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.

if it is possible it would be easier if you write a new class that implements Runnable and to pass to it all the parameters you need, in the constructor maybe. — giocarmine, Dec 17 '12 at 08:21
In IntelliJ it has an auto-correct for this. It assigns variables used in anonymous classes to final variables for you. — Peter Lawrey, Dec 17 '12 at 08:31
I would suggest you to use THRAEDLOCAL , its a better way to keep and pass variables to threads , give it a try , might end up better — Hussain Akhtar Wahid 'Ghouri', Dec 17 '12 at 08:37
Jon's answer works, you would just have to assign the value of i to a final local variable inside the loop. I would second the suggestion to look into using a ExecutorService though. — Gustav Grusell, Dec 17 '12 at 08:45

Jon Skeet · Accepted Answer · 2012-12-17T09:10:33.933

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
         }
    });
}

@TerryLi: See my edit for how you can use my first approach within a loop. Or just use a named class... — Jon Skeet, Dec 17 '12 at 09:10

score 5 · Answer 2 · answered Dec 17 '12 at 08:36

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);
}

Passing parameter to Java Thread

2 Answers2