Java Multithreading Program with Example

import java.lang.*;
import java.util.Scanner;

public class Main { 
    // turned to be true when user entered
    // something on terminal
    public static boolean stop = false;

    // Parent thread
    public static void main(String args[])
    { 
        // object of class which do writing task.
        WriteOnFile newFile = new WriteOnFile(
            "Anjali loves GeeksforGeeks.", "File.txt");
        // execute it on different thread...child
        // thread...it will execute run method of
        // WriteOnFile class
        newFile.start();

        Scanner sc = new Scanner(System.in);

        System.out.print("Enter Something : ");
        // waits for user input
        sc.nextLine();
        // turned on after entering input by user and it
        // tells another thread to stop writing
        Main.stop = true;

        return;
    }
}

import java.io.File;
import java.io.FileWriter;
import java.lang.*;
import java.util.Scanner;

class WriteOnFile extends Thread {
    private String mssg, fileName;

    // read text of file and returns it so that we can
    // append our string to this text
    private String readFile()
    {
        String currText = "";
        try {
            // object of File class
            File myFile = new File(this.fileName);
            Scanner myReader = new Scanner(myFile);
            while (myReader.hasNextLine()) {
                // reads lines of text file and concatenate
                // it to currText
                currText += myReader.nextLine();
                currText += "\n";
            }
        }
        catch (Exception err) {
        }
        return currText;
    }

    // helps to write on file
    private void write()
    { 
        // writes until Main.stop is false. if it's value
        // becomes true then it will stop writing
        while (Main.stop == false) {
            try {
                // gives the current text present in our
                // file. It will help us to append the text
                // in our text file and not overwriting on
                // file
                String currText = readFile();

                // object of FileWriter class which helps to
                // write on file
                FileWriter myWriter
                    = new FileWriter(fileName);

                // writes on given file
                myWriter.write(currText + this.mssg);

                // closing the writing stream
                myWriter.close();

                // puts this thread on sleep state for 1.5
                // seconds...it will gives more good
                // experience of writing
                Thread.sleep(1500);
            }
            catch (Exception err) {
            }
        }
        return;
    }

    // constructor
    public WriteOnFile(String mssg, String fileName)
    {
        this.mssg = mssg;
        this.fileName = fileName;
    }
    // start method calls this method (start calling is at
    // Main.java)
    public void run()
    {
        // run method calls write method which writes on
        // file
        this.write();
    }
}