Core Java Tutorial

Introduction

The do-while loop in Java is a control flow statement that allows code to be executed repeatedly based on a given boolean condition. Unlike the while loop, the do-while loop guarantees that the code block is executed at least once before the condition is tested. This makes the do-while loop particularly useful for scenarios where you need to ensure that the code block executes at least once.

Table of Contents

• What is a do-while Loop?
• Syntax of do-while Loop
• How do-while Loop Works
• Simple do-while Loop Example
• Infinite do-while Loop
• do-while Loop with a Break Statement
• do-while Loop with a Continue Statement
• Nested do-while Loop
• Using a do-while Loop for Input Validation
• Conclusion

What is a do-while Loop?

A do-while loop repeatedly executes a block of code as long as the specified condition evaluates to true. The key difference from a while loop is that the condition is evaluated after the loop body, ensuring that the loop body is executed at least once.

Syntax of do-while Loop

        do {
            // body of loop
        } while (condition);
    

• condition: A boolean expression that is evaluated after each iteration of the loop.
• body of loop: The block of code that is executed at least once and as long as the condition is true.

How do-while Loop Works

• The loop body is executed.
• The condition is evaluated.
• If the condition is true, the loop body is executed again.
• Steps 2 and 3 are repeated until the condition becomes false.
• When the condition is false, the loop terminates, and control moves to the next statement after the loop.

Simple do-while Loop Example

        public class SimpleDoWhileLoop {
            public static void main(String[] args) {
                int count = 1;
                do {
                    System.out.println("Count: " + count);
                    count++;
                } while (count <= 5);
            }
        }
    

Explanation: This loop prints the numbers from 1 to 5. The body of the loop is executed first, and then the condition count <= 5 is checked. Since the condition is true, the loop continues to execute until count exceeds 5.

Infinite do-while Loop

        public class InfiniteDoWhileLoop {
            public static void main(String[] args) {
                do {
                    System.out.println("This is an infinite loop");
                } while (true);
            }
        }
    

Explanation: This loop will print "This is an infinite loop" indefinitely because the condition true never changes.

do-while Loop with a Break Statement

        public class DoWhileWithBreak {
            public static void main(String[] args) {
                int count = 1;
                do {
                    if (count == 5) {
                        break;
                    }
                    System.out.println("Count: " + count);
                    count++;
                } while (count <= 10);
            }
        }
    

Explanation: This loop prints numbers from 1 to 4. When count equals 5, the break statement exits the loop.

do-while Loop with a Continue Statement

        public class DoWhileWithContinue {
            public static void main(String[] args) {
                int count = 0;
                do {
                    count++;
                    if (count % 2 == 0) {
                        continue;
                    }
                    System.out.println("Odd number: " + count);
                } while (count < 10);
            }
        }
    

Explanation: This loop prints odd numbers from 1 to 9. The continue statement skips the even numbers.

Nested do-while Loop

        public class NestedDoWhileLoop {
            public static void main(String[] args) {
                int i = 1;
                do {
                    int j = 1;
                    do {
                        System.out.println("i: " + i + ", j: " + j);
                        j++;
                    } while (j <= 3);
                    i++;
                } while (i <= 3);
            }
        }
    

Explanation: This loop prints pairs of i and j values, iterating over all combinations of i and j from 1 to 3.

Using a do-while Loop for Input Validation

        import java.util.Scanner;

        public class InputValidation {
            public static void main(String[] args) {
                Scanner scanner = new Scanner(System.in);
                int number;

                do {
                    System.out.print("Enter a positive number: ");
                    number = scanner.nextInt();
                    if (number <= 0) {
                        System.out.println("Invalid input. Please try again.");
                    }
                } while (number <= 0);

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

Explanation: This loop repeatedly prompts the user to enter a positive number until a valid input is provided.

Conclusion

The do-while loop in Java is a powerful control flow statement for performing repeated tasks based on a condition. Unlike the while loop, the do-while loop guarantees that the code block is executed at least once. Understanding how to use the do-while loop effectively, including its variations with break and continue statements, as well as nested loops, is essential for writing robust and efficient Java programs.