Runnable run() vs. Thread start(): What's the Key Difference?

Runnable run() vs. Thread start()

Question:

In concurrent Java programming, the Runnable interface defines a run() method which is the entry point for a task. Similarly, the Thread class has a start() method that initiates a new thread of execution. What's the key difference between these two methods?

Answer:

The fundamental distinction lies in thread creation and execution:

Runnable run(): No Thread Creation

• Calling Runnable.run() directly does not create a new thread.
• Instead, it executes the run() method within the current thread of execution.
• This approach is suitable when the task is lightweight and does not require its own dedicated thread.

Thread start(): Thread Creation

• Calling Thread.start() initiates a new thread of execution.
• Within the new thread, the run() method of the associated Runnable object is executed.
• Using Thread.start() allows for parallelism by dividing the task into separate threads that run concurrently.

Example:

Consider two classes, R1 and R2, implementing the Runnable interface:

class R1 implements Runnable {
    public void run() { ... }
}

class R2 implements Runnable {
    public void run() { ... }
}

In the following code snippet, r1.run() and r2.run() are invoked directly, which means no new threads are created:

public static void main() {
    R1 r1 = new R1();
    R2 r2 = new R2();

    r1.run();
    r2.run();
}

In contrast, the following code creates two new threads, t1 and t2, and starts them:

public static void main() {
    R1 r1 = new R1();
    R2 r2 = new R2();
    Thread t1 = new Thread(r1);
    Thread t2 = new Thread(r2);

    t1.start();
    t2.start();
}

In this code, the tasks associated with r1 and r2 will be executed concurrently within the newly created threads.

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