如何在 Java 中返回随机访问文件中一行的位置？

Question

I create a random access file and I write some info in it. I would like to store in an array the position of each line in the random access file. I'm going to have an index which points to every line of the random access file. So I need to store in my index the positions of the lines the random access file.

My program is as follows

package randomaccessfile;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.RandomAccessFile;

public class raf {

   public static void main(String[] args) throws FileNotFoundException, IOException {
      File file = new File("DocumentsFile.txt");
      RandomAccessFile raf = new RandomAccessFile(file, "rw");
      for (int i = 0; i <= 10 ; i++) {
         String sentence = "1 Info Name Surname"; //i create sentences in the random access file
         raf.seek(file.length());
         raf.writeBytes(sentence);
         raf.writeBytes("\r\n");
      }
      raf.close();
   }
}

For each of the lines that are created in the Random access file, I would like to store in an array their positions.

The positions then are going to be stored in the index.

Is there any method that I can use in order to return the position of a line in the random access file?

Answer 1

RandomAccessFile already provides everything you want.

For one, you don't need seek() here: the file pointer advances past the bytes written into the file on each write operation.

This means that, after you have written line n , for whatever value of n , grabbing the result of .getFilePointer() from the RandomAccessFile object will give you the starting offset of line n+1 . Line 1 starts at offset 0 and that's pretty much the only thing you have to really account for here.

Answer 2

You can use a temporary variable to store the last FilePointer. If the readline must be reversed, you can set the reader pointer using this variable.

RandomAccessFile myReader = new RandomAccessFile(fileObject, "r");    
long current_postion = myReader.getFilePointer();
myReader.readLine();
if (line should be reversed){
   myReader.seek(current_postion); //Point to the previous line
}

Answer 3

On the other hand, if what you really care about are line numbers, you could use a LineNumberReader to map every line in the written file to a given line number.

File file = new File("jedi.txt");

try(LineNumberReader lnr = new LineNumberReader(new FileReader(file))){
    String line = null;

    while ( (line = lnr.readLine()) != null){
        System.out.println(lnr.getLineNumber() + " " + line );
    }
}catch(IOException e){
    e.printStackTrace();
}

Of course this has nothing to do with RandomAccessFiles, but it could be an alternative depending on what you are doing.