Cannot write “i+2” as a post-condition in my for-each loop [Java]

Question

Can someone explain to me why i can write "i+2" as a post-condition while iterating through a List<> in a for-each loop, but have to write "i=i+2" while iterating through an Array?

    for(int i = 0; i < numbers.length; i+2)
    {
        numbers[i] = 2;
        System.out.println(numbers[i]);
    }

Answer 1

The ForUpdate has to be a StatementExpressionList , ie a list of StatementExpression s.

i+2 is an expression, but not a statement expression.

Statement expressions can be informally (*) thought of as expressions which might have a side effect, and thus it makes sense for them to stand alone in a statement by themselves. i+2 doesn't have a side effect, so there is no point in evaluating it.

---

(*) Informally, because method1() + method2() can have a side effect, because methodN() can have a side effect; and yet, it is not a statement expression because the "main" expression here - the addition - has no side effect in and of itself.

You could write this as for (;; method1(), method2()) { ... } (with no addition) if this was what you wanted in your ForUpdate .

Answer 2

i+2 it's an expression, you have to assign a result to somewhere, so the correct syntax is:

for(int i = 0; i < numbers.length; i=i+2)
{
    numbers[i] = 2;
    System.out.println(numbers[i]);
}