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do-while Loop

The do-while loop in Java is a control flow statement that executes a block of code at least once, and then repeats execution as long as a given condition remains true. This loop is best used when the code must run at least one time, regardless of the condition.

What Is a do-while Loop?

  • Executes loop body before checking the condition
  • Condition is evaluated after each iteration
  • Loop executes at least once

Basic Syntax

do {
    // code to execute
} while (condition);

⚠️ Semicolon is mandatory after while(condition);

Simple Example

int i = 1;
do {
    System.out.println(i);
    i++;
} while (i <= 5);

Output:

1
2
3
4
5

Guaranteed One-Time Execution Example

int x = 10;
do {
    System.out.println("Executed once");
} while (x < 5);

Why it matters: Condition is false, but code executes once.

Execution Flow of do-while Loop

  1. Loop body executes
  2. Condition is checked
  3. If condition is true, loop repeats
  4. If condition is false, loop exits

do-while with break

do {
    if (conditionMet) {
        break;
    }
} while (true);

do-while with continue

int i = 0;
do {
    i++;
    if (i == 3) {
        continue;
    }
    System.out.println(i);
} while (i <= 5);

Nested do-while Loop

int i = 1;
do {
    int j = 1;
    do {
        System.out.println(i + "," + j);
        j++;
    } while (j <= 2);
    i++;
} while (i <= 3);

More do-while Loop Examples

1. Basic do-while Loop (Print 1 to 5)

int i = 1;
do {
    System.out.println(i);
    i++;
} while (i <= 5);

Explanation

  • Loop executes at least once.
  • Condition is checked after execution.
  • Output: 1 2 3 4 5

2. do-while Loop Executes Even When Condition Is False

int i = 10;
do {
    System.out.println(i);
} while (i < 5);

Explanation

  • Condition is false initially.
  • Still executes once.
  • Output: 10

3. do-while Loop Printing Even Numbers

int i = 1;
do {
    if (i % 2 == 0) {
        System.out.println(i);
    }
    i++;
} while (i <= 10);

Explanation

  • Prints only even numbers.
  • Output: 2 4 6 8 10

4. do-while Loop Printing Odd Numbers

int i = 1;
do {
    if (i % 2 != 0) {
        System.out.println(i);
    }
    i++;
} while (i <= 10);

Explanation

  • Prints only odd numbers.
  • Output: 1 3 5 7 9

5. Reverse do-while Loop

int i = 5;
do {
    System.out.println(i);
    i--;
} while (i >= 1);

Explanation

  • Loop runs backward.
  • Output: 5 4 3 2 1

6. do-while Loop with break

int i = 1;
do {
    if (i == 3) {
        break;
    }
    System.out.println(i);
    i++;
} while (i <= 5);

Explanation

  • Loop terminates early when i == 3.
  • Output: 1 2

7. do-while Loop with continue

int i = 0;
do {
    i++;
    if (i == 3) {
        continue;
    }
    System.out.println(i);
} while (i <= 5);

Explanation

  • Skips printing when i == 3.
  • Output: 1 2 4 5

8. Infinite do-while Loop (Controlled with break)

do {
    System.out.println("Running");
    break;
} while (true);

Explanation

  • Condition always true.
  • break is required to stop execution.

9. do-while Loop for Array Traversal

int[] nums = {10, 20, 30};
int i = 0;
do {
    System.out.println(nums[i]);
    i++;
} while (i < nums.length);

Explanation

  • Traverses array elements.
  • Output: 10 20 30

10. do-while Loop to Skip Negative Values

int[] nums = {5, -1, 8, -3, 10};
int i = 0;
do {
    if (nums[i] < 0) {
        i++;
        continue;
    }
    System.out.println(nums[i]);
    i++;
} while (i < nums.length);

Explanation

  • Skips negative numbers.
  • Output: 5 8 10

11. do-while Loop for Sum Calculation

int i = 1;
int sum = 0;
do {
    sum += i;
    i++;
} while (i <= 5);
System.out.println(sum);

Explanation

  • Accumulates values.
  • Output: 15

12. do-while Loop for Factorial

int num = 5;
int fact = 1;
do {
    fact *= num;
    num--;
} while (num > 0);
System.out.println(fact);

Explanation

  • Calculates factorial.
  • Output: 120

13. do-while Loop for String Traversal

String s = "JAVA";
int i = 0;
do {
    System.out.println(s.charAt(i));
    i++;
} while (i < s.length());

Explanation

  • Iterates character by character.
  • Output:
J
A
V
A

14. do-while Loop for Digit Extraction

int num = 1234;
do {
    System.out.println(num % 10);
    num /= 10;
} while (num > 0);

Explanation

  • Extracts digits from right to left.
  • Output: 4 3 2 1

15. do-while Loop for Palindrome Check

int num = 121;
int temp = num;
int rev = 0;
do {
    rev = rev * 10 + temp % 10;
    temp /= 10;
} while (temp > 0);
System.out.println(num == rev);

Explanation

  • Reverses number.
  • Checks palindrome.
  • Output: true

16. Nested do-while Loop

int i = 1;
do {
    int j = 1;
    do {
        System.out.println("i=" + i + ", j=" + j);
        j++;
    } while (j <= 3);
    i++;
} while (i <= 3);

Explanation

  • Inner loop completes for each outer loop.
  • Used in matrix/grid logic.

17. do-while Loop with Boolean Flag

int i = 1;
boolean found = false;
do {
    if (i == 4) {
        found = true;
        break;
    }
    i++;
} while (i <= 5);
System.out.println(found);

Explanation

  • break exits loop.
  • Flag holds result after loop.

18. do-while Loop for Menu Simulation

int choice = 3;
do {
    System.out.println("Menu shown");
} while (choice != 3);

Explanation

  • Menu displays at least once.
  • Common in menu-driven programs.

19. do-while Loop Skipping Empty Strings

String[] data = {"A", "", "B", "", "C"};
int i = 0;
do {
    if (data[i].isEmpty()) {
        i++;
        continue;
    }
    System.out.println(data[i]);
    i++;
} while (i < data.length);

Explanation

  • Skips empty values.
  • Output: A B C

20. Interview Summary Example (do-while Guarantee)

int i = 5;
do {
    System.out.println("Executed once");
} while (i < 3);

Explanation

  • Output: Executed once
  • Demonstrates:
  • Guaranteed execution
  • Condition checked after body
  • Very common interview concept.

Common Use Cases

  • Menu-driven programs
  • Input validation
  • User interaction loops
  • Retry mechanisms

do-while vs while

Feature do-while while
Condition Check After loop body Before loop body
Minimum Execution Once Zero or more
Semicolon Required Not required

Common Beginner Mistakes

  • Forgetting semicolon after while()
  • Infinite loops due to missing updates
  • Confusing do-while with while
  • Overusing nested loops

Interview-Ready Answers

Short Answer

The do-while loop executes the loop body at least once and then repeats as long as the condition is true.

Detailed Answer

In Java, the do-while loop checks the condition after executing the loop body, ensuring at least one execution. It is commonly used in menu-driven and user-interaction programs.

Key Takeaway

The do-while loop guarantees at least one execution, making it ideal for interactive and retry-based logic.