java.lang.Runnable is an interface that is to be implemented by a class whose instances are intended to be executed by a thread. It contains a single method, run(), which defines the code that constitutes the task.

Key Method in Runnable

run(): This method contains the code that constitutes the task to be executed by the thread.

public interface Runnable {
    void run();
}

Implementing Multithreading with Runnable

Step 1: Create a Runnable implementer and implement the run() method.

Step 2: Instantiate the Thread class and pass the implementer to the Thread, Thread has a constructor which accepts Runnable instances.

Step 3: Invoke start() of Thread instance, start internally calls run() of the implementer. Invoking start() creates a new Thread that executes the code written in run(). Calling run() directly doesn’t create and start a new Thread, it will run in the same thread. To start a new line of execution, call start() on the thread.

class MyRunnable implements Runnable {
    public void run() {
        System.out.println("Thread is running");
    }
}

public class Main {
    public static void main(String[] args) {
        MyRunnable myRunnable = new MyRunnable();
        Thread thread = new Thread(myRunnable);
        thread.start(); // Start the thread
    }
}