Difference Between while and do-while Loop in Java

Java while Loop

The while loop is a pre-test loop, meaning that it evaluates the condition before entering the loop body. If the condition is true, the loop body is executed. If the condition is false from the start, the loop body is not executed at all.

Syntax:

The loop will continue to execute the code block repeatedly as long as the condition remains true. If the condition becomes false at any point, the loop terminates, and the program continues with the next statement following the loop.

Example of a While Loop

Output:

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5

Explanation

In the above example, the loop will print the numbers from 1 to 5. The condition i <= 5 is checked before each iteration, and as long as it evaluates to true, the loop will keep executing.

Java do-while Loop

The do-while loop is a post-test loop; it means it executes the loop body at least once before checking the condition. It ensures that the loop body is executed at least once, regardless of whether the condition is true or false.

Syntax:

The loop will repeatedly execute the code block while the condition remains true. It will stop only when the condition becomes false.

Example of a do-while Loop

Output:

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5

Explanation

In the above example, the loop will also print the numbers from 1 to 5. However, the key difference here is that the loop body is executed first before checking the condition j <= 5.

Condition Evaluation

while Loop: The condition is checked before each iteration. If the condition is initially false, the loop body will not be executed at all.

do-while Loop: The condition is checked after each iteration. The loop body is executed at least once, even if the condition is false from the start.

Execution Count

while Loop: Executes zero or more times (0 if the condition is false initially).

do-while Loop: Executes one or more times (at least once).

Use Cases

while Loop: We should use it when we want to repeat a block of code based on a condition, but the code should not run if the condition is false initially.

do-while Loop: We should use it when we need to execute a block of code at least once, irrespective of the condition, and then repeat it based on the condition.

Which one to use?

The choice between the while loop and the do-while loop depends on the specific requirements of your programme. If we wish to ensure that the loop body is executed at least once, then the do-while loop is more suitable. Conversely, if the loop may not need to run at all based on an initial condition check, then the while loop is a better option.

Both loops are invaluable tools in a Java developer's toolkit, providing flexibility for managing various situations in code.

The while loop and the do-while loop in Java share similarities in functionality; however, their behaviour regarding condition evaluation and initial execution makes them distinct. Understanding these differences is crucial for writing efficient and error-free code. Choose the loop that best fits the specific requirements of your programme, and you will be well-equipped to handle repetitive tasks effectively.

Java while Vs. do-while Loop

Java while and do-while Loop MCQs

1. What is the difference between a while loop and a do-while loop in Java?

1. There is no difference; they are interchangeable.
2. A while loop executes the body at least once; a do-while loop may not execute at all.
3. A while loop checks the condition before the body execution, the do-while loop checks the condition after the body execution.
4. A while loop is used for infinite loops, do-while loop for finite loops.

2. Which of the following correctly describes the behavior of a do-while loop in Java?

1. The loop body is executed at least once, regardless of the condition.
2. The loop body may not execute at all if the condition is false initially.
3. The loop body is executed only if the condition is true initially.
4. The loop does not have a condition and executes indefinitely.

3. When is the condition checked in a while loop?

1. After the code block is executed
2. Before the code block is executed
3. Only once at the beginning
4. At the end of the loop's execution

4. What happens if the loop variable is not updated inside a while loop?

1. The loop may execute more times than expected
2. The loop might not execute at all
3. The loop could become an infinite loop
4. There is no risk

5. What is the primary function of the while loop in Java?

1. To execute a block of code a fixed number of times
2. To execute a block of code as long as a specified condition is true
3. To iterate over elements of an array
4. To execute a block of code at least once, regardless of condition