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Computational Problem Solving with Java and Interactive Object Graphics

Mark F. Russo, PhD

Section 6.5 Ternary Operator

The last way to "branch" on a condition that we’ll discuss is not a statement at all. It is an expression that makes use of a special ternary operator. Rather than modify the flow of control in a program, this ternary operator evalutes to one of two a values, depending upon whether its conditional subexpression evaluates to true of false.

Subsection 6.5.1 Parts of a Ternary Operator

The Java ternary operator is composed of the symbols ? and :. Like any other expression, one that makes use of the ternary operator evalutes to a value. What makes this kind of expression unique is that it evaluates to a different value depending upon an internal subexpression. This operator is ternary, meaning it has three operands. The first operand is the conditional expression to be evaluated. If the subexpression evaluates to true, the second operand is evaluated and its value returned. If the first operand subexpression evaluates to false the third operand of the ternary operator is evaluated and returned. The first two operands of the ternary operator are separated by ? and the second two by :.
The parts of an expression making use of a ternary operator are illustrated in Figure 6.5.1 It is useful to think of the first operand of the ternary operator as the question being asked. The first part of the ternary operator (?) is a good reminder of that. If the answer to the question (the conditional subexpression result) evaluates to true, then the second operand is evaluated and returned, otherwise the third operand is evaluated and returned.
Figure 6.5.1. Expression using a Ternary Operator

Subsection 6.5.2 When to use a Ternary Operator

Ternary operators are particularly useful when you want to substitute different values based on the value of a subexpression. Consider the following code snippet.
Listing 6.5.2. Is the number even or odd?
Let’s test this in JShell. What you’ll see below is that the exact same expression produces a different output depending upon the result of the subexpression num % 2 == 0.
jshell> int num = 12;
num ==> 12
jshell> System.out.println( num + " is " + (num % 2 == 0? "even" : "odd" ) );
12 is even
jshell> num = 13;
num ==> 13
jshell> System.out.println( num + " is " + (num % 2 == 0? "even" : "odd" ) );
13 is odd
jshell>
Think for a moment about the amount of code we’d need to produce the same output using an if-statement or a switch-statement. Either alternative would require several lines of code to accomplish. The ternary operator is a very compact way to do the same thing, only within the context of a single expression.
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