ncw - 7 months ago 65
Java Question

# Java precedence for multiple + and - operators

This is more of a theoretical question to understand Java's evaluation of arithmetic operations. Since

`+`
and
`-`
have the same precedence, I don't quite understand how Java evaluates the following expressions (where there are more than one
`+`
and
`-`
operators between the two operands).

``````public static void main(String[] args) {
int a = 1;
int b = 2;
System.out.println(a+-b);    // results in -1
System.out.println(a-+b);    // results in -1
System.out.println(a+-+b);   // results in -1
System.out.println(a-+-b);   // results in  3
System.out.println(a-+-+b);  // results in  3
System.out.println(a+-+-b);  // results in  3
System.out.println(a-+-+-b); // results in -1
System.out.println(a+-+-+b); // results in  3
}
``````

From the Java 8 Language Specification (§15.8.2):

The binary + operator performs addition when applied to two operands of numeric type, producing the sum of the operands.

The binary - operator performs subtraction, producing the difference of two numeric operands.

[...]

Addition is a commutative operation if the operand expressions have no side effects.

Integer addition is associative when the operands are all of the same type.

What I also noticed, is that everytime the #operators is even, the result is the same and the order doesn't matter.
But when the #operators is odd, this doesn't necessarily influence the result. E.g. in the following two expressions there is one more
`-`
than
`+`
, however the result is different.

``````System.out.println(a-+-b);   // results in 3
System.out.println(a-+-+-b); // results in -1
``````

With all that information I still don't see the pattern or the way how this works.

In math, how would you evaluate this?

``````a - + - b
``````

Adding some brackets helps:

``````a - (+ (-b))
``````

We can do this, because this doesn't violate the rules of precedence.

Then we can start reducing: `+ (-b)` is really `-b`, and `a - -b` is really `a + b`, so the result is `1 + 2 = 3`.

Let's see the second one:

``````a - + - + - b
a - (+ (- (+ (-b))))
a - (+ (- (-b)))
a - (+ b)
a - b
1 - 2 = -1
``````

So simple rules of math work naturally.

Source (Stackoverflow)