Using assignment operator inside of expression

Question

Running this code will return 11 while I was expecting 20. Why is that so?

int x = 1;
int y = x + (x = 10);
System.out.println(y);

@NicholasK since i dont assignment (x = 10) and i thought assignment will be evaluated first and then x value will be 10 — bhusak
– bhusak, Commented Dec 28, 2018 at 16:44
@TomHawtin-tackline got it. so (x = 10) parenthesis doesn't count? — bhusak
– bhusak, Commented Dec 28, 2018 at 16:45
@bhusak Parenthesis just affect parsing, so 'x = 10 + x` is different from (x = 10) + x. — Tom Hawtin - tackline
– Tom Hawtin - tackline, Commented Dec 28, 2018 at 16:48

2 revs · Accepted Answer · 2018-12-28 16:51:23Z

3

Evaluation is from left to right. So

int y = x + (x = 10);

is (with x initially 1):

int y = 1 + 10;

Putting the assignment in () doesn't make it come first. It just ensures that it's a valid expression, since y = x + x = 10 would be y = (x + x) = 10 which would require assigning to something (x + x) that wasn't a variable.

If you want 20, put the assignment first:

int y = (x = 10) + x;

Of course, the vast majority of the time, it's best to avoid these kinds of side-effects and assign x a value outside the expression, breaking the expression up if necessary. Assignment-within-expression can be useful sometimes (particularly while ((blah = getNextBlah()) != null) sort of things), but only in limited situations.

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