NullPointerException when storing lines in an arraylist

Question

I'm getting a NullPointerException in java (at this line :FileReader fstream = new FileReader(fileName)) when I try to read a text file line by line and store them in an arraylist with this code :

public ArrayList<String> StoreLineByLine(String fileName) throws IOException {
    String str;

    ArrayList<String> Line = new ArrayList<String>();
    FileReader fstream = new FileReader(fileName);
    BufferedReader myFileReader = new BufferedReader(fstream);

    while ((str = myFileReader.readLine()) != null) {
        Line.add(str);
    }
    myFileReader.close();
    return Line;
}

Could anyone help me understand the problem ? Thanks a lot !

Take a look at [this](http://stackoverflow.com/questions/1217449/how-can-i-debug-this-nullpointer-exception) and [this](http://msmvps.com/blogs/jon_skeet/archive/2010/08/29/writing-the-perfect-question.aspx) — isaias-b, Jun 23 '14 at 14:17
Please add an error log and tell us which line exactly throws the nullPointerException. — rhbvkleef, Jun 23 '14 at 14:17
Try researching like I did, http://stackoverflow.com/questions/218384/what-is-a-null-pointer-exception-and-how-do-i-fix-it — Rick S, Jun 23 '14 at 14:18
Works perfectly for me, tried it on my system, calling `StoreLineByLine(String fileName)` from `main()` . Check your `fileName`. — Mustafa sabir, Jun 23 '14 at 14:25
Tested on OS X. If `fileName` is NULL, it yields a NPE on that line. See my answer. — Alvin Thompson, Jun 23 '14 at 14:57

Alvin Thompson · Accepted Answer · 2014-06-23T15:35:29.387

2

If you're getting a NPE on that line, then fileName must be null.

BTW, if you're using JDK 8 then this may be a better way to load the lines. Replace the contents of your method with this:

return Files.readAllLines(Paths.get(fileName), Charset.defaultCharset());

...and change the method's return type from ArrayList<String> to List<String>.

edited Jun 23 '14 at 15:35

answered Jun 23 '14 at 14:25

Alvin Thompson

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Hello, it's the first time I use this website so I don't know how to log the error. The error is on that line : fstream = new FileReader(fileName); I don't understand how come it is null! – user3767791 Jun 23 '14 at 15:06
Well it still doesn't work :/ – user3767791 Jun 23 '14 at 15:13
whats new issue you are getting? – Jayaram Jun 23 '14 at 15:14
I still get the NPE error on that line – user3767791 Jun 23 '14 at 15:19
@user3767791 never pass fileName as null ... have a null check first in the method – Jayaram Jun 23 '14 at 15:21
@user3767791: That's because the method is being called with a null value for `fileName`. If you're running from an IDE, then likely the command line arguments are not being supplied to your `main` method. – Alvin Thompson Jun 23 '14 at 15:21
Yes I had a problem with the opening, it's all good now ! – user3767791 Jun 23 '14 at 15:35

NullPointerException when storing lines in an arraylist

1 Answers1