While Vs Do While loop in Java [In-Depth Tutorial]

Looping constructs are the cornerstone of programming, allowing developers to control the flow of execution and repeatedly perform tasks within their programs. In the world of Java, a powerful and widely-used programming language, various looping mechanisms exist, such as 'for', 'while', and 'do-while'. In this article, we will focus on understanding the differences between 'while' and 'do-while' loops in Java, using the keyword "while vs do while in java" as a guiding theme. By examining their unique execution behavior, syntax, and use cases, we aim to empower you with the knowledge necessary to make informed decisions when designing and implementing your programs.

The key distinction between 'while' and 'do-while' loops lies in their approach to executing code blocks based on given conditions. A 'while' loop checks the condition before running the code block, potentially skipping the loop entirely if the condition is false from the outset. Conversely, a 'do-while' loop guarantees at least one execution of the code block, as it checks the condition after running it. By exploring these differences and their implications, you will be better equipped to create efficient and readable code in Java, ultimately enhancing your programming expertise.

What is a while loop?

A while loop in Java is a control flow statement that allows a block of code to be executed repeatedly based on a given Boolean condition. The code within the while loop will continue to execute as long as the condition remains true. The syntax for a while loop in Java is as follows:

while (condition) {
    // code to be executed
}

In the syntax above, the condition is the Boolean expression that is evaluated at the beginning of each iteration of the loop. If the condition is true, the code within the loop will be executed. If the condition is false, the loop will be exited and the program will continue to execute from the point immediately following the loop.

Example-1: Example of while loop for printing numbers in reverse order

Let us take an example of while loop in Java and then we will explain the code.

public class Main {

public static void main(String[] args) {
    int i = 10;
    while (i >= 1) {
        System.out.println(i);
        i--;
    }
}
}

Output:

10
9
8
7
6
5
4
3
2
1

In this example, the loop execute 10 times because the condition i >= 1 is true for the values of i from 10 to 1. The code within the loop prints the value of i to the console and decrements the value of i by 1 in each iteration. Once the value of i becomes 0, the condition i >= 1 becomes false and the loop is exited.

Example-2: Example of while loop for computing the factorial of a number

Now, we will use the while loop to find the factorial of a number and then will explain the code.

public class Main {
    public static void main(String[] args) {
        int n = 5;
        int factorial = 1;
        int i = 1;
        while (i <= n) {
            factorial *= i;
            i++;
        }
        System.out.println("The factorial of " + n + " is " + factorial);
    }
    
}

Output:

The factorial of 5 is 120

In this example, the loop executes n times because the condition i <= n is true for the values of i from 1 to n. The code within the loop multiplies the value of factorial by i and increments the value of i by 1 in each iteration. Once the value of i becomes greater than n, the loop is exited. Finally, the value of the factorial is printed to the console, which gives the factorial of n. In this case, the output will be "The factorial of 5 is 120".

What is a do-while loop?

A do-while loop in Java is a control flow statement that allows a block of code to be executed repeatedly based on a given Boolean condition, similar to a while loop. However, the key difference is that a do-while loop will execute the code within the loop at least once, even if the condition is false from the beginning.

The syntax for a do-while loop in Java is as follows:

do {
    // code to be executed
} while (condition);

In the syntax above, the condition is the Boolean expression that is evaluated at the end of each iteration of the loop. If the condition is true, the loop will continue to execute. If the condition is false, the loop will be exited and the program will continue to execute from the point immediately following the loop.

Example-1: Example of the do-while loop for asking user input

Let us take an example of the do-while loop and then we will explain the code.

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String input;
        do {
            System.out.print("Please enter a word (type 'exit' to quit): ");
            input = scanner.nextLine();
            System.out.println("You entered: " + input);
        } while (!input.equals("exit"));
    }
}

In this example, the loop executes at least once because it is a do-while loop, and the condition !input.equals("exit") is checked at the end of each iteration. The code within the loop prompts the user to enter a word, reads the input using a Scanner object, and then prints the input to the console. If the input is "exit", the condition becomes false and the loop is exited. Otherwise, the loop continues to execute, prompting the user to enter another word.

Example-2: Example of the do-while loop for generating a random number between 1 and 100

Let us now use the do-while loop to generate random numbers.

import java.util.Random;

public class Main {
    public static void main(String[] args) {
        Random random = new Random();
        int number;
        do {
            number = random.nextInt(100) + 1;
            System.out.println("Generated number: " + number);
        } while (number != 42);
        System.out.println("Found the answer!");
    }
}

Output:

Generated number: 29
Generated number: 64
Generated number: 27
Generated number: 33
Generated number: 55
Generated number: 91
Generated number: 52
Generated number: 42
Found the answer!

In this example, the loop will execute until the generated number is equal to 42. The code within the loop generates a random number using a Random object and prints it to the console. If the generated number is not equal to 42, the loop continues to execute and generates another random number. Once the generated number is equal to 42, the loop is exited and the message "Found the answer!" is printed to the console.

Key Differences between while Vs do while loop in Java

Here are the main differences between a while loop and a do-while loop in Java:

// while loop
while (condition) {
  // code block
}

// do-while loop
do {
  // code block
} while (condition);

// while loop example
int count = 0;
while (count < 5) {
  System.out.println("Count: " + count);
  count++;
}

// do-while loop example
int count2 = 0;
do {
  System.out.println("Count: " + count2);
  count2++;
} while (count2 < 5);

Summary

In summary, the key differences between while and do-while loops in Java revolve around their execution behavior and use cases. A while loop checks the condition before executing the code block, meaning it may not run at all if the condition is false initially. On the other hand, a do-while loop ensures the code block is executed at least once, as it checks the condition after running the block. By understanding these fundamental differences, you can make informed decisions when choosing the appropriate loop structure for your programming scenarios, leading to more efficient and readable code.

Further Readings

While and do-while loop in java