Scanner to reset pointer at previous line

Question

My problem could be solved if Scanner class had previous() method on it. I am asking this question to know if there are any methods to achieve this functionality.

Input: a file with contents like

a,1
a,2
a,3
b,1
c,1
c,2
c,3
c,4
d,1
d,2
d,3
e,1
f,1

I need to create a list of all lines that has same alphabet.

try {
            Scanner scanner = new Scanner(new File(fileName));
            List<String> procList = null;
            String line =null;
            while (scanner.hasNextLine()){
                line = scanner.nextLine();
                System.out.println(line);
                String[] sParts = line.split(",");
                procList = new ArrayList<String>();
                procList.add(line);
                boolean isSamealpha = true;
                while(isSamealpha){
                    String s1 = scanner.nextLine();
                    if (s1.contains(sParts[0])){
                        procList.add(s1);
                    }else{
                        isSamealpha = false;
                        System.out.println(procList);
                    }
                }
            }
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }

I get output like

a,1
[a,1, a,2, a,3]
c,1
[c,1, c,2, c,3, c,4]
d,2
[d,2, d,3]
f,1
[f,1]

As you can see it missed list for b and e. If I has scanner.previous() method, I would have put it in else of second while loop. Because there is no previous method, I am stuck.

Please let me know if there are any methods I can use. I can't use FileUtils.readLines() because its a 3GB file and I don't want to use my java memory to store all the file.

Answer 1

Below is some sample code that should help. (I only typed this; I did no checking.)

Scanner scanner = new Scanner(new File(fileName));
List<String> procList = null;
String line = null;
String previousAlpha = null;
while (scanner.hasNextLine()){
    line = scanner.nextLine();

    if (previousAlpha == null) {
        // very first line in the file
        procList = new ArrayList<String>();
        procList.add(line);
        System.out.println(line);
        previousAlpha = line.split(",")[0];
    }
    else if (line.contains(previousAlpha)) {
        // same letter as before
        procList.add(line);
    }
    else {
        // new letter, but not the very first
        // line
        System.out.println(procList);

        procList = new ArrayList<String>();
        procList.add(line);
        System.out.println(line);
        previousAlpha = line.split(",")[0];

    }
}

Answer 2

I would suggest reconsidering your algorithm instead. You are missing tokens because your algorithm involves reading ahead to determine when the sequence has broken, yet you aren't collecting that next line of input into the same structures that you are placing "duplicate" entries.

You can solve this without needing to read backwards. If you know that the input is always sorted, just read line by line and keep a reference to the last line (to compare with the current one).