There are two ways to create threads in Java

1. By implementing Runnable interface
2. By extending the Thread Class

public class ThreadCreationDemo {
    public static void main(String[] args) {
        
        // Create threads by instantiating extended Thread class
        ThreadByThreadClass t1 = new ThreadByThreadClass();
        t1.setName("t1"); // Sets name of thread
        
        ThreadByThreadClass t2 = new ThreadByThreadClass();
        t2.setName("t2"); // Sets name of thread
        
        // Create threads by instantiating Thread class by passing Runnable as argument
        Thread t3 = new Thread(new ThreadByRunnableInterface(),"t3");
        Thread t4 = new Thread(new ThreadByRunnableInterface(),"t4");
        
        t1.start();
        t2.start();
        t3.start();
        t4.start();
    }
}

class ThreadByRunnableInterface implements Runnable {

    @Override
    public void run() {
        for (int i = 0; i < 10; i++) {
            System.out.println(Thread.currentThread().getName() + ":" + i);
        }
    }

}

class ThreadByThreadClass extends Thread {

    @Override
    public void run() {
        for (int i = 0; i < 10; i++) {
            System.out.println(Thread.currentThread().getName() + ":" + i);
        }
    }
}

Difference between Thread and Runnable

• Preferred way of creating threads is by implementing Runnable interface. Through this we are not really modifying the behavior of thread. We are just submitting a task to the thread to execute.

Favor Composition over Inheritance

• Java only supports single inheritance, so we can loose the leverage of extending another class if create thread by extending Thread class.
• Implementing an interface gives a cleaner separation between our code.
• By implementing Runnable interface, we actually create a task which can be passed to Executor service to execute but created threads by extending Thread class cannot be used with Executor service.