Passing parameter to Java Thread

java multithreading runnable
Terry Li picture Terry Li · Dec 17, 2012 · Viewed 15.6k times · Source 
Thread t = new Thread(new Runnable() { public void run() {} });

I'd like to create a thread this way. How can I pass parameters to the run method if possible at all?

Edit: To make my problem specific, consider the following code segment:

for (int i=0; i< threads.length; i++) {
    threads[i] = new Thread(new Runnable() {public void run() {//Can I use the value of i in the method?}});
}

Based on Jon's answer it won't work, since i is not declared as final.

Answer

Jon Skeet picture Jon Skeet · Dec 17, 2012

No, the run method never has any parameters. You'll need to put the initial state into the Runnable. If you're using an anonymous inner class, you can do that via a final local variable:

final int foo = 10; // Or whatever

Thread t = new Thread(new Runnable() {
    public void run() {
        System.out.println(foo); // Prints 10
    }
});

If you're writing a named class, add a field to the class and populate it in the constructor.

Alternatively, you may find the classes in java.util.concurrent help you more (ExecutorService etc) - it depends on what you're trying to do.

EDIT: To put the above into your context, you just need a final variable within the loop:

for (int i=0; i< threads.length; i++) {
    final int foo = i;
    threads[i] = new Thread(new Runnable() {
         public void run() {
             // Use foo here
         }
    });
}

Related questions

"implements Runnable" vs "extends Thread" in Java

From what time I've spent with threads in Java, I've found these two ways to write threads: With implements Runnable: public class MyRunnable implements Runnable { public void run() { //Code } } //Started with a "new Thread(new MyRunnable()).start()" call Or, with …

Read More
The difference between the Runnable and Callable interfaces in Java

What is the difference between using the Runnable and Callable interfaces when designing a concurrent thread in Java, why would you choose one over the other?

Read More
What's the difference between Thread start() and Runnable run()

Say we have these two Runnables: class R1 implements Runnable { public void run() { … } … } class R2 implements Runnable { public void run() { … } … } Then what's the difference between this: public static void main() { R1 r1 = new R1(); R2 r2 = new R2(); r1.run(); …

Read More