Read text file in Java

Question

I have a text file. I would like to retrieve the content from one line to another line. For example, the file may be 200K lines. I want to read the content from line 78 to line 2735. Since the file may be very large, I do not want to read the whole content into the memory.

Solution 1

Here's a start of a possible solution:

public static List<String> linesFromTo(int from, int to, String fileName)
        throws FileNotFoundException, IllegalArgumentException {
    return linesFromTo(from, to, fileName, "UTF-8");
}

public static List<String> linesFromTo(int from, int to, String fileName, String charsetName)
        throws FileNotFoundException, IllegalArgumentException {

    if(from > to) {
        throw new IllegalArgumentException("'from' > 'to'");
    }
    if(from < 1 || to < 1) {
        throw new IllegalArgumentException("'from' or 'to' is negative");
    }

    List<String> lines = new ArrayList<String>();
    Scanner scan = new Scanner(new File(fileName), charsetName);
    int lineNumber = 0;

    while(scan.hasNextLine() && lineNumber < to) {
        lineNumber++;
        String line = scan.nextLine();
        if(lineNumber < from) continue;
        lines.add(line);
    }

    if(lineNumber != to) {
        throw new IllegalArgumentException(fileName+" does not have "+to+" lines");
    }

    return lines;
}

Solution 2

Use BufferedReader.readLine() and count the lines. You'll keep only the buffer size and the current line in memory.

And no, it's not possible to get to line 3412 without reading the whole file up to that point (unless your lines all have a fixed size).