Get the contents of the last line of a Java file

Question

I need to recover a file always the last line written. I know one way to do this would be:

import java.io.FileInputStream;
import java.io.InputStreamReader;
import java.io.LineNumberReader;
public class LineReader {
    public static void main(String[] args) throws Exception {
        LineNumberReader lineCounter = new LineNumberReader(new InputStreamReader(new FileInputStream("C:\\MyFile.txt")));
        String nextLine = null;
        try {
            while ((nextLine = lineCounter.readLine()) != null) {
                if (nextLine == null)
                    break;
                System.out.println(nextLine);
            }
            System.out.println("Total number of line in this file " + lineCounter.getLineNumber());
        } catch (Exception done) {
            done.printStackTrace();
        }
    }
}

But is there any method ready in java to get this line without having to go through all the lines of the file? Even more than I’ll ever know how many lines he already has.

Answer 1

Try to use the method Files.readAllLines(Path):

import java.io.File;
import java.io.IOException;
import java.nio.file.Files;
import java.util.List;

/**
 * @author Victor Stafusa
 */
public class LerArquivo {

    public static void main(String[] args) throws IOException {
        List<String> linhas = Files.readAllLines(new File("C:\\MyFile.txt").toPath());
        System.out.println("Número de linhas: " + linhas.size());
        System.out.println("Última linha: " + linhas.get(linhas.size() - 1));
    }
}

Answer 2

Since you don’t know the number of lines or the size of the file: Randomaccessfile.

You can make your own implemetation or use the Apache Commons IO implementation: https://commons.apache.org/proper/commons-io/javadocs/api-release/org/apache/commons/io/input/ReversedLinesFileReader.html

ReversedLinesFileReader reader = new ReversedLinesFileReader(new File("seu-arquivo.txt"), StandardCharsets.UTF_8);
System.out.println(reader.readLine());
reader.close();

Answer 3

In Java version 7 the Files.readAllLines(Path p, Charset Cs) method requires the final attribute to specify the character that will serve as a reference for the line break. I suggest you add:

    List<String> lines = Files.readAllLines(
          new File("C:\\MyFile.txt").toPath(),     
          Charset.defaultCharset()
    );

Answer 4

For files that are very large (a few gigabytes), most approaches would be to read it sequentially, which would be too slow and could give a OutOfMemoryError if the program stored all this in memory.

To resolve this issue, follow an approach that reads the file backwards:

import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;

/**
 * @author Victor Stafusa
 */
public class LerArquivoDeTrasParaFrente {
    public static void main(String[] args) throws IOException {
        File f = new File("C:\\MyFile.txt");
        try (RandomAccessFile raf = new RandomAccessFile(f, "r")) {
            byte b = 0;
            long t = raf.length();
            for (long n = 0; t - n >= 0 && b != '\r' && b != '\n'; n++) {
                raf.seek(t - n);
                b = (byte) raf.read();
            }
            System.out.println("A última linha é: " + raf.readLine());
        }
    }
}