.get(i)).indexOf(questionDI);
            unavialbeQuestions.get(cells.get(i)).remove(index);
        }
    }

the exception on the index parameter, what am i doing wrong?

It would help to know what line the NPE occurred on. The obvious answer is "you're dereferencing something that's null", but without line numbers it's impossible to say whether getCellsIntersectedWithCell() returned null, or unavialbeQuestions.get(), or cells.get(i), or something else... — Andrzej Doyle
– Andrzej Doyle, Commented Jul 6, 2012 at 11:28
When you receive the NPE (NullPointerException), it shows you the line on which it is thrown - that is an important information because you know where the problem is. — assylias
– assylias, Commented Jul 6, 2012 at 11:29

JB Nizet · Accepted Answer · 2012-07-06 11:30:40Z

2

You don't check that the returned list is not null here:

unavialbeQuestions.get(cells.get(i)).indexOf(questionDI);

and here:

unavialbeQuestions.get(cells.get(i)).remove(index);

And you use different keys for your map here:

if (unavialbeQuestions.containsKey(cellID)) {

and here:

unavialbeQuestions.get(cells.get(i)).add(questionID);

(and your code is not thread-safe, also)

answered Jul 6, 2012 at 11:30

JB Nizet

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9 Comments

Totti Over a year ago

unavialbeQuestions.get(cells.get(i)).indexOf(questionDI); for sure it is not null, because i add it in the setUnavialbeQuestion,

Totti Over a year ago

the thread will never fire if it is null, check the loop, the tread is begin working in the loop

JB Nizet Over a year ago

Read my answer: you set cellID in the map, but get cells.get(i).

Totti Over a year ago

@MarkoTopolnik what is the solution please?

Marko Topolnik Over a year ago

To thread-safety? Mark the whole method as synchronized and use a HashMap instead of HashTable.

|

Kalai Selvan Ravi · Accepted Answer · 2012-07-06 11:28:35Z

1

unavialbeQuestions or cells list may be null. If you are trying to access it's method, it will throw NullPointerException.

answered Jul 6, 2012 at 11:28

Kalai Selvan Ravi

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2 Comments

Totti Over a year ago

the exception in the index variable

Kalai Selvan Ravi Over a year ago

@Totti Just print unavialbeQuestions before the statement unavialbeQuestions.get(cells.get(i)).indexOf(questionDI);

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