Difference between start and run method in Thread

Difference between start and run method in Thread

Main difference is that when program calls start() method a new Thread is created and code inside run() method is executed in new Thread while if you call run() method directly no new Thread is created and code inside run() will execute on current Thread. Most of the time calling run() is bug or programming mistake because caller has intention of calling start() to create new thread and this error can be detect by many static code coverage tools like findbugs. If you want to perform time consuming task than always call start() method otherwise your main thread will stuck while performing time consuming task if you call run() method directly. Another difference between start vs run in Java thread is that you can not call start() method twice on thread object. once started, second call of start() will throw IllegalStateException in Java while you can call run() method twice.

public class StartVsRunCall{

    public static void main(String args[]) {
       
        //creating two threads for start and run method call
        Thread startThread = new Thread(new Task("start"));
        Thread runThread = new Thread(new Task("run"));
       
        startThread.start(); //calling start method of Thread - will execute in new Thread
        runThread.run();  //calling run method of Thread - will execute in current Thread

    }

    /*
     * Simple Runnable implementation
     */
    private static class Task implements Runnable{
        private String caller;
       
        public Task(String caller){
            this.caller = caller;
        }
       
        @Override
        public void run() {
            System.out.println("Caller: "+ caller + " and code on this Thread is executed by : " + Thread.currentThread().getName());
           
        }        
    } }

Output:

Caller: start and code on this Thread is executed by : Thread-0
Caller: run and code on this Thread is executed by : main