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how to compare elements in a string array in java?

I am trying to set the words in an array to a string, and then calculate the number of words not including duplicates, but when I try to check for duplicates with an if statement, it says "NullPointerException" meaning there is a null in the array. Why does the array have null values?

Here is the code for setting the string array, the input is "DO UNTO OTHERS AS YOU WOULD HAVE THEM DO UNTO YOU":

   String[] stringArray = new String[wordCount];
   while (!line.equals("DONE"))
   {
       for ( int k = 0 ; k < wordCount ; k++)
       {
           //put tokens into string array
           StringTokenizer tokens = new StringTokenizer(line);
           stringArray[k] = tokens.nextToken();
       }
   }

Here is the code for the comparison and if statement that causes NullPointerException:

   for ( int j = 0 ; j < wordCount ; j++)
   {    
       for (int i = j+1 ; i < wordCount ; i++)
       {       
            if (stringArray[i] == null)
            {
                stringArray[i] = "null";
            } 
            else if (stringArray[i].compareTo(stringArray[j]) == 0 && i!=j) 
            {
                //duplicate
                System.out.print("\n" + stringArray[j]);
                duplicates++;
            }
       }
   }
   wordCount -= duplicates;
   System.out.print("\nNumber of words, not including duplicates: " + wordCount);

I am trying to null check but the result is still way off because it adds more to duplicates because when i change stringArray[i] to "null" it changes stringArray[j] also

please help! i've been trying to solve this for so long

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2  
Posting full stack trace would be helpful. – Rohit Jain Oct 18 '12 at 5:16
1  
On what line does the null pointer Exception occur? – Cory Kendall Oct 18 '12 at 5:18
Did you try to debug it? What did you find out? – w4rumy Oct 18 '12 at 5:19
You're creating the StringTokenizer on every iteration of the loop. – Frank Pavageau Oct 18 '12 at 5:25

marked as duplicate by Sean Owen, skolima, Michal Klouda, lunaryorn, Chathuranga Chandrasekara Oct 18 '12 at 11:55

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3 Answers

up vote 3 down vote

You should use equals() rather than compareTo(). compareTo() raises NullPointerException if passed null.

Java doc of Comparable

Note that null is not an instance of any class, and e.compareTo(null) should throw a NullPointerException even though e.equals(null) returns false.

          if (stringArray[i] == null) {
                continue;
            } else if (stringArray[i].equals(stringArray[j]) && i != j) {
                // duplicate
                duplicates++;
            }

So complete source code should look like. I have not changed any logic inside your for loop.

    String line = "DO UNTO OTHERS AS YOU WOULD HAVE THEM DO UNTO YOU";
    String[] stringArray = line.split("\\s");//Use split StringTokenizer use is discouraged in new code
    int duplicates = 0;
    int wordCount = stringArray.length;
    for (int j = 0; j < stringArray.length; j++) {
        for (int i = j + 1; i < stringArray.length; i++) {
            if (stringArray[i] == null) {
                stringArray[i] = "null";
            } else if (stringArray[i].equals(stringArray[j]) && i != j) {
                // duplicate
                System.out.print("\n" + stringArray[j]);
                duplicates++;
            }
        }
    }
    wordCount -= duplicates;
    System.out.print("\nNumber of words, not including duplicates: "
            + wordCount);
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Yes - but documentation reference of the behavior? – pst Oct 18 '12 at 5:22
I tried equals() and still got NullPointerException – Katherine Oct 18 '12 at 5:26
@Katherine. Check the code snippet. – AmitD Oct 18 '12 at 5:28
up vote 1 down vote

Drop your if (stringArray[i] == null) null check. It's not necessary. Instead, try printing out the array right after you initialize it and see whether wordCount may be too big, or perhaps the tokenizer is not breaking the string where you think it should.

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up vote 0 down vote

Code by @AmitD looks great!! The only issue i guess you having with your code is about wordcount..I think you are not initializing the string array which might give you the exception, thats what i understood..Just initiate the wordcount with some value..Check if wordcount is getting somthing or not..May be that can help...

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