Exploring While and DoWhile in Java: Master Loop Structures with Practical Examples

This article will delve into two of Java's most fundamental programming structures: While and DoWhile loops. Understanding when and how to use them is essential for writing efficient code, solving dynamic problems, and manipulating data intelligently. Let’s explore their applications with practical examples.

Previously in this series, we learned how to use the For loop in Java. Today, we will focus on While and DoWhile loops. How should you choose between these options in your daily programming tasks? Here's a tip: “Use the For loop when you know the required number of iterations. Use While when the number of iterations is unknown.”

Let's get started with some code!

While Loop

The following example demonstrates how a While loop iterates until a condition is satisfied. This is particularly useful when the exact number of iterations is not predetermined.

public class WhileExample {
    public static void main(String[] args) {
        int totalSubscribers = 100; // Example data
        int availableCoupons = 50;
        int currentSubscriber = 1;

        while (currentSubscriber <= availableCoupons) {
            // Print message that the subscriber won a coupon
            printMessage("Subscriber number " + currentSubscriber + " won a coupon!");
            // Increment the number of processed subscribers
            currentSubscriber++;
        }
    }

    static void printMessage(String message) {
        System.out.println(message);
    }
}

In the above code, we simulate a promotion where only the first 50 subscribers receive a discount coupon for a product. As long as currentSubscriber is less than or equal to the number of coupons, a message is printed in the terminal, and currentSubscriber is incremented by 1. This process continues until the condition is no longer met.

DoWhile Loop

The logic for the DoWhile loop is similar to While but with one significant difference: the validation is performed at the end of the loop. In other words, the block of code executes first, and then the condition is checked. Here’s an example:

public class WhileExample {
    public static void main(String[] args) {
        int totalSubscribers = 100; // Example data
        int availableCoupons = 50;
        int currentSubscriber = 1;

        while (currentSubscriber <= availableCoupons) {
            // Print message that the subscriber won a coupon
            printMessage("Subscriber number " + currentSubscriber + " won a coupon!");
            // Increment the number of processed subscribers
            currentSubscriber++;
        }
    }

    static void printMessage(String message) {
        System.out.println(message);
    }
}

In this example, a school tries to contact selected candidates for a course, with a maximum of three contact attempts. The line answered = new Random().ints(0, 2).findFirst().getAsInt(); generates a random number using the Random class. This number falls between 0 (inclusive) and 2 (exclusive), effectively simulating whether the candidate answered the call (1) or not (0). The process repeats until the candidate answers or the maximum attempts are reached.

Both While and DoWhile loops are essential for scenarios such as validating user input, batch data processing, or algorithms requiring iteration until a specific condition is met. Examples include the school scenario above or loops for validating user inputs.

We hope this article clarified the differences between While and DoWhile loops. How do you use these structures in your code? Share your experiences or questions in the comments! If you enjoyed this content, follow me for more articles in this series and other Java topics. See you next week—before the New Year—with more Java concepts!