Java Enthusiast Java Enthusiast - 2 months ago 11
Java Question

Why Thread.start() is needed?

I was working on Threads when a question struck to my mind..If we can directly call run() method with the object of a class like any ordinary method then why do we need to call Thread.start() to call run() method..I tried both the methods as shown and got same result with both of them

First attempt by calling run() method directly

class Abc extends Thread
{
public void run()
{
for(int i=0;i<5;i++)
{
System.out.println("Abc");
}
try
{
Thread.sleep(100);
}catch(Exception e)
{
System.out.println("Error : "+ e);
}
}
}

class Xyz extends Thread
{
public void run()
{
try
{
for(int i=0;i<5;i++)
{
System.out.println("Xyz");
}
Thread.sleep(100);
}catch(Exception e)
{
System.out.println("Error : "+ e);
}
}
}

public class ThreadDemo
{
public static void main(String[] args)
{
Abc ob=new Abc();
Xyz oc=new Xyz();
ob.run();
oc.run();
}
}


Second attempt by calling Thread.start()

public class ThreadDemo
{
public static void main(String[] args)
{
Abc ob=new Abc();
Xyz oc=new Xyz();
Thread t1,t2;
t1=new Thread(ob);
t2=new Thread(oc);
t1.start();
t2.start();
}
}

Answer

If you call run() directly, the code gets executed in the calling thread. By calling start(), a new Thread is created and executed in parallel.