Java Multithreading - One Thread to Take Input, Another to Print on Console

Last Updated : 28 Apr, 2025

Multithreading is a concept in which our program can do two or more tasks at the same time. Thread is the execution unit of any process. Every process has at least one thread which is called main thread. In this article, we will create a Java program that will do printing on the console until the user gives input. Here, we need two threads. One thread do printing and another waits for user input.

Java Program and Explanation

Here, one thread is printing on the console and while the other thread will wait for user input. Main class is executed on the parent thread and also it creates a child thread that does printing on the console.

Tasks of Parent thread:

Create a new thread. (child thread)
Waits for user input.

Tasks of child thread:

Do printing.

Java

// fileName - GeeksForGeeks.java

import java.lang.*;

// needs to be inherited from
// Thread class for multithreading
public class GeeksForGeeks extends Thread {
    private String mssg;

    // constructor
    GeeksForGeeks(String mssg)
    {
        this.mssg = mssg;
    }

    // invoked when we
    // call start() method
    public void run()
    {
        // prints only when start variable
        // is true (variable present in Main class)
        while (Main.start == true) {
            System.out.println(this.mssg);
            try {
                // put thread on waiting
                // state for 1500ms
                Thread.sleep(1500);
            }
            catch (Exception err) {
            }
        }
    }
}

Explanation of the above code:

We extend our class to Thread class to make our program multithreaded.
its run() method is invoked internally whenever we invoke the start() method.
run() methods do printing until the start variable value is true.

Java

// fileName - Main.java

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

public class Main {
    // public static variable and it's
    // value will be false when user enter input
    public static boolean start = true;
    public static void main(String args[])
    {
        GeeksForGeeks newThread = new GeeksForGeeks("I Love GFG!");

        // child thread execution
        // starts here
        newThread.start();

        Scanner sc = new Scanner(System.in);

        // parent thread waiting for user input here
        // parent thread is at waiting state at this line
        sc.nextLine();

        // when user gave any input then the value
        // of this variable changed to be false
        // it will tells the child thread that
        // user gave input and now stop printing
        Main.start = false;
    }
}

Explanation of the above code:

It creates an object of GeeksForGeeks Class which extends the Thread class.
After this, it invokes the start() method which executes all the code of the GeeksForGeeks run() method on a different thread.
After that, It waits for user input.

Output: printing and waiting for user input at the same time

Both classes in the same Java file:

Java

/*package whatever // do not write package name here */

// This program may not work on at gfg IDE
// run it on your local system
import java.io.*;
import java.lang.*;
import java.util.Scanner;

// needs to be inherited from Thread
// class for multithreading
class GeeksForGeeks extends Thread {
    private String mssg;

    GeeksForGeeks(String mssg)
    {
        this.mssg = mssg;
    }

    // invoked when we
    // call start() method
    public void run()
    {
        // prints only when start variable
        // is true (variable present in Main class)
        while (Main.start == true) {
            System.out.println(this.mssg);
            try {
                // put thread on waiting
                // state for 1000ms
                Thread.sleep(1000);
            }
            catch (Exception err) {
            }
        }
    }
}

public class Main {
    // public static variable and it's value
    // will be false when user enter input
    public static boolean start = true;
    public static void main(String[] args)
    {
        GeeksForGeeks newThread = new GeeksForGeeks("Manish Loves GFG!");

        // child thread execution
        // starts here
        newThread.start();

        Scanner sc = new Scanner(System.in);

        // parent thread waiting for user input here
        // parent thread is at waiting state at this line
        sc.nextLine();

        // when user gave any input then the value
        // of this variable changed to be false
        // it will tells the child thread that
        // user gave input and now stop printing
        Main.start = false;
    }
}