A final counter in a for loop?

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.

Since a loop counter for obvious reasons can never be made final, I think your approach of copying the value into a final variable is the only way to go (but I'm interested in alternatives I may be missing). — ftr, Mar 13 '12 at 11:21

mikera · Accepted Answer · 2012-03-13T13:50:36.677

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 :-)

I think this is the cleanest way so far. – assylias Mar 13 '12 at 11:32 — assylias, Mar 13 '12 at 11:32

score 3 · Answer 2 · answered Mar 13 '12 at 11:21

3

This is what IntelliJ does for you as a fix. The only difference is I would do

ExecutorService es = 
for(int i = 0; i < 10; i++) {
    final int i_final = i;
    es.execute(new Runnable() {

answered Mar 13 '12 at 11:21

Peter Lawrey

525,659
79
751
1,130

score 2 · Answer 3 · answered Mar 13 '12 at 11:21

2

(Less-than optimal) alternative: create a small inner class that implements Runnable:

class Printer implements Runnable {
    private int index;

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

    public void run() {
        System.out.println(index);
    }
}

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

answered Mar 13 '12 at 11:21

Tudor

61,523
12
102
142

1

@assylias: True, it doesn't. Normally, I would also use your initial version, since the code in the run method is so small... – Tudor Mar 13 '12 at 11:25
1

@assylias, actually non-anon classes [but declared with a method body] are significantly better in very important cases: class histograms and stack traces. I try to avoid the anonymous ones if I can help it. – bestsss Aug 07 '12 at 16:14

score 1 · Answer 4 · answered Mar 13 '12 at 11:22

1

I'm afraid there's no other way than copy your counter to a second final variable and use that in your anonymous inner class. This is one of the "deficiencies" of Java around the topic of closures and an advertised advantage of sibling languages like Groovy.

answered Mar 13 '12 at 11:22

vagelis

334
3
9

1

It's not really a deficiency: it stops the user from making certain kinds of errors. If the Java compiler didn't enforce such variables as being final, then a lot of people might assume (mistakenly) that they can update a local variable and that the "closure" would see the result. And you can bet that that would cause a lot of bugs and a lot confused SO questions! – mikera Mar 13 '12 at 11:27

score 1 · Answer 5 · answered Mar 13 '12 at 11:24

1

Looks fine to me. You want to use the value of the loop variable inside your anonymous class and the loop variable obviously can't be final (as its value changes).

Creating a new final local variable is a good solution.

answered Mar 13 '12 at 11:24

Andreas Dolk

113,398
19
180
268

score 1 · Answer 6 · answered Mar 13 '12 at 11:24

Your solution isn't bad. You could do other stuff, like define your own subclass of Runnable and initialize it with i in the constructor or initialization blocks, but in this case I think it would be just adding complexity without a good cause.

BTW: I assume your example is a syntehetic one, in practice creating a new Runnable just to print an integer wouldn't seem a good idea.

A final counter in a for loop?

6 Answers6

Linked

Related