Assignment operator explanation in Java

Question

public class Conversions {

        public static void main(String[] args) {

                    int index = 3;
                    int[] arr = new int[] { 10, 20, 30, 40};

                    arr[index] = index = 2; //(1)
                    System.out.println("" + arr[3] + " " + arr[2]);

            }
    }

I have this and it gives:

2 30 

I was hoping it will give

40 2

At (1) Why was the value of the index in assignment not changed to 2 ( and kept as 3). ?

Answer 1

The right-associativity of = implied by section 15.26 of the Java Language Specification (JLS) means that your expression can be represented as a tree, thus:

           =
    +------+-------+
    |              |
arr[index]         =
              +----+----+
              |         |
            index       2

But then, section 15.7 states:

The Java programming language guarantees that the operands of operators appear to be evaluated in a specific evaluation order, namely, from left to right.

Therefore, arr[index] is evaluated before index = 2 is, i.e. before the value of index is updated.

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Obviously, you should never write code that relies on this fact, as it relies on rules that almost no reader understands.