Java : Read last n lines of a HUGE file

Question

I want to read the last n lines of a very big file without reading the whole file into any buffer/memory area using Java.

I looked around the JDK APIs and Apache Com

Answer 1

package com.uday;

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

public class TailN {
    public static void main(String[] args) throws Exception {
        long startTime = System.currentTimeMillis();

        TailN tailN = new TailN();
        File file = new File("/Users/udakkuma/Documents/workspace/uday_cancel_feature/TestOOPS/src/file.txt");
        tailN.readFromLast(file);

        System.out.println("Execution Time : " + (System.currentTimeMillis() - startTime));

    }

    public void readFromLast(File file) throws Exception {
        int lines = 3;
        int readLines = 0;
        StringBuilder builder = new StringBuilder();
        try (RandomAccessFile randomAccessFile = new RandomAccessFile(file, "r")) {
            long fileLength = file.length() - 1;
            // Set the pointer at the last of the file
            randomAccessFile.seek(fileLength);

            for (long pointer = fileLength; pointer >= 0; pointer--) {
                randomAccessFile.seek(pointer);
                char c;
                // read from the last, one char at the time
                c = (char) randomAccessFile.read();
                // break when end of the line
                if (c == '\n') {
                    readLines++;
                    if (readLines == lines)
                        break;
                }
                builder.append(c);
                fileLength = fileLength - pointer;
            }
            // Since line is read from the last so it is in reverse order. Use reverse
            // method to make it correct order
            builder.reverse();
            System.out.println(builder.toString());
        }

    }
}