Thread:

A thread is a light-weight process which can run independently. A java program can contain multiple threads. Every  thread is created and controlled by the java.lang.Thread class. We can create threads in two ways:

1. By extending Thread class
2. By implementing Runnable interface.

Creating Thread by extending Thread class:

class ThreadDemo extends Thread{  
public void run(){  
System.out.println("thread is running...");  
}  
public static void main(String args[]){  
ThreadDemo tr=new ThreadDemo();  
tr.start();  
 }  
} 

Here we are extending Thread class to Create a thread.

Creating Thread by implementing Runnable interface:

    class ThreadDemo implements Runnable{  
    public void run(){  
    System.out.println("thread is running...");  
    }  
      
    public static void main(String args[]){  
    ThreadDemo m1=new ThreadDemo();  
    Thread t1 =new Thread(m1);  //passing  the object of ThreadDemo class that implements Runnable
    t1.start();  
     }  
    }  

If we implement runnable interface then we have  to explicitly create Thread class object