Loops in Java are used to execute a block of code repeatedly until a condition becomes false. Loops help reduce code repetition and make programs more efficient.

In Java, the most commonly used loops are:

• for loop

• while loop

• do-while loop (we will briefly cover this too)

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1. for Loop in Java

The for loop is used when we know how many times we want to run the loop.

Syntax:

for (initialization; condition; update) {
    // code to execute
}

How for loop works:

1. Initialization → Runs only once

2. Condition → Checked before every iteration

3. Code inside loop executes

4. Update → Increases/decreases variable

5. Loop repeats until condition becomes false

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Example 1: Print numbers from 1 to 5

public class ForLoopExample {
    public static void main(String[] args) {
        for (int i = 1; i <= 5; i++) {
            System.out.println(i);
        }
    }
}

Output:

1
2
3
4
5

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Example 2: Sum of first 5 numbers

public class SumExample {
    public static void main(String[] args) {
        int sum = 0;

        for (int i = 1; i <= 5; i++) {
            sum = sum + i;
        }

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

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Example 3: Reverse loop

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

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2. while Loop in Java

The while loop is used when we do not know how many times the loop will run. It runs until the condition becomes false.

Syntax:

while (condition) {
    // code
}

Example 1: Print numbers from 1 to 5

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

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

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Example 2: Sum of numbers until user enters 0

import java.util.Scanner;

public class WhileSumExample {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int sum = 0;
        int num = 1;

        while (num != 0) {
            System.out.print("Enter number: ");
            num = sc.nextInt();
            sum += num;
        }

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

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3. do-while Loop (Important)

This loop executes at least once, even if the condition is false.

Syntax:

do {
    // code
} while (condition);

Example:

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

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

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for vs while Loop Difference

for loop	while loop
Used when number of iterations is known	Used when number of iterations is unknown
Initialization, condition, update in one line	Initialization outside
Mostly used for arrays, fixed loops	Mostly used for user input
Syntax shorter	Syntax longer

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Real Life Example

ATM Example using while loop

import java.util.Scanner;

public class ATMExample {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int pin = 1234;
        int enteredPin;

        while (true) {
            System.out.print("Enter PIN: ");
            enteredPin = sc.nextInt();

            if (enteredPin == pin) {
                System.out.println("Login Successful");
                break;
            } else {
                System.out.println("Wrong PIN, try again");
            }
        }
    }
}

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Conclusion

Loop	Use
for	When iterations are fixed
while	When condition-based loop
do-while	Runs at least once

Simple rule to remember:

• Use for loop → when you know the count

• Use while loop → when you don't know the count

• Use do-while → when code must run at least once

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Practice Questions

1. Print table of a number

2. Factorial of a number

3. Reverse a number

4. Count digits in a number

5. Sum of digits

6. Prime number check

Practice these using both for loop and while loop.

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