What does start() function do in multithreading in Java?

In Java, threads enable concurrent execution of multiple tasks within a program. They help improve performance and responsiveness in applications.

• Threads can be created by extending the Thread class or implementing the Runnable interface, both requiring the run() method.
• Use the start() method to begin execution, as it internally calls run() and creates a new thread.

start() Method

The purpose of start() is to create a separate call stack for the thread. A separate call stack is created by it, and then run() is called by JVM. The main purpose of the start() method is to create a separate call stack for a new thread. When start() is invoked:

• JVM creates a new thread.
• A separate call stack is allocated for that thread.
• The JVM internally calls the run() method.

Because of this, multiple threads can execute simultaneously.

Example: Java code to see that all threads are pushed on same stack if we use run() instead of start().

class ThreadTest extends Thread
{
  public void run()
  {
    try
    {
      // Displaying the thread that is running
      System.out.println ("Thread " +
                Thread.currentThread().getId() +
                " is running");

    }
    catch (Exception e)
    {
      // Throwing an exception
      System.out.println ("Exception is caught");
    }
  }
}

// Main Class
public class Main
{
  public static void main(String[] args)
  {
    int n = 8;
    for (int i=0; i<n; i++)
    {
      ThreadTest object = new ThreadTest();

      // start() is replaced with run() for
      // seeing the purpose of start
      object.run();
    }
  }
}

Output: 
 

Thread 1 is running
Thread 1 is running
Thread 1 is running
Thread 1 is running
Thread 1 is running
Thread 1 is running
Thread 1 is running
Thread 1 is running

Explanation:

• The class ThreadTest extends the Thread and overrides the run() method.
• In the main() method, multiple objects of ThreadTest are created in a loop.
• The program calls run() directly instead of start().
• Because run() is called directly, no new threads are created.
• All executions run in the same main thread, so the same thread ID is printed each time.