Question

This is the snippet of Java code.

class Test{  
    public static void main(String[ ] args){
        int[] a = { 1, 2, 3, 4 };       
        int[] b = { 2, 3, 1, 0 };     
        System.out.println( a [ (a = b)[3] ] );  
    }
}

Why does it print 1? This is not a homework! I am trying to understand Java. That is related to OCA Java 7 exam.

Answer 1

At the moment you refer to a[ ... ], a still points to the first array. When the index itself is evaluated, there’s an assignment of b to a. So at that moment, a becomes b, of which the 3rd item is fetched, which is 0.

This 0 is used as an index of the array that was already found before. This is the array that a pointed to, although a itself in the mean time has changed. Therefor it prints the 1, even though you might expect 2.

I think that is what this example is trying to show: The array reference is already evaluated and doesn’t change once you modify the array variable during the evaluation of the index.

But I wouldn’t use this ‘feature’ in production code. Very unclear.