What happens if we start a thread twice in java multithreading?

What happens if we start a thread twice is a basic multithreading concepts in java. In fact, we cannot start a thread twice on the same instance of a thread. It is illegal to call start() method of a thread class twice on same instance of a thread. On first call of start() method thread will start working properly, but, on the second call of start() method IllegalThreadStateException will be thrown.

Here is the java code example and output:

public class Sample extends Thread  {

	public static void main(String[] args) {	
		
		Thread t = new Thread( new Sample());
		t.start(); // will create a new thread and call run() method internally.
		t.start();

	}
	
	@Override
	public void run() {
		
		System.out.println("This method is executed by : " + Thread.currentThread().getName()+"\n");
		
	}
}

Output:

Exception in thread “main” This method is executed by : Thread-1

java.lang.IllegalThreadStateException

    at java.lang.Thread.start(Unknown Source)

     at Sample.main(Sample.java:8)



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