The syntax of the while loop

while (booleanExpression)
{
statements
}

A loop that continues while choice is "y" or "Y"

String choice = "y";
while (choice.equalsIgnoreCase("y"))
{
  // get the invoice subtotal from the user
  Scanner sc = new Scanner(System.in);
  System.out.print("Enter subtotal:");
  double subtotal = sc.nextDouble();

  // the code that processes
  // the user's entry goes here

  // see if the user wants to continue
  System.out.print("Continue? (y/n):");
  choice = sc.next();
  System.out.println();
}

A loop that adds the numbers 1 through 4 to sum

int i = 1;
int sum = 0;
while (i < 5)
{
    sum = sum + i;
    i = i + 1;
}

How to code while loops

• A while statement executes the block of statements within its braces as long as the Boolean expression is true. When the expression becomes false, the program exits the while statement.
• The statements in a while statement can be called a while loop.
• Any variables that are declared in the block of a while statement have block scope.
• If the Boolean expression in a while statement never becomes false, the statement never ends, so the program goes into an infinite loop. To cancel an infinite loop, close the console window or press Ctrl+C.

Code that uses while loop.

import java.util.*;

public class WhileLoop
{
    static Scanner console =
    	new Scanner(System.in);

    static final int SENTINEL = -999;

    public static void main (String[] args)
    {
        int number;  //variable to store number
        int sum = 0;  //variable to store sum
        int count = 0; //variable to store total
                       //numbers read

        System.out.println("Line 1:
        	Enter positive integers "
          + "ending with " + SENTINEL);//Line 1

        number = console.nextInt();    //Line 2

        while (number != SENTINEL)   //Line 3
        {
            sum = sum + number;        //Line 4
            count++;                   //Line 5
            number = console.nextInt(); //Line 6
        }

        System.out.printf("Line 7:
        	The sum of the %d " +
            "numbers = %d%n",
            count, sum);              //Line 7

        if (count != 0)                 //Line 8
            System.out.printf("Line 9:
            The average = %d%n",
                (sum / count));         //Line 9
        else                            //Line 10
            System.out.println("Line 11:
            No input.");          //Line 11
    }
}