Check Harshad Number in Java
What you’ll learn
- Harshad rule:
n % digitSum(n) == 0for positive numbers. - Single-number check and range listing in Java.
- How to avoid division-by-zero edge cases.
Prerequisites
- Basic Java syntax, loops, conditionals, and methods.
- Comfort with integer arithmetic and console input/output.
Math definition
This problem is driven by a deterministic numeric rule: for each valid input, apply the same formula repeatedly and classify the result.
Intuition with examples
Work a small input by hand first, then map each step to one Java loop iteration. This makes the control flow and stopping condition clear.
Live preview
Algorithm
Goal: decide whether n is divisible by the sum of its digits.
Compute digit sum
Use a loop with % 10 and / 10 to build sum.
Apply divisibility check
Return false for n <= 0 or sum == 0; otherwise test n % sum == 0.
📜 Pseudocode
function isHarshad(n):
if n <= 0: return false
sum = digitSum(n)
if sum == 0: return false
return (n mod sum) == 0Single value: 18
public class Main {
static int digitSumPositive(int n) {
int sum = 0;
while (n > 0) {
sum += n % 10;
n /= 10;
}
return sum;
}
static boolean isHarshad(int number) {
if (number <= 0) return false;
int sum = digitSumPositive(number);
return sum != 0 && number % sum == 0;
}
public static void main(String[] args) {
int number = 18;
if (isHarshad(number)) {
System.out.println(number + " is a Harshad number.");
} else {
System.out.println(number + " is not a Harshad number.");
}
}
}18 is a Harshad number.
Harshad numbers in [1, 20]
public class Main {
static int digitSumPositive(int n) {
int sum = 0;
while (n > 0) {
sum += n % 10;
n /= 10;
}
return sum;
}
static boolean isHarshad(int num) {
if (num <= 0) return false;
int sum = digitSumPositive(num);
return sum != 0 && num % sum == 0;
}
public static void main(String[] args) {
System.out.println("Harshad numbers in the range 1 to 20:");
for (int i = 1; i <= 20; i++) {
if (isHarshad(i)) System.out.print(i + " ");
}
System.out.println();
}
}Harshad numbers in the range 1 to 20: 1 2 3 4 5 6 7 8 9 10 12 18 20
Optimization notes
Keep it simple. Prefer clear loops and method extraction before micro-optimizations.
Reuse computed values. Avoid recomputing the same sub-result inside nested loops.
❓ FAQ
🔄 Input / output examples
Use the sample programs as reference: provide valid numeric input and verify the output pattern matches the problem definition.
Edge cases
Smallest valid input
Confirm behavior for minimum accepted values.
Invalid input
Guard against non-numeric or out-of-range values when input is user-provided.
⏱️ Time and space complexity
| Task | Time | Extra space |
|---|---|---|
| One number check | O(log n) digits | O(1) |
Scan range [1, N] | O(N log N) | O(1) |
Summary
- Use a direct Java implementation of the numeric rule.
- Validate input and test boundary cases.
- Discuss complexity clearly in interviews.
Harshad numbers are also called Niven numbers. In base 10, a positive integer n is Harshad if n is divisible by the sum of its digits.
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