Java: pre-,postfix operator precedences

Question

I have two similar questions about operator precedences in Java.

First one:

int X = 10;
System.out.println(X++ * ++X * X++); //it pr         


        

Answer 1

The confusion stems from the fact that the operands are evaluated from left to right. This is done first, before any attention is paid to operator precedence/order of operations.

This behavior is specified in JLS 15.7.2. Evaluate Operands before Operation

So X++ * ++X * X++ is first evaluated as 10 * 12 * 12 which yields, as you saw, 1440.

To convince yourself of this, consider the following:

X = 10; System.out.println(X++ * ++X);
X = 10; System.out.println(++X * X++);

If X++ were done first, then ++X second, then multiplication, both should print the same number.

But they do not:

X = 10; System.out.println(X++ * ++X); // 120
X = 10; System.out.println(++X * X++); // 121

So how does this make sense? Well if we realize that operands are evaluated from left to right, then it makes perfect sense.

X = 10; System.out.println(X++ * ++X); // 120 (10 * 12)
X = 10; System.out.println(++X * X++); // 121 (11 * 11)

The first line looks like

X++       * ++X
10 (X=11) * (X=12) 12
10        * 12 = 120

and the second

++X       * X++
(X=11) 11 * 11 (X=12)
11        * 11 = 121

---

So why are prefix and postfix increment/decrement operators in the table?

It is true that increment and decrement must be performed before multiplication. But what that is saying is that:

Y = A * B++

// Should be interpreted as
Y = A * (B++)

// and not
Y = (A * B)++

Just as

Y = A + B * C

// Should be interpreted as
Y = A + (B * C)

// and not
Y = (A + B) * C

It remains that the order of the evaluation of the operands occurs left-to-right.

---

If you're still not conviced:

Consider the following program:

class Test
{
    public static int a(){ System.out.println("a"); return 2; }
    public static int b(){ System.out.println("b"); return 3; }
    public static int c(){ System.out.println("c"); return 4; }

    public static void main(String[] args)
    {
        System.out.println(a() + b() * c());
        // Lets make it even more explicit
        System.out.println(a() + (b() * c()));
    }
}

If the arguments were evaluated at the time they were needed, either b or c would come first, the other next, and lastly a. However, the program outputs:

a
b
c
14
a
b
c
14

Because, regardless of the order that they're needed and used in the equation, they're still evaluated left to right.

Helpful reading:

• What are the rules for evaluation order in Java?
• a += a++ * a++ * a++ in Java. How does it get evaluated?
• Appendix A: Operator Precedence in Java