There's nothing special about a Runnable. It's doesn't have anything to do with threads. All it is is an interface, that's it:

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

The interface takes no parameters and returns no parameters.

You can create a runnable so you have something to send on a separate thread. You don't need a runnable to do it, it's just standard convention everybody seems to have agreed upon. Here's a nice friendly runnable to play with:

  public class MyRunnable implements Runnable {
 
    public void run(){
       Log.v("test","MyRunnable running");
    }
  }

3 ways to create an instance of this runnable:

• Define the class as above and create an instance: Runnable myRunnable = new MyRunnable
• Skip naming the class but still define it:

Runnable myRunnable =
    new Runnable(){
        public void run(){
            Log.v("Runnable running");
        }
    }

• do “the lambda”: Runnable myRunnable = () -> { Log.v("Lambda Runnable running"); };

Once you have a runnable or really any function wrapped up all pretty like this you can create a new thread to run it:

Thread thread = new Thread(myRunnable);
thread.start();

Borrowed and reworded from here.