Assigning an array reference to another array in Java

Question

int a[]={1, 2, 3, 4, 5};
int b[]={4, 3, 2, 1, 0};

a=b;

System.out.println("a[0] = "+a[0]);

This displays a[0] = 4 as obvious because a is assigned a reference to b.

---

If it is modified as follows

int a[]={1, 2, 3, 4, 5};
int b[]={4, 3, 2, 1, 0};        

System.out.println("a[(a=b)[0]] = "+a[(a=b)[0]]);  //<-------

then, it displays a[(a=b)[0]] = 5.

---

Why doesn't this expression - a[(a=b)[0]] yield 4, the 0^th element of b even though it appears to be the same as the previous case?

Answer 1

The second expression features an assignment expression inside an array indexer expression. The expression evaluates as follows:

• The target of the indexer expression is selected. That's the original array a
• The index expression is evaluated by first assigning b to a, and then taking the element of b at index zero
• The index of the outer indexer is evaluated as b[0]
• The index is applied to a, returning 5
• The assignment of b to a takes effect. Subsequent accesses to a[i] will reference b, not the original a.

Essentially, your single-line expression is equivalent to this two-line snippet:

System.out.println("a[(a=b)[0]] = "+a[b[0]]); // Array reference completes first
a=b;                                       // Array assignment is completed last