Scalable Human Blog

Hello, Java enthusiasts! Today, we’re threading our way through a commonly asked question in Java: What is the difference between calling the start() and run() methods of a Thread? Let’s unravel this mystery together.

Understanding Threads in Java

Threads are the smallest unit of a process that can be executed concurrently in Java. When creating a thread, we typically extend the Thread class or implement the Runnable interface, and override the run() method. This method contains the code that will be executed when the thread starts. But how do we start the thread? Here’s where the start() and run() methods come into play.

Start vs. Run Method

Java Thread provides two methods to kick off the execution of thread: start() and run(). However, they behave quite differently:

1. start() Method: When we call the start() method on a thread object, the Java Virtual Machine (JVM) creates a new thread of execution and calls the run() method on this new thread. This results in concurrent execution, meaning the thread’s run() method and the method that called start() can run simultaneously.
2. run() Method: When we call the run() method directly, no new thread is created, and the run() method is executed in the current thread, just like any other method call. This results in sequential execution, meaning the method that called run() must wait for run() to return before it can continue executing.

Example

Here is a simple example that demonstrates the difference:

class MyThread extends Thread {
    @Override
    public void run() {
        System.out.println("Inside thread: " + Thread.currentThread().getId());
    }
}

public static void main(String[] args) {
    MyThread t1 = new MyThread();
    MyThread t2 = new MyThread();

    System.out.println("Calling start() method:");
    t1.start();
    t2.start();

    System.out.println("Calling run() method:");
    t1.run();
    t2.run();
}

In the output, you’ll see different thread IDs for start() calls (indicating different threads), and the same thread ID for run() calls (indicating the main thread).

Final Note

Threads are an integral part of Java, enabling us to write concurrent programs. The choice between the start() and run() methods depends on whether we want a new thread of execution. If we want concurrent execution, we use start(). If sequential execution suits our needs, we call run() directly. Remember, understanding these subtle differences makes us better programmers. Until next time, keep threading and happy coding!

📚 Further Reading & Related Topics

If you’re exploring Java multithreading and thread lifecycle management, these related articles will provide deeper insights:

• Difference Between wait() and notify() in Java – Learn how inter-thread communication works and how it relates to thread execution control.

• Structured Concurrency in Java 21: Simplifying Multithreaded Programming – Discover Java’s modern approach to managing threads, making concurrent programming safer and more predictable.