Java lambda - for loop counter is not effectively

问题:

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.

回答1:

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++;
}

回答2:

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());
});

回答3:

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.

回答4:

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.