allows to seek to any line in the file

LineReader

import com.google.common.primitives.Longs;
import java.io.BufferedReader;
import java.io.ByteArrayInputStream;
import java.io.File;
import java.io.IOException;
import java.io.InputStreamReader;
import java.nio.ByteBuffer;
import java.nio.channels.SeekableByteChannel;
import java.nio.file.Files;
import java.util.ArrayList;
import java.util.List;

public static class LineReader {
  private final File file;
  private final long offsets[];
  private final static int BUFFER_SIZE = 1024;

  public LineReader(File file) throws IOException {
    this.file = file;
    List<Long> offsetsList = new ArrayList<>();
    try (SeekableByteChannel sbc = Files.newByteChannel(file.toPath())) {
      byte[] buf = new byte[BUFFER_SIZE];
      long offset = 0;
      offsetsList.add(0L);
      while (sbc.position() < sbc.size()) {
        ByteBuffer dst = ByteBuffer.wrap(buf);
        sbc.read(dst);
        InputStreamReader reader = new InputStreamReader(new ByteArrayInputStream(buf));
        int processedOffset = 0;
        while (reader.ready()) {
          Character ch = (char) reader.read();
          int length = ch.toString().getBytes().length;
          offset += length;
          processedOffset += length;
          if (ch == '\n') {
            offsetsList.add(offset);
          }
          if (processedOffset > BUFFER_SIZE - 10) {
            sbc.position(offset);
            break;
          }
        }
      }
    }
    offsets = Longs.toArray(offsetsList);
  }

  public int getLinesCount() {
    return offsets.length;
  }

  public String getLine(int line) throws IOException {
    try (SeekableByteChannel sbc = Files.newByteChannel(file.toPath())) {
      long expectedSize = (line < offsets.length - 1 ? offsets[line + 1] : sbc.size()) - offsets[line];
      byte[] buf = new byte[(int) expectedSize];
      ByteBuffer dst = ByteBuffer.wrap(buf);
      sbc.position(offsets[line]);
      sbc.read(dst);
      BufferedReader br = new BufferedReader(new InputStreamReader(new ByteArrayInputStream(buf)));
      return br.readLine();
    }
  }
}