The difference between += and =+

Question

I\'ve misplaced += with =+ one too many times, and I think I keep forgetting because I don\'t know the difference between these two, only that one

Answer 1

Some historical perspective: Java inherited the += and similar operators from C. In very early versions of C (mid 1970s), the compound assignment operators had the "=" on the left, so

x =- 3;

was equivalent to

x = x - 3;

(except that x is only evaluated once).

This caused confusion, because

x=-1;

would decrement x rather than assigning the value -1 to it, so the syntax was changed (avoiding the horror of having to surround operators with blanks: x = -1;).

(I used -= and =- in the examples because early C didn't have the unary + operator.)

Fortunately, Java was invented long after C changed to the current syntax, so it never had this particular problem.

Answer 2

a += b is short-hand for a = a + b (though note that the expression a will only be evaluated once.)

a =+ b is a = (+b), i.e. assigning the unary + of b to a.

Examples:

int a = 15;
int b = -5;

a += b; // a is now 10
a =+ b; // a is now -5

Answer 3

Because =+ is not a Java operator.

Answer 4

a += b equals a = a + b. a =+ b equals a = (+b).

Answer 5

+= → Add the right side to the left

=+ → Don't use this. Set the left to the right side.

Answer 6

x += y 

is the same as

x = x + y

and

x =+ y

is wrong but could be interpreted as

x = 0 + y