Question

I am writing some Java code to read each line of a data file into an array list, shuffle that list, and then perform some further operations on the shuffled data. The problem is that, after shuffling, I am getting null elements for some of the elements in the array list.

//this file contains the data
String input = ...;
//this file contains the number of rows in the data file
String input2 = ...;
FileInputStream fstream2 = new FileInputStream(input2);
DataInputStream in2 = new DataInputStream(fstream2);
BufferedReader brIndex = new BufferedReader(new InputStreamReader(in2));

for(int i = 1; i <= N; i++) {
        FileInputStream fstream = new FileInputStream(input+(i+1)+".txt");
        DataInputStream in = new DataInputStream(fstream);
        BufferedReader brData = new BufferedReader(new InputStreamReader(in));
        num = Integer.parseInt(brIndex.readLine());

        ArrayList<String> temp = new ArrayList<String>();
        for(int j = 0; j < num; j++) {
            temp.add(brData.readLine());
        }
        brData.close();
        Collections.shuffle(temp);

        //read in shuffled data
        for(int j = 0; j < num; j++) {
            rowData = temp.get(j); ...
}

In executing the code, I get a NullPointerException after this when working with rowData. The original file has 17,169 rows (none of them are null). temp.size() is also 17,169 but when I printed temp out to the console, temp.get(j) was null for some j and not for others.

Can anyone explain to me why this is the case and how to avoid it?

Answer 1

The problem might be there are not enough lines in the file as num.

Try:

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

You could also check for null elements in the list before you actually shuffle the list, as shuffle does not insert any new element to the list.