Programming Tutorials

The ? Operator

By: aathishankaran in Java Tutorials on 2007-02-07  

Java includes a special ternary (three-way) operator that can replace certain types of if-then-else statements. This operator is the ?, and it works in java much like it does in C and C++. It can seem somewhat confusing at first, but the ? can be used very effectively once mastered. The ? has this general form:

expression1? expression2 : expression3

Here, expression1 can be any expression that evaluates to a boolean value. If expression1 is true, then expression2 is evaluated; otherwise, the expression3 is evaluated. The result of the ? operation is that of the expression evaluated. Both expression2 and expression3 are required to return the same type, which can't be void.

Here is an example of the way that the ? is employed:

ratio = denom == 0 ? 0 : num/denom;

When java evaluates this assignment expression, it first looks at the expression to the left of the question mark. If denom equals zero, then the expression between the question mark and the colon is evaluated and used as the value of the entire ? expression. If denom does not equal zero, then the expression after the colon is evaluated and used for the value of the entire ? expression. The result produced by the ? operator is then assigned to ratio.

Here is a program that demonstrates the ? operator. It uses it to obtain the absolute value of a variable.

// Demonstrate ?.

class Ternary {

 public static void main (String args[]) {

  int count, k;
  count = 10;
  k = count < 0 ? -count : count;
  System.out.print("Absolute value of ");
  System.out.println(count+ " is " + k);

  Count = -10;
  k = count < 0 ? -count : count;

  System.out.print("Absolute value of ");
  System.out.println(count+ " is " + k);

 }

}

The output generated by the program is shown here:

Absolute value of 10 is 10
Absolute value of -10 is 10