Prime numbers aren’t just abstract math concepts — they’re practical tools used to test logic, optimization skills, and problem-solving abilities during coding interviews. Whether you’re a student learning Java or a job-seeker preparing for technical rounds, mastering the Prime Number Program in Java is an essential milestone. It helps you build foundational skills in loops, conditionals, and algorithmic thinking — all of which are core to real-world programming.
What Is a Prime Number?
A prime number is a natural number greater than 1 that has no divisors other than 1 and itself.
Examples: 2, 3, 5, 7, 11, 13, 17, 19, 23...
Note: 2 is the only even prime number.
Method 1: Java Program Using a For Loop (Efficient Version)
import java.util.Scanner;
public class PrimeChecker {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.print("Enter a number: ");
int num = input.nextInt();
input.close();
if (isPrime(num)) {
System.out.println(num + " is a prime number.");
} else {
System.out.println(num + " is not a prime number.");
}
}
// Prime-checking function
public static boolean isPrime(int number) {
if (number <= 1) return false;
for (int i = 2; i <= Math.sqrt(number); i++) {
if (number % i == 0) return false;
}
return true;
}
}
Why It’s Better:
Cleaner, interview-ready structure.
Uses Math.sqrt(number) instead of number/2 for better performance.
Extracted logic into a function — great for reuse and testing.
Method 2: Using a While Loop
import java.util.Scanner;
public class PrimeUsingWhile {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
System.out.print("Enter a number: ");
int n = sc.nextInt();
sc.close();
int i = 2;
boolean isPrime = true;
while (i <= Math.sqrt(n)) {
if (n % i == 0) {
isPrime = false;
break;
}
i++;
}
if (n <= 1) isPrime = false;
if (isPrime)
System.out.println(n + " is a prime number.");
else
System.out.println(n + " is not a prime number.");
}
}
Suitable For:
- Beginners practicing loops
- Competitive coding basics
Method 3 – Find All Prime Numbers in a Range
public class PrimeInRange {
public static void main(String[] args) {
int lower = 10, upper = 50;
for (int i = lower; i <= upper; i++) {
if (isPrime(i)) {
System.out.print(i + " ");
}
}
}
public static boolean isPrime(int n) {
if (n <= 1) return false;
for (int i = 2; i <= Math.sqrt(n); i++) {
if (n % i == 0) return false;
}
return true;
}
}Common Mistakes Beginners Make (And How to Fix Them)
Learning how to find prime numbers is a fundamental exercise, but beginners often fall into the same pitfalls. Here’s how to recognize and fix them.
Mistake 1: Not checking numbers ≤ 1
Many new programmers forget that numbers like 0, 1, and negatives are not prime.
Fix:
Always start your logic with:
if (n <= 1) return false;
Mistake 2: Using n/2 instead of Math.sqrt(n)
Checking all the way up to n/2 causes unnecessary iterations, especially for large numbers.
Fix:
Use the square root approach:
for (int i = 2; i <= Math.sqrt(n); i++)
This reduces your loop to only what’s mathematically necessary.
Mistake 3: Over-complicating logic
Beginners often create deeply nested conditions, hard-to-read loops, or skip commenting.
Fix:
Keep it simple and structured. Use descriptive variable names, comment where needed, and avoid overthinking logic that can be written clearly in a few lines.
Simple code is professional code.
How to Talk About Prime Number Programs in Interviews
Knowing how to talk through your solution is just as important as writing it. These sample Q&As can help you frame your logic effectively.
Q: Why did you use a separate method?
A: “I used a dedicated method to encapsulate the prime-checking logic. This improves readability and allows easy reuse or testing.”
Q: How can you optimize your loop further?
A: “Instead of looping till n/2, I used Math.sqrt(n). It significantly reduces iteration count while maintaining correctness.”
Q: Can this be scaled to large datasets?
A: “For bulk prime checks, I would implement the Sieve of Eratosthenes. For even larger sets, parallel processing or segmenting ranges would improve performance.”
Exercise for You- Prime Number Program in Java
Take your learning further with these hands-on challenges:
Task 1: Modify the Code to Handle a List of Inputs
Accept multiple numbers and check if each one is prime.
int[] numbers = {17, 20, 23, 0, -3};
for (int n : numbers) {
System.out.println(n + ": " + (isPrime(n) ? "Prime" : "Not Prime"));
}
Task 2: Implement the Sieve of Eratosthenes
Write a Java program that prints all prime numbers up to 10,000 using the Sieve algorithm. It’s fast and ideal for large prime searches.
Task 3: Handle Edge Cases
Update your program to handle:
0,1, and negative numbers- Very large inputs (consider using
BigIntegerif needed) - Repeated or sorted inputs
These exercises will strengthen your understanding of logic, edge case handling, and algorithm optimization — all of which are valuable in real-world coding roles.
We hope that our blog post on “Prime Number Program in Java” was informative to you and answer your Java-related queries.To keep enhancing your coding skills, do check out Newtum’s website and explore our online coding courses covering Java,html, PHP, and more.
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