Java BufferedReader.readLine() returning null when reading file

Question

I need some help. This is my function:

public String[] getLines(String filename) {
    String[] returnVal = null;
    int i = 0;

    try {
        BufferedReader br = new BufferedReader(new FileReader(new File(filename)));

        for(String line; (line = br.readLine()) != null; ) {
            // process the line.
            returnVal[i] = line;
            i++;
        }

        br.close();
    }
    // Catches any error conditions
    catch (Exception e)
    {
        debug.error("Unable to read file '"+filename+"'");
        debug.message(e.toString());
    }

    return returnVal;
}

which should return me String[] array with all the lines from a specified file. But I only get exception as return:

java.lang.NullPointerException

and when I try to print the result it is null. Any ideas? Thanks!

returnVal[i] -->returnVal is null. You have to initialize it. — TheLostMind
– TheLostMind, Commented Feb 2, 2015 at 14:06

Maroun · Accepted Answer · 2015-02-02 14:07:09Z

3

You're setting the value to null explicitly:

String[] returnVal = null;

Since you don't know how many elements it'll contain, you should use an ArrayList instead^*:

ArrayList<String> returnVal = new ArrayList<>();

^{* See the API to know what methods you should use to add objects to it}

answered Feb 2, 2015 at 14:07

Maroun

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1 Comment

dodancs Over a year ago

Thanks this helped me! But I wasn't setting the return to null explicitly I just initialized it. But now with the ArrayList it works great :)

khachik · Accepted Answer · 2015-02-02 14:09:28Z

you have returnVal as null, String[] returnVal = null; and trying to write to it. If you know number of lines in advance, initialize it as returnVal = new String [N_LINES];, and change your loop condition to take into account number lines read. Otherwise you can use a list of strings and append to it as you read:

List<String> returnVal = new ArrayList<>();
...
while((line = br.readLine()) != null) {
    returnVal.add(line);
}

Not related to the original question but still: br.close(); should be in finally and if you're using 1.7, you can benefit from try-with-resources:

List<String> returnVal = new ArrayList<>();
try(BufferedReader br = 
    new BufferedReader(new FileReader(new File(filename)))) {
    while((line = br.readLine()) != null) {
        returnVal.add(line);
    }
} catch (Exception e) {
    debug.error("Unable to read file '"+filename+"'");
    debug.message(e.toString());
}

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