Creating thread

By extending java.lang.Thread class

Extend the Thread class and override run() method.

public class Main extends Thread {

    @Override
    public void run() {
        // Perform task here
    }
}

To start performing task, create and object of Main and call start() method.

Main main = new Main();
main.start();

By implementing java.lang.Runnable interface

Implement the Runnable interface and override run() method.

public class Main implements Runnable {

    @Override
    public void run() {
      // Perform task here
    }
}

To start performing task, create an object to java.lang.Thread class by passing an object of your thread class which implements Runnable interface and call start() method.

Thread t = new Thread(new Main());
t.start();

Keynotes

Extending Thread class means you will limit your class to thread behavior as you cannot extend any other class (Multiple inheritance).

Extending Thread class means you will inherit all the methods of Thread which you may not need.

Implementing Runnable interface will separate actual task from the runner. Runnable interface represents only the task, and you can pass this task to any type of runner, either a thread or any executors.

Implementing Runnable makes your code loosely coupled. Because it separates the task from the runner. Extending Thread will make your code tightly coupled. Because, single class will act as both task container and runner.