该表达式如何计算

Question

I have an expression with compound assignment as

x += 2*5

so how it will be evaluated is it

x = (x + 2) * 5

or

x = x + (2 * 5)

and why?

Answer 1

An expression of the form

x += expr;

is equivalent to

x = x + (expr);

So in this case it's

x = x + (2 * 5);

It would be very weird and confusing if the current value of x was used for part of the expression implicitly.

Answer 2

Any expression in the form of var op= expr is evaluated to var = var op expr . This is defined in the java specification 15.26.2 Compound Assignment Operators .

Answer 3

I see the phrase "operator precedence" mentioned, and I believe this is confusing the issue. Consider this:

x *= a + b;

Even though * has a higher precedence than + , this is still evaluated as

x = x * (a + b)

The full explanation is given in JLS 15.26.2 Compound Assignment Operatos :

E1 op= E2 is equivalent to E1 = (T)((E1) op (E2)) , where T is the type of E1 , except that E1 is evaluated only once