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?

Array index has the highest priority.

PM 77-1
– PM 77-1

2013-10-13 17:24:57 +00:00
Commented Oct 13, 2013 at 17:24 — PM 77-1
– PM 77-1, Commented Oct 13, 2013 at 17:24

Sergey Kalinichenko · Accepted Answer · 2013-10-13 17:23:50Z

7

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

answered Oct 13, 2013 at 17:23

Sergey Kalinichenko

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3 Comments

Amith Over a year ago

so in the end its nothing but a[4]..:)

AlexR Over a year ago

@dasblinkenlight, but why assignment of b to a takes effect only at the end of the process?

Sergey Kalinichenko Over a year ago

@AlexR It does not matter when the assignment actually happens in order to decide what's the outcome of the expression a[something] is going to be. The only thing that matters is what object is picked as the first operand of the [] expression. In this example, the object is the old a, because the assignment is to the right of the operand, so the assignment's effect is not visible at that point of the expression.

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