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Thread: Map containsKey() giving Null Pointer Exception

  1. #1
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    Default Map containsKey() giving Null Pointer Exception

    Hi everyone I'm having an issue with my code that needs an urgent fix. I'll begin by slapping in my code.

    private void readTweetFile(String filename){
            try{
                Scanner input = new Scanner(new FileInputStream(filename));	
                ArrayList<String> tempList;
                ArrayList<String> tempListTwo;
                while(input.hasNext()){
                    String temp = input.nextLine();
                    Scanner sc = new Scanner(temp);
                    while(sc.hasNext()){
                        String word = sc.next();
                        if(wordFrequency.containsKey(word)){
                            wordFrequency.put(word, wordFrequency.get(word)+1);
                            tempListTwo = wordIndex.get(word);
                            tempListTwo.add(temp);
                            wordIndex.put(word, tempListTwo);
                        }
                        else if(!wordFrequency.containsKey(word)){
                            wordFrequency.put(word, 1);
                            tempList = new ArrayList<String>();
                            tempList.add(temp);
                            wordIndex.put(word, tempList);
                        }
                    }
                }
            }
            catch(IOException e){System.out.println("Fail: " + e);}
        }

    The idea of this code is to read a file of tweets and map how many times each word was said by setting the key to the map the word and the value to that key as the frequency. The second map is a map of strings to arraylists where for every word in the file, it has a list of tweets that the word was in, (Obviously it is an arraylist of strings.)

    The null pointer exception is pointing at these lines...

    if(wordFrequency.containsKey(word)){

    and...

    readTweetFile(File);

    The latter line of code being in a separate method that calls on this method so I believe it is of no consequence to the exception. Sorry if the answer is obvious I'm still a beginner programmer.


  2. #2
    Senior Member PhHein's Avatar
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    Default Re: Map containsKey() giving Null Pointer Exception

    Where and how did you declare wordFrequency?

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    Default Re: Map containsKey() giving Null Pointer Exception

    It is a field in the class, funny you should ask I was just looking at it...

    private Map<String, Integer> wordFrequency;

  4. #4
    Senior Member PhHein's Avatar
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    Default Re: Map containsKey() giving Null Pointer Exception

    There you go. It's null.

  5. The Following User Says Thank You to PhHein For This Useful Post:

    Blinktwink (July 31st, 2013)

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    Default Re: Map containsKey() giving Null Pointer Exception

    Oh wow I just realised that -.- I passed over it before because,

    private Map<String, Integer> wordFrequency = new Map<String, Integer>();

    wasn't working so I removed it to keep the compiler happy. I only just realised I needed to use HashMap.

    Must get some sleep before I embarrass myself on the forums further thanks a lot!

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    Default Re: Map containsKey() giving Null Pointer Exception

    Hello.
    You may be making one more mistake in your code.
    In the beginning when you are using scanner for file, you are checking for next word using scanner.hasNext() but reading the entire line. You may have to use scanner.next(). It will not be a problem is you are having one word in every line.
    Does you code still contain errors?

    Syed.

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