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while Loop

The while loop in Java is a control flow statement used to execute a block of code repeatedly as long as a given condition remains true. It is best suited when the number of iterations is not known in advance. This loop is widely used in input-driven logic, polling, and condition-controlled execution.

What Is a while Loop?

  • Executes code while a condition is true
  • Condition is evaluated before each iteration
  • Loop may execute zero or more times

Basic Syntax

while (condition) {
    // code to execute
}

Simple Example

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

Output:

1
2
3
4
5

Execution Flow of while Loop

  1. Condition is evaluated
  2. If condition is true, loop body executes
  3. Loop variable is updated
  4. Control returns to condition check
  5. Loop stops when condition becomes false

while Loop with Decrement

int i = 5;
while (i > 0) {
    System.out.println(i);
    i--;
}

Infinite while Loop

while (true) {
    System.out.println("Infinite loop");
}

Use case: Server listeners, background processes (with break conditions).

while Loop with break

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

while Loop with continue

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

Nested while Loop

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

Common Use Cases

  • Reading user input
  • Polling conditions
  • Iterating until a condition is met
  • File reading, network checks

while vs for Loop

Feature while for
Condition Check Before loop Before loop
Iterations Known Usually no Usually yes
Structure Simple Compact
Use Case Condition-based Count-based

Common Beginner Mistakes

  • Forgetting to update loop variable (infinite loop)
  • Using wrong condition
  • Confusing while with do-while
  • Overcomplicating loop logic

Interview-Ready Answers

Short Answer

The while loop executes a block of code repeatedly as long as the condition is true.

Detailed Answer

In Java, the while loop evaluates a condition before each iteration and executes the loop body only when the condition is true. It is best used when the number of iterations is not predetermined.

while Loop Examples

1. Basic while Loop (Print 1 to 5)

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

Explanation

  • Initializes i before the loop.
  • Loop runs while condition is true.
  • Output: 1 2 3 4 5

2. while Loop Printing Even Numbers

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

Explanation

  • Checks even condition inside the loop.
  • Output: 2 4 6 8 10

3. while Loop Printing Odd Numbers

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

Explanation

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

4. Reverse while Loop

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

Explanation

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

5. while Loop with break

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

Explanation

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

6. while Loop with continue

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

Explanation

  • Skips only iteration where i == 3.
  • Output: 1 2 4 5

7. Infinite while Loop (Controlled with break)

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

Explanation

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

8. while Loop for Array Traversal

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

Explanation

  • Uses index to traverse array.
  • Output: 10 20 30

9. while Loop with Enhanced Logic (Skip Negatives)

int[] nums = {5, -2, 8, -1, 10};
int i = 0;
while (i < nums.length) {
    if (nums[i] < 0) {
        i++;
        continue;
    }
    System.out.println(nums[i]);
    i++;
}

Explanation

  • Skips negative numbers.
  • Output: 5 8 10

10. while Loop for Summation

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

Explanation

  • Accumulates sum of numbers.
  • Output: 15

11. while Loop for Factorial Calculation

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

Explanation

  • Calculates factorial.
  • Output: 120

12. while Loop for String Traversal

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

Explanation

  • Iterates over each character.
  • Output:
J
A
V
A

13. while Loop for Digit Extraction

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

Explanation

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

14. while Loop for Palindrome Check

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

Explanation

  • Reverses number using while.
  • Output: true

15. Nested while Loop

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

Explanation

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

16. while Loop with Boolean Flag

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

Explanation

  • Flag stores result after loop.
  • Output: true

17. while Loop for User Validation (Simulation)

int attempts = 0;
while (attempts < 3) {
    System.out.println("Trying login");
    attempts++;
}

Explanation

  • Limits attempts.
  • Common real-world pattern.

18. while Loop to Skip Blank Values

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

Explanation

  • Skips empty strings.
  • Output: A B C

19. while Loop with Decrement Logic

int i = 10;
while (i > 0) {
    System.out.println(i);
    i -= 2;
}

Explanation

  • Decreases by 2 each iteration.
  • Output: 10 8 6 4 2

20. Interview Summary Example

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

Explanation

  • Demonstrates:
  • Initialization outside loop
  • Condition check
  • Increment inside loop
  • Very common interview question.

Key Takeaway

The while loop is ideal for condition-controlled execution. Correct management of the loop condition and updates is crucial to avoid infinite loops.