Run() vs. start() in Java Threads: What's the Difference in Multithreading Behavior?

Run() vs. start() in Threading: A Comprehensive Explanation

In multithreaded programming, effectively understanding the distinctions between Thread.run() and Runnable.run() is crucial.

Question:

Consider the following code snippets:

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

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

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();
}

Answer:

First Example: No Multithreading

In the first snippet, r1.run() and r2.run() are executed directly without creating new threads. Consequently, both R1 and R2 are executed sequentially within the main thread.

Second Example: Multithreading

In the second snippet, Thread objects (t1 and t2) are created to encapsulate the R1 and R2 instances, respectively. When t1.start() and t2.start() are invoked, new threads are initiated, each executing the run() method of the corresponding Runnable implementation concurrently in parallel with the main thread.

Key Differences:

• Thread Creation: start() creates a new thread, while run() executes the Runnable within the calling thread.
• Execution Context: start() executes the Runnable within a newly created, independent thread, providing true parallel execution. run() executes the Runnable within the invoking thread, limiting concurrency.

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