Reading a specific line from a text file in Java

Question

Is there any method to read a specific line from a text file ? In the API or Apache Commons. Something like :

String readLine(File file, int lineNumber)


        

Answer 1

Because files are byte and not line orientated - any general solutions complexity will be O(n) at best with n being the files size in bytes. You have to scan the whole file and count the line delimiters until you know which part of the file you want to read.

Answer 2

According to this answer, Java 8 enables us to extract specific lines from a file. Examples are provided in that answer.

Answer 3

String line = FileUtils.readLines(file).get(lineNumber);

would do, but it still has the efficiency problem.

Alternatively, you can use:

 LineIterator it = IOUtils.lineIterator(
       new BufferedReader(new FileReader("file.txt")));
 for (int lineNumber = 0; it.hasNext(); lineNumber++) {
    String line = (String) it.next();
    if (lineNumber == expectedLineNumber) {
        return line;
    }
 }

This will be slightly more efficient due to the buffer.

Take a look at Scanner.skip(..) and attempt skipping whole lines (with regex). I can't tell if it will be more efficient - benchmark it.

P.S. with efficiency I mean memory efficiency