Java Masterclass for Software Developers/

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Boolean Variables in Loops

Boolean Variables in Loops

In this lesson, we will explore the use of Boolean variables to control a loop.

We'll cover the following...

The sum of positive numbers

Let’s recall the loop that sums the positive numbers entered by a user, as given earlier in this chapter:

import java.util.Scanner;
public class Example
{
   public static void main(String[] args) 
   {
      Scanner keyboard = new Scanner(System.in);
      
      System.out.println("This program computes the sum of positive numbers you enter as input.");
      System.out.println("A nonpositive number will signal the end of the data.");
 
      double sum = 0;
      System.out.print("Type the next positive number or 0 to end: ");
      double number = keyboard.nextDouble();
      while (number > 0)
      {
         sum = sum + number;
         System.out.print("Type the next positive number or 0 to end: ");
         number = keyboard.nextDouble();
      } // End while
      
      System.out.println("The sum is " + sum);
      System.out.println("All Done!");
	} // End main
} // End Example

The while loop is controlled by the Boolean expression number > 0, whose value is either true or false. We can describe the logic of this loop with the following pseudocode:

sum = 0
while (we are not done)
{
   number = the next number that the user types
   if (number > 0)
      sum = sum + number
   else
      We are done
}

A Boolean variable—like a Boolean expression—has a value that is either true or false, and we can use such a variable to control a loop. So instead of implementing the pseudocode:

while (we are not done)

as:

while (number > 0)

let’s write:

while (!done)

where done is a Boolean variable. We read this as “while not done.” We can initialize done to false. Since !done is true, the loop will execute until done becomes true. ...