Is the array index or the assigned value evaluated first?

Question

If I have the following code:

array[index] = someValue;

does the someValue or the index get evaluated first?

Answer 1

The index is evaluated first. See JLS section 15.26.1, in particular:

15.26.1. Simple Assignment Operator =

...

If the left-hand operand is an array access expression (§15.13), possibly enclosed in one or more pairs of parentheses, then:

1. First, the array reference subexpression of the left-hand operand array access expression is evaluated. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason; the index subexpression (of the left-hand operand array access expression) and the right-hand operand are not evaluated and no assignment occurs.

2. Otherwise, the index subexpression of the left-hand operand array access expression is evaluated. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason and the right-hand operand is not evaluated and no assignment occurs.

3. Otherwise, the right-hand operand is evaluated. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason and no assignment occurs.

TL;DR: The order is 1[2]=3

Answer 2

It looks like the index expression is evaluated first.

This test program confirms it:

public class Main {
   public static void main(String[] args) {
      int[] array = new int[10];
      array[getIndex()] = getValue();
   }


   private static int getIndex() {
      System.out.println("getIndex!");
      return 0;
   }

   private static int getValue() {
      System.out.println("getValue!");
      return 1;
   }
}

Output:

getIndex!
getValue!

The value is evaluated even if an ArrayIndexOutOfBoundsException is thrown. Changing getIndex():

private static int getIndex() {
   System.out.println("getIndex!");
   return -1;  // Hey!
}

Output:

getIndex!
getValue!
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: -1
    at Main.main(Main.java:11)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:90)