Java lambda - for loop counter is not effectively final

Question

Given this situation where a lambda is inside a for loop I would expect the counter i to be effectively final.

The compiler complains that i is not effectively final so I had to use i2.

for (int i = 0; i < x.getBooks().size(); i++){
   //The compiler needs i to be effectively final.
   int i2 = i;
   List<Book> books = bookstore.stream()
                    .filter(c -> c.getAuthors().get(i2).equals("xxx"))
                    .collect(Collectors.toList());
}

So the question is why i is not effectively final inside the scope of the for loop and is this the simplest workaround.

effectively final? I'm not sure what that means. Your code doesn't even use `i` in the lambda — Ruan Mendes, Jul 22 '14 at 17:37
This is what effectively final means http://stackoverflow.com/questions/20938095/difference-between-final-and-effectively-final — Dan, Jul 22 '14 at 18:01
IMO they should allow `final int i` which would then prevent modification of `i` outside the for construct. — rghome, Aug 09 '21 at 16:45

score 6 · Answer 1 · edited May 23 '17 at 12:31

TLDR: i is not final because it is modified (i++) at each iteration of the for loop.

why i is not effectively final inside the scope of the for loop ?

The for loop syntax is

for (initialization; termination; increment) {
    statement(s)
}

The increment expression is invoked after each iteration through the loop. In your case, increment is i++ so i is modified after each iteration.

You could confirm this by declaring i final:

for (final int i = 0; i < x.getBooks().size(); i++) {
}

you will get this compilation error:

The final local variable i cannot be assigned.
It must be blank and not using a compound  assignment

is this the simplest workaround ?

In the case of a for loop: yes.

But you could use a while loop as shown by @dkatzel or a foreach:

int i = 0;
for (Book book: x.getBooks()) {
    int i2 = i;
    ...  
    i++;
}

In your `foreach` example the semantics of `i` have been changed. It was 0 the first time around the loop. It will now be 1 the first time around. Also, the same problem with it not being effectively final exists, so `i2` would still be needed. — David Conrad, Jul 24 '14 at 17:48

score 6 · Answer 2 · answered Jul 23 '14 at 13:18

As the other answers mention, effectively final means, that a variable only can be assigned once and will not be reassigned. That's not the case in a for-loop. effectively final exists, because otherwise developers have to explicitly mark a variable as final to use it in a lambda.

However, the reason i'm answering is a solution, to write your code without duplicating i:

IntStream.range (0, x.getBooks().size()).forEach (i -> {
    List<Book> books = bookstore.stream()
                                .filter(c -> c.getAuthors().get(i).equals("xxx"))
                                .collect(Collectors.toList());
});

Sotirios Delimanolis · Answer 3 · 2014-07-25T03:55:54.150

From the Java Language Specification

The scope of a local variable declared in the ForInit part of a basic for statement (§14.14.1) includes all of the following:

Its own initializer

Any further declarators to the right in the ForInit part of the for statement

The Expression and ForUpdate parts of the for statement

The contained Statement

And about effectively final

A local variable or a method, constructor, lambda, or exception parameter is effectively final if it is not declared final but it never occurs as the left hand operand of an assignment operator (§15.26) or as the operand of a prefix or postfix increment or decrement operator

Your i variable occurs as the operand of postfix increment operator

i++

It is therefore not effectively final and can not be captured by the lambda.

score 0 · Answer 4 · answered Jul 22 '14 at 18:11

0

You have to remember that a for loop actually equivalent to :

int i=0;
while(i < x.getBooks().size()){
  //execute your block
  i++;
}

So you see that i is declared only once and updated therefore not effectively final.

It's a little unclear the relationship between x.getBooks() and Book.getAuthors() but apparently they have the same size? Without a better understanding, I don't think I can show you a better way to do it. But this might also show you that your design is poor.

answered Jul 22 '14 at 18:11

dkatzel

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The actual example is somewhat manufactured so don't worry about that too much. The main question pertains to the scope of the counter in the for loop. – Dan Jul 22 '14 at 18:28
@Dan OK, then maybe the first part of my answer is sufficient? – dkatzel Jul 22 '14 at 18:40

Java lambda - for loop counter is not effectively final

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