The Thread class itself implements Runnable with an empty implementation of run() method. For creating a new thread, create an instance of the class that implements Runnable interface and then pass that instance to Thread(Runnable target) constructor.

What interface does the thread class implement?

The Runnable interface marks an object that can be run as a thread. It has only one method, run, that contains the code that's executed in the thread. (The Thread class itself implements Runnable, which is why the Thread class has a run method.)

Can a thread be formed by implementing runnable interface only?

A thread can be formed by implementing Runnable interface only. A thread can be formed by a class that extends Thread class. start() method is used to begin execution of the thread. run() method is used to begin execution of a thread before start() method in special cases.