import java.io.*;

public class WriteAtSpecificLine {

        public static void main(String[] args) {
                String line = "";
                int lineNo;
                try {
                    File f=new File("c:/file.txt");
                        FileWriter fw = new FileWriter(f,true);
                        BufferedWriter bw = new BufferedWriter(fw);
                        LineNumberReader  lnr = new LineNumberReader(new FileReader(f));
                        lnr.setLineNumber(4);

                        for(int i=1;i<=lnr.getLineNumber();i++){
                        bw.newLine();
                        }
                        bw.write("Hello World");
                        bw.close();
                        lnr.close();
                        } catch (IOException e) {
                        e.printStackTrace();
                }
           }
}