Question

I have this code:

    List<Runnable> r = new ArrayList<>();
    for(int i = 0; i < 10; i++) {
        r.add(new Runnable() {

            @Override
            public void run() {
                System.out.println(i);
            }
        });
    }

It obviously does not compile because i would need to be final to be used in the anonymous class. But I can’t make it final because it is not. What would you do? A solution is to duplicate it but I thought there might be a better way:

    List<Runnable> r = new ArrayList<>();
    for(int i = 0; i < 10; i++) {
        final int i_final = i;
        r.add(new Runnable() {

            @Override
            public void run() {
                System.out.println(i_final);
            }
        });
    }

EDIT just to make it clear, I used a Runnable here for the sake of the example, the question is really about anonymous classes, which could be anything else.

Answer 1

I think your solution is the simplest way.

Another option would be to refactor the creation of the inner class into a factory function that does it for you, then your loop itself could be something clean like:

List<Runnable> r = new ArrayList<>();
for(int i = 0; i < 10; i++) {
    r.add(generateRunnablePrinter(i));
}

And the factory function could just declare a final parameter:

private Runnable generateRunnablePrinter(final int value) {
    return new Runnable() {
       public void run() {
           System.out.println(value);
       }
    };
}

I prefer this refactored approach because it keeps the code cleaner, is relatively self descriptive and also hides away all the inner class plumbing.

Random digression: if you consider anonymous inner classes to be equivalent to closures, then generateRunnablePrinter is effectively a higher order function. Who said you can’t do functional programming in Java 🙂