While Loop in Java

The while loop repeats a block of code as long as a condition is true. It is ideal when the number of iterations is not known up front — for example reading input until the user types 'exit', or processing items until a queue is empty.

• Syntax: while (condition) { body }
• The condition must produce a boolean.
• If the condition is false at start, the body never runs.
• Always ensure the loop variable changes inside the body.

Let's start with a hands-on example. The program below shows the core idea behind While Loop in just a few lines of Java. Read it line by line and observe how Java executes each statement in order.

class Main
{
    public static void main(String args[])
    {
        int i = 1;

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

The following examples progress from simple to more practical patterns. Try each in your IDE, change the inputs and observe the output. This is the fastest way to internalize the concept.

class Main
{
    public static void main(String args[])
    {
        int n = 1234, sum = 0;

        while (n > 0)
        {
            sum += n % 10;
            n /= 10;
        }

        System.out.println("Sum = " + sum);
    }
}

class Main
{
    public static void main(String args[])
    {
        int n = 5829, rev = 0;

        while (n > 0)
        {
            rev = rev * 10 + n % 10;
            n /= 10;
        }

        System.out.println("Reversed = " + rev);
    }
}

import java.util.Scanner;

class Main
{
    public static void main(String args[])
    {
        Scanner sc = new Scanner(System.in);
        int n = -1, total = 0;

        while (n != 0)
        {
            System.out.print("Enter (0 to stop): ");
            n = sc.nextInt();
            total += n;
        }

        System.out.println("Total = " + total);
    }
}

class Main
{
    public static void main(String args[])
    {
        int i = 1;

        while (true)
        {
            if (i > 4) break;
            System.out.println(i);
            i++;
        }
    }
}

• Always update the loop variable to avoid an unintended infinite loop.
• Use while(true) with break for event loops and servers.
• Be careful when reading input — verify hasNextInt() before nextInt().
• Prefer for loops for fixed counts; while loops shine for conditional repetition.
• Use continue sparingly — too many can make the loop hard to read.

Reading is not enough — practice solidifies knowledge. Try every question below in your own editor before peeking at any solution.

• Q1. Print the first N natural numbers using a while loop.
• Q2. Find the GCD of two numbers using a while loop.
• Q3. Reverse a string using a while loop.
• Q4. Count the vowels in a string using a while loop.
• Q5. Find whether a number is an Armstrong number using a while loop.

These are the most common questions asked in Java interviews on this topic. Memorize the concept, not just the answer — interviewers often follow up with edge cases.

Even experienced developers slip on the same pitfalls. Watch out for these classic mistakes while working with While Loop in Java:

• Forgetting the semicolon ; at the end of statements while using While Loop.
• Mixing up similar method names — read Java docs before using new APIs related to While Loop.
• Ignoring compiler warnings — they often hint at bugs that will appear later in production.
• Hard-coding values that should come from configuration files or environment variables.
• Not handling edge cases: empty inputs, very large inputs, negative numbers and null references.
• Skipping unit tests — small tests prevent big regressions, especially around control flow.