What is do-while Loop in Java? (Explained With Examples)

Introduction

In Java, loops are control flow statements that allow code to be executed repeatedly based on a condition. Java provides several types of loops: for, while, and do-while. Among these, the do-while loop stands out because it guarantees the execution of the loop body at least once, making it ideal for scenarios where an action must be taken before checking a condition.

What is do-while Loop in Java?

The do-while loop is considered a post-test loop since the condition is evaluated after the loop body has executed. This feature makes it useful in real-world applications like menu-driven programs, input validation, and retry mechanisms.

Syntax of do-while Loop

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do {

// block of statements

} while (condition);

? Explanation:

do: Starts the loop.

{}: Contains the code to be executed.

while(condition): After the first execution, the condition is checked. If true, the loop repeats.

Important: Don’t forget the semicolon (;) after while(condition)—this is a common syntax mistake.

Flowchart of do-while Loop

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┌──────────────┐

│ Statement │

└─────┬────────┘

↓

┌────────────┐

│ Condition │

└─────┬──────┘

↓ Yes

┌──────┐

↓ ↑

Repeat ──┘

↑ No

Exit

As shown in the flowchart, the statement block is executed before the condition is checked.

Characteristics of do-while Loop

Guaranteed Execution: The code inside the do block is always executed at least once.

Post-condition Check: The condition is evaluated after execution.

Controlled Iteration: Useful for repeat-until scenarios.

Flexible Input Handling: Often used to handle user input validation.

Read: What is a while Loop in Java?

How do-while Differs from Other Loops

Loop Type	Condition Checked	Executes At Least Once	Use Case
for	Before	No	Counting, known iterations
while	Before	No	Condition-based
do-while	After	Yes	Must-run-once situations

Real-world Analogy: Imagine you're tasting soup. You taste first (do), then check if it needs salt (while). If it needs salt, you repeat the process.

Basic Example

Let’s start with a simple program that prints numbers from 1 to 5 using a do-while loop.

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public class DoWhileExample {

public static void main(String[] args) {

int i = 1;

do {

System.out.println("Number: " + i);

i++;

} while (i <= 5);

}

Output:

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Number: 1

Number: 2

Number: 3

Number: 4

Number: 5

Explanation:

The loop starts by executing System.out.println("Number: " + i);

Then it increments i and checks if i <= 5.

If true, the loop continues. Otherwise, it stops.

Using do-while Loop with User Input

This type of loop is ideal for validating user input—prompting the user until they enter a correct value.

Example: Accepting only positive numbers

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import java.util.Scanner;

public class PositiveInput {

public static void main(String[] args) {

Scanner sc = new Scanner(System.in);

int number;

do {

System.out.print("Enter a positive number: ");

number = sc.nextInt();

} while (number <= 0);

System.out.println("You entered: " + number);

}

Explanation:

The loop prompts the user to enter a number.

It continues until a positive number is entered.

This guarantees at least one input prompt.

Nested do-while Loop

You can use a do-while loop inside another do-while loop.

Example: Displaying a multiplication table (1 to 3)

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public class NestedDoWhile {

public static void main(String[] args) {

int i = 1;

do {

int j = 1;

do {

System.out.print((i * j) + "\t");

j++;

} while (j <= 5);

System.out.println();

i++;

} while (i <= 3);

}

Output:

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1 2 3 4 5

2 4 6 8 10

3 6 9 12 15

Explanation:

Outer loop iterates for rows (1 to 3)

Inner loop prints the multiplication values

Common Mistakes and Debugging

? Infinite Loops

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int i = 1;

do {

System.out.println(i);

// i++; // forgot to increment

} while (i <= 5);

This causes an infinite loop since i never increases.

❌ Missing Semicolon

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do {

System.out.println("Hello");

} while (true) // missing semicolon here!

You’ll get a syntax error. Correct way:

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} while (true);

? Tip: Always test your condition carefully and include increment/decrement logic to avoid logical bugs.

do-while with Arrays

The do-while loop can be used to iterate through arrays, although it's more common to use for or while.

Example: Print array values

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public class ArrayDoWhile {

public static void main(String[] args) {

int[] numbers = {10, 20, 30, 40, 50};

int i = 0;

do {

System.out.println(numbers[i]);

i++;

} while (i < numbers.length);

}

Explanation:

do-while starts from index 0.

Runs until i reaches the array length.

do-while in Menu-Driven Programs

Menu-based programs often use do-while to keep displaying the menu until the user decides to exit.

Example: Simple Calculator

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import java.util.Scanner;

public class MenuCalculator {

public static void main(String[] args) {

Scanner sc = new Scanner(System.in);

int choice;

do {

System.out.println("Menu:");

System.out.println("1. Add");

System.out.println("2. Subtract");

System.out.println("3. Exit");

System.out.print("Enter choice: ");

choice = sc.nextInt();

switch (choice) {

case 1:

System.out.println("Result: " + (10 + 5));

break;

case 2:

System.out.println("Result: " + (10 - 5));

break;

case 3:

System.out.println("Exiting...");

break;

default:

System.out.println("Invalid choice!");

}

} while (choice != 3);

}

Performance Considerations

Use do-while only when necessary—i.e., when at least one execution is required.

For known iteration counts, prefer for loop.

Avoid using do-while for indefinite processes without exit conditions.

Interview Questions Related to do-while

Q1. What is the difference between while and do-while?

A: do-while executes the block first, then checks the condition. while checks the condition first.

Q2. Will this run?

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int i = 10;

do {

System.out.println("Hello");

} while (i < 5);

A: Yes. It prints "Hello" once since the body runs before the condition is checked.

Advanced do-while Use Cases

Simulate a countdown with delay

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public class Countdown {

public static void main(String[] args) throws InterruptedException {

int i = 5;

do {

System.out.println("Countdown: " + i);

Thread.sleep(1000); // 1-second delay

i--;

} while (i > 0);

System.out.println("Go!");

}

Game Loop Concept

Game engines or simple animations often use do-while for loops that run until an exit condition is met.

Best Practices

Ensure the exit condition is reachable.

Use only when one-time execution is mandatory.

Don’t misuse for standard iteration patterns better suited for for or while.

Summary

The do-while loop in Java is a powerful tool when you want your block of code to execute at least once before checking a condition. It’s commonly used in user input validation, menu-based systems, and retry loops. However, its misuse can lead to infinite loops and unreadable code.

Key Points: