Java 7 for Absolute Beginners

CHAPTER 4 ■ OPERATORS

Continued

Category Operators

logical OR ||

ternary? :

assignment = += -= *= /= %= &= ^= |= <<= >>= >>>=

Operator Precedence

The first thing to know about operators is that they have precedence. I can’t forget Mr. Smith in junior

high algebra class teaching us to memorize Please Excuse My Dear Aunt Sally. That odd phrase is an

effective mnemonic for the order of operations (another name for operator precedence) in algebra. It

shortens to PEMDAS, which gives us Parentheses, Exponent, Multiplication,Division, Addition, and

Subtraction. Thanks to operator precedence (and, in my case, Mr. Smith), when we work with algebra

equations, we know to resolve parentheses before we resolve exponents, exponents before

multiplication, and so on.

The same kind of thing holds true in Java (and many other programming languages). However, as

shown previously, Java has a lot more than six operators. Also, Java has some operators that have the

same level of precedence. In those cases, precedence proceeds from left to right for binary operators

(except assignment operators) and right to left for assignment operators. That's probably as clear as

mud, but we get to some examples shortly that clarify the order of operations and identify some of the

problem spots where people often trip.

The Missing Operator: Parentheses........................................................................................................

Parentheses aren't in the list of Java operators, but they act as an operator with the highest precedence.

Anything in parentheses is resolved first. When a line has several sets of parentheses, they are resolved

from innermost to outermost and from left to right. Let's consider some examples in Listing 4-1.

Listing 4-1. Parentheses as an operator

System.out.println(2 + 4 / 2); // division processed first, so prints 4

System.out.println((2 + 4) / 2); // addition processed first, so prints 3

System.out.println((2 + 4) / 3 * 2); // prints 4, not 1 – see below for why

System.out.println((2 + 4) / (3 * 2)); // prints 1

System.out.println((2 + 4) / (2 * 2)); // prints 1 – note the truncation

System.out.println((2.0 + 4.0) / (2.0 * 2.0)); // prints 1.5

The bolded line prints 4 because the division operator gets processed before the multiplication

operator. That happens because operators of equal precedence get processed from left to right. In

algebra, multiplication comes before division. However, Java is not algebra, and that sometimes trips up

new Java developers who remember their algebra. (I've tripped over that difference at least once.)