Ternary Operator

• Core Java: Volume I—Fundamentals

  • Chapter 3.8

Syntax

The ternary operator is another conditional control structure, often used to selectively assign one of two values to a variable at runtime. It functions as a condensed version of an if/else statement, while also returning a value.

The syntax behind the ternary operators is little unfamiliar.

(condition) ? expression1 : expression2;

First, the contion is evaluated. If it is true, then expression1 is returned. Otherwise, expression2 is returned. Typically, the result of expression1 or expression2 is conditionally assigned to a variable.

Usage

If you wanted to print a sentence that states how many apples are left in a supermarket's inventory, a ternary operator might be helpful in getting the verb conjugation and pluralization right. The lazy way would look like this.

int apples = /* read from a database */;

// remaining apples might be 0, 1, or greater
System.out.println("There is (are) " + apples + " apple(s) in stock.");

// prints: There is (are) X apple(s) in stock.

The sentence will be grammatically correct, but not the most elegant. What if you wanted to clean it up?

int apples = /* read from a database */;
String status = (apples == 1)
  ? "is " + apples + " apple"
  : "are " + apples + " apples";

System.out.println("There " + status + " in stock.");

// prints: There is 1 apple in stock.
//         There are 0 apples in stock.
//         There are X apples in stock. (where X is greater than 1)

Now, the ternary operator will assign "is 1 apple" or "are X apples" to the status variable. If the value of apples is 1, then the ternary operator returns "is 1 apple". If the value of apples is 0 or greater than 1, then the ternary operator returns "are X apples", where "X" is the value of apples.

Nesting Ternary Operators

Like anything else, you can nest ternary operators. However, this can become messy and hard to read. For shorter expressions, it's acceptable to put the entire ternary operator (condition, and two expressions) on a single line.

double bonusRate = (experience > 10) ? 0.15 : 0.05;

However, for longer or nested expressions, it's helpful to break this out across several lines. The condition should go on the first line (along with the variable assignment), then each expression should be placed on its own line.

String accessLevel = role.equals("manager")
    ? "elevated"
    : "standard";

// if role is manager, accessLevel = elevated
// if role is not manager, accessLevel = standard

This isn't the longest expression, but it makes the point. Simplify your code for those who may read it. The variable assignment and condition go on the first line, the ? and the first expression go on the second line, and the : and the second expression go on the third line.

You should probably do the same for any nested statements, regardless of length.

double bonusRate = role.equals("manager")
    ? experience > 10 ? 0.20 : 0.10
    : experience > 10 ? 0.10 : 0.05;

// if role is manager and experience > 10, bonusRate = 0.2
// if role is manager and experience <= 10, bonusRate = 0.1
// if role is not manager and experience > 10, bonusRate = 0.1
// if role is not manager and experience <= 10, bonusRate = 0.05

Again, the expressions are rather short. The top-level ternary operator is broken out across three lines as detailed earlier. The nested ternary operators, provided they aren't too long, can go on a single line.

Problem Set 5