Can we call run() method without start() method in thread?

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Can we call run() method without start() method in thread?

Yes, you can call the run() method of a Thread or Runnable object directly, but doing so will not create a new thread of execution. Instead, it will simply execute the run() method in the current thread, just like calling any other method.

Understanding run() vs start()

run() Method

The run() method contains the code that constitutes the new thread's task. When you call the run() method directly, it does not create a new thread; it simply executes the method in the current thread.

public class MyRunnable implements Runnable {
    @Override
    public void run() {
        System.out.println("Running in: " + Thread.currentThread().getName());
    }

public static void main(String[] args) {
MyRunnable myRunnable = new MyRunnable();
myRunnable.run(); // This runs in the main thread
}


}

Output:

Running in: main

start() Method

The start() method creates a new thread and then calls the run() method in that new thread. This is the correct way to start a new thread of execution.

public class MyRunnable implements Runnable {
    @Override
    public void run() {
        System.out.println("Running in: " + Thread.currentThread().getName());
    }

public static void main(String[] args) {
Thread thread = new Thread(new MyRunnable());
thread.start(); // This starts a new thread
}


}

Output:

Running in: Thread-0

Key Differences

  1. Thread Creation:
  2. run(): No new thread is created; the method runs in the current thread.
  3. start(): A new thread is created, and run() is executed in that new thread.
  4. Concurrency:
  5. run(): No concurrent execution.
  6. start(): Allows concurrent execution.

Why It Matters

  • Using run() directly: This is essentially just a normal method call. There is no parallelism; the code in run() executes sequentially within the current thread.
  • Using start(): This leverages the Java concurrency framework to run tasks in parallel, allowing multiple threads to execute simultaneously.

Conclusion

To achieve actual multithreading and run tasks in parallel, you must call start() on a Thread object, which internally calls the run() method in a new thread of execution. Calling run() directly is useful only for testing or when you do not need concurrent execution.