What happens if we call run directly instead start() method?

When you call the run() method directly instead of the start() method in a threading context, the code within run() executes on the current thread rather than creating a new, separate thread. This means that no new thread is initiated, and the tasks run sequentially on the existing thread.

Understanding run() vs. start()

Calling start()

The start() method is responsible for creating a new thread of execution. When you invoke start(), the Java Virtual Machine (JVM) allocates a new thread and then calls the run() method within that new thread. This allows multiple threads to execute concurrently, enabling parallel processing and improved application performance.

Example:

class MyThread extends Thread {
    public void run() {
        System.out.println("Thread is running.");
    }
}

public class Main {
    public static void main(String[] args) {
        MyThread t = new MyThread();
        t.start(); // Creates a new thread and executes run()
    }
}

Output:

Thread is running.

Calling run() Directly

On the other hand, if you call the run() method directly, it does not create a new thread. Instead, the run() method executes in the context of the current thread, behaving like a regular method call. This means that the tasks within run() run sequentially on the existing thread, without any concurrency benefits.

Example:

class MyThread extends Thread {
    public void run() {
        System.out.println("Thread is running.");
    }
}

public class Main {
    public static void main(String[] args) {
        MyThread t = new MyThread();
        t.run(); // Executes run() on the main thread
    }
}

Output:

Thread is running.

Implications of Calling run() Directly

1. No New Thread Creation:

   • The primary difference is that start() creates a new thread, while run() does not. This means that without using start(), your application does not benefit from multithreading.

2. Sequential Execution:

   • Tasks execute sequentially on the main thread, potentially leading to performance bottlenecks, especially in applications that require concurrent processing.

3. Lack of Concurrency:

   • Without initiating a new thread, the ability to perform parallel operations is lost, reducing the efficiency and responsiveness of the application.

Real-Life Example

Imagine you have a kitchen with one chef. If you ask the chef to prepare a dish (calling run()), the chef will do it step by step. However, if you hire an additional chef (calling start()), both chefs can work on different dishes simultaneously, speeding up the overall cooking process.

Conclusion

Calling the run() method directly does not initiate a new thread; instead, it executes the run() method within the current thread, resulting in sequential task execution. To leverage the benefits of multithreading, such as improved performance and concurrency, it's essential to use the start() method, which properly creates and manages new threads.

For a deeper understanding of multithreading and how to implement it effectively, consider enrolling in the Grokking Multithreading and Concurrency for Coding Interviews course by DesignGurus.io. Additionally, the Grokking Advanced Coding Patterns for Interviews can further enhance your ability to manage complex multithreading scenarios effectively.