Java Program to Find Sum of Digits (With Explanation & Example)

Introduction

In programming, working with numbers is a fundamental skill. One common problem asked in exams, interviews, and basic coding practice is finding the sum of digits of a number.

For example:

• Input: 1234

• Output: 1 + 2 + 3 + 4 = 10

In this article, you’ll learn:

• Logic behind the program

• Step-by-step explanation

• Java implementation

• Dry run example

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🧠 Concept Behind Sum of Digits

To find the sum of digits, we repeatedly:

1. Extract the last digit of the number

2. Add it to a sum variable

3. Remove the last digit from the number

Key Operations:

• n % 10 → gives the last digit

• n / 10 → removes the last digit

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💻 Java Program

import java.util.Scanner;

public class SumOfDigits {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("Enter a number: ");
        int num = sc.nextInt();

        int sum = 0;

        while (num > 0) {
            int digit = num % 10;  // Extract last digit
            sum = sum + digit;    // Add to sum
            num = num / 10;       // Remove last digit
        }

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

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🔍 Step-by-Step Explanation

Let’s break down the logic:

1. Input the number

   • User enters a number like 1234

2. Initialize sum

   • sum = 0

3. Loop until number becomes 0

   • Extract digit using %

   • Add it to sum

   • Remove digit using /

4. Print result

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🧪 Dry Run Example

Input: 1234

Step	num	digit (num % 10)	sum
1	1234	4	4
2	123	3	7
3	12	2	9
4	1	1	10
End	0	—	10

Output: Sum = 10

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⚡ Alternative Approach (Using String)

You can also solve this by converting the number into a string:

public class SumDigitsString {
    public static void main(String[] args) {
        String num = "1234";
        int sum = 0;

        for (int i = 0; i < num.length(); i++) {
            sum += num.charAt(i) - '0';
        }

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

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📌 Key Points to Remember

• % operator extracts digits

• / operator reduces the number

• Loop continues until number becomes zero

• Works for any positive integer

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🎯 Conclusion

The sum of digits problem is simple but very important for building logic in programming. It helps you understand loops, arithmetic operations, and number manipulation.

Once you master this, you can move on to advanced problems like:

• Sum of even/odd digits

• Product of digits

• Palindrome numbers