while loop vs do-while loop in Java

1. Introduction

In Java, loops are used to execute a set of statements repeatedly until a particular condition is satisfied. The while loop and the do-while loop are two forms of conditional loops that serve this purpose. 

A while loop evaluates its expression at the beginning of each loop cycle, so the code inside the loop may not execute at all if the condition is not met. 

A do-while loop evaluates its expression at the end of each loop cycle, which guarantees that the code inside the loop will execute at least once.

2. Key Points

1. A while loop checks the condition before the code within the loop is executed.

2. A do-while loop checks the condition after the code within the loop is executed.

3. As a result of the check timing, a while loop may not execute its code at all if the condition is initially false.

4. A do-while loop always executes its code block at least once.

3. Differences

while loop	do-while loop
The while loop checks the condition at the beginning of the loop.	The do-while loop checks the condition at the end of the loop.
If the condition is false at the first iteration, the loop body does not execute even once.	The loop body is guaranteed to execute at least once, regardless of the condition.
More commonly used when the number of iterations is unknown and depends solely on the condition.	More suitable when the loop body needs to be executed at least once, such as when displaying a menu or prompting for user input.
If the condition is initially false, It can result in zero loop iterations.	Always results in at least one iteration, even if the condition is initially false.
Syntax: while (condition) { statements; }	Syntax: do { statements; } while (condition);

4. Example

public class LoopComparison {
    public static void main(String[] args) {
        int count = 0;

        // Step 1: Using a while loop
        System.out.println("while loop output:");
        while (count < 0) {
            System.out.println("This will not print");
        }

        // Step 2: Using a do-while loop
        System.out.println("do-while loop output:");
        do {
            System.out.println("This will print at least once");
            count++;
        } while (count < 0);
    }
}

Output:

while loop output:
do-while loop output:
This will print at least once

Explanation:

1. The while loop with the condition count < 0 does not execute because the condition is false at the start.

2. The do-while loop, on the other hand, executes once before checking the condition count < 0, which is why the statement within the loop's body is printed.

5. When to use?

- Use a while loop when you want to execute code only if the condition is true at the start of the loop.

- Use a do-while loop when you want to ensure that the code within the loop executes at least once, regardless of the condition being true or false.