writing to file gives me NullPointerException

Question

I'm programming a code that writes an array of integers to a file. However, it keeps giving me a NullPointerException. Which I think is in one of two methods that I have, I know it's somewhere in there, but I can't spot it. Here's the code.

public static void WriteToFile (int[] n)throws IOException{
    try{
        BufferedWriter bw=new BufferedWriter(new FileWriter("C:\\sortfile2.txt"));
        for(int i=0;i<=25;i++){
            bw.write(sortArray(n)+" ");
        }
        bw.flush();
        bw.close();
    }
    catch(IOException e){
        System.out.println("Could not write to file");
    }
}

This is the first method that the error could be located in. The second is here:

public static int sortArray(int[] a){
    int sort=0;
    for(int i=0;i<a.length;i++){
        int max=IndexOfMaxInRange(a,i,a.length-1);
        sort=SwapElement(a,max,max++);
        a[i]=sort;
    }
    return sort;
}

Where IndexOfMaxInRange is a method that finds the highest integer of an array, and where SwapElement is a method that implements IndexOfMaxInRange, and swaps the highest number in the array, with the lowest.

Could you share the stack trace, so know line number where you getting null pointer — Panther, Apr 28 '15 at 12:41
The exception trace should give you the exact line in which the error happens, so you don't need to guess. Can you spot that? — Luan Nico, Apr 28 '15 at 12:41
To get a stack trace, you need to replace `System.out.println("Could not write to file");` with `e.printStackTrace();` though. — Siguza, Apr 28 '15 at 12:42
looking at the code, I suppose in sortArray the array is null — Saurabh Jhunjhunwala, Apr 28 '15 at 12:42
try to use try catch while programming,it is eaiser to figure out where the error is.. — Miller, Apr 28 '15 at 12:44
do catch NullPointerException or use generic Exception to get stack trace. currently you are catching IOException — Vigneshwaran, Apr 28 '15 at 12:44
Also, you should either declare your method with `throws IOException`, saying it can throw an exception, or `catch (IOException e) { ... }` and handle the error, but not both. — Luan Nico, Apr 28 '15 at 12:44
you can check its length before processing like, if(n.length > 0) { bw.write(sortArray(n)+" "); } — Vigneshwaran, Apr 28 '15 at 12:48
Can you please add code for SwapElement and IndexOfMaxInRange method? — Mohammad Ashfaq, Apr 28 '15 at 12:48

Miller · Answer 1 · 2015-04-28T13:11:49.843

0

Try this code,and edit according to yours(this works for me)...

 public static void write (String filename, int[]x) throws IOException{
      BufferedWriter outputWriter = null;
      outputWriter = new BufferedWriter(new FileWriter(filename));
      for (int i = 0; i < x.length; i++) {
        // Maybe:
        outputWriter.write(x[i]+"");
        // Or:
        outputWriter.write(Integer.toString(x[i]);
        outputWriter.newLine();
      }
      outputWriter.flush();  
      outputWriter.close();  
    }

edited Apr 28 '15 at 13:11

answered Apr 28 '15 at 12:49

Miller

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Any reason why not simply do `BufferedWriter outputWriter = new BufferedWriter(new FileWriter(filename));`? I didn't follow your `make sure your buffered writer was declared as null`... – Luan Nico Apr 28 '15 at 12:51

score 0 · Answer 2 · answered Apr 28 '15 at 13:16

Your custom sort doesn't work and will be causing the issue. In particular

for(int i=0;i<a.length;i++)

means i becomes the last element of the array

so

    int max=IndexOfMaxInRange(a,i,a.length-1);
    sort=SwapElement(a,max,max++);

has some rather worrying index bounds issues around max, and max++ seems to be pointless.

Try it without the sorting to pinpoint your issue.

writing to file gives me NullPointerException

2 Answers2