Can you jump a scanner to a location in file or scan backwards?

Question

I have a very large text file and I need to gather data from somewhere near the end. Maybe Scanner isn't the best way to do this but it would be very wasteful to start at the top and grab 6000 lines before getting to the part of the file I am interested in. Is there a way to either tell Scanner to jump to say 7/8ths down the document or start from the bottom and scan upwards grabbing line by line?

Thanks

Answer 1

Scanner wraps an InputStream, you can use the stream's skip(long) method to skip the lines you don't want and then start scanning.

Read more in the InputStream javadoc

Answer 2

You should probably use RandomAccessFile instead.

Answer 3

The underlying input source for a java.util.Scanner is a java.lang.Readable. Beyond the Scanner(File) constructor, a Scanner neither knows nor cares of the fact that it's scanning a file.

Also, since it's regex based on java.util.regex.*, there's no way it can scan backward.

To accomplish what you want to do, it's best to do it at the input source level, e.g. by using InputStream.skip of the source before passing it to the constructor of Scanner.

---

On Scanner.skip

Scanner itself does have a skip, and a pattern like "(?s).{10}" would skip 10 characters (in (?s) single-line/Pattern.DOTALL mode), but this is perhaps a rather roundabout way of doing it.

Here's an example of using skip to skip a given number of lines.

    String text =
        "Line1 blah blah\n" +
        "Line2 more blah blah\n" +
        "Line3 let's try something new \r\n" +
        "Line4 meh\n" + 
        "Line5 bleh\n" + 
        "Line6 bloop\n";
    Scanner sc = new Scanner(text).skip("(?:.*\\r?\\n|\\r){4}");
    while (sc.hasNextLine()) {
        System.out.println(sc.nextLine());
    }

This prints (as seen on ideone.com):

Line5 bleh
Line6 bloop