java中变量的赋值优先级

Question

I have a code snippet example

int i1 = 2;
int i2 = 3;
int i3 = i2 + (i2 = i1); 

Please can anyone explain to me why i3 will be initialized with 5 in this case? I thought that i2 after (i2 = i1) will be 2. And i2 + i2 will be 4

Answer 1

You are making the common error of confusing operator priority with order of evaluation of operands.

Priority affects the structure (ie, meaning) of the expression only. Since you use explicit parentheses, there's no real doubt in i2 + (i2 = i1) -- it can't possibly mean (i2 + i2) = i1 .

HOWEVER, in Java, evaluation of operands is always left-to-right . So `i2 + (i2 = i1)' means:

1. Take value of i2, call it 'tmp1'
2. Take address of i2, call it 'tmp2'
3. Take value of i1
4. Store a copy of that value at address 'tmp2'
5. Add the value in hand to the value of 'tmp1'
6. which is the result of the expression

This sequence is didactic only, I do not mean this is the actual object code produced by the compiler. But it is how to understand what result you'll get.

In practice, though, you don't want to be writing code that's combining assignments to a variable and using the prior value of the variable. If I were doing the code review, I'd advise you to reformulate.