Java while and do while Loop

Use a while loop when you do not know how many times you need to repeat, and you only know the condition.

while loop

The condition is checked before each round. If it is false at the start, the body never runs.

public class Main {
    public static void main(String[] args) {
        int i = 1;
        while (i <= 5) {
            System.out.println("i = " + i);
            i++;              // do not forget to change the variable
        }
    }
}

do while loop

The body runs first and then the condition is checked. So it runs at least one time.

public class Do {
    public static void main(String[] args) {
        int i = 10;
        do {
            System.out.println("Runs once, i = " + i);
            i++;
        } while (i < 5);

        int j = 10;
        while (j < 5) {
            System.out.println("never printed");
        }
        System.out.println("done");
    }
}

Example: digits of a number

public class Digits {
    public static void main(String[] args) {
        int number = 9475;
        int sum = 0;
        while (number > 0) {
            sum += number % 10;     // last digit
            number /= 10;           // remove last digit
        }
        System.out.println("Sum of digits: " + sum);
    }
}

Example: ask again until the input is right

import java.util.Scanner;

public class Valid {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int age = -1;
        while (age < 0 || age > 120) {
            System.out.print("Enter your age: ");
            if (sc.hasNextInt()) {
                age = sc.nextInt();
            } else {
                sc.next();      // throw away the bad input
            }
        }
        System.out.println();
        System.out.println("Age saved: " + age);
    }
}

for or while?

Loop	Use when
for	You know the number of rounds, or you count
while	You repeat until a condition changes
do while	The body must run at least once (menus, input)

Infinite loop

while (true) {
    // runs for ever, use break to stop
}

• Previous Page
• Next Page

---

Exercises & Assignments
No Content Found.

Interview Questions & Answers
No Content Found.