Variable assignment: int x = x = 1

Question

This is allowed by the Java compiler, what is it doing?

int x = x = 1;

I realize that x is assigned to x, but how can it have two =s?

Did you compile it? Than you know the answer to the first question without asking us — moffeltje, Jun 11 '15 at 13:50

score 7 · Accepted Answer · answered Jun 11 '15 at 13:50

7

x = 1 both assigns the value 1 to x and also 'returns' 1, it allows for things like this:

while ((line = reader.readLine()) != null)

answered Jun 11 '15 at 13:50

Karrde

471
2
9

P.S. In java, that sort of thing is considered a code smell in most situations - the only time it's sometimes OK (depends on who you talk to) is similar to the example above – Karrde Jun 11 '15 at 13:58

score 3 · Answer 2 · answered Jun 11 '15 at 14:01

Read assignment statement from right to left:

Acording to Assignment Operators

There are 12 assignment operators; all are syntactically right-associative (they group right-to-left). Thus, a=b=c means a=(b=c), which assigns the value of c to b and then assigns the value of b to a.

So,

int x = x = 1;

is the same as

x = (x = 1);

then

x = 1; x = x;

score 1 · Answer 3 · answered Jun 11 '15 at 13:53

1

int x puts x on the stack.

The right hand part x = 1 assigns 1 to x. But this is an expression with value 1.

Finally this is re-assigned to x.

answered Jun 11 '15 at 13:53

Bathsheba

231,907
34
361
483

Variable assignment: int x = x = 1

3 Answers3