Passing parameter to Java Thread

Question

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 1

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
         }
    });
}

Answer 2

You may create a custom thread object that accepts your parameter, for example :

public class IndexedThread implements Runnable {
    private final int index;

    public IndexedThread(int index) {
        this.index = index;
    }

    public void run() {
        // ...
    }
}

Which could be used like this :

IndexedThread threads[] = new IndexedThread[N];

for (int i=0; i<threads.length; i++) {
    threads[i] = new IndexedThread(i);
}