Have you ever sat, staring at your computer, with your coffee in your hand, thinking- how am I supposed to write a prime number program in Java and not screw it up!

Well, I’ve been there. A long time before I learned how to write code, I was haunted by the sight of prime numbers in Java program assignments. Sounds simple on paper, right? A whole that can be divided into only 1. However, once you begin writing the logic, then you suddenly fall into the trap of an infinite loop (pun intended 😉).

This is why today I am going to take you through prime numbers in Java program the most human way. No jargon overload. Clear examples, stories, and precise mistakes I made (so you do not need to repeat them).

What are Prime Numbers (Quick Refresher 🧮)

In general, a prime number is just a number that is greater than 1, and cannot be divided into two parts, other than the parts of 1 and the parts of itself.

Examples:

2, 3, 5, 7, 11, 13 ✅ (prime)

4, 6, 8, 9, 10 ❌ (not prime)

The only even prime number is 2. This small fact served me wrong in my Java program. I’ll explain later.

Why Prime Numbers in Java Program

In case you think, Why should I even bother with prime numbers in Java program, here it is:

• They are the blocks of cryptography. Your WhatsApp messages would not be safe without them.
• A well known interview question is prime number checks. Believe me I was even asked to write one on my very first job interview.
• They sharpen your problem solving abilities. It is the physical training of coders.

And thus, not to pass exams, it is about the development of your coding muscles 💪.

Prime Numbers in Java Program: The Basic Approach

The following is the simplest I wrote as a beginner. Well, it is not the most efficient, but it works.

public class PrimeCheck {
    public static void main(String[] args) {
        int num = 29; // you can try with any number
        boolean isPrime = true;

        if (num <= 1) {
            isPrime = false;
        } else {
            for (int i = 2; i < num; i++) {
                if (num % i == 0) {
                    isPrime = false;
                    break;
                }
            }
        }

        if (isPrime)
            System.out.println(num + " is a prime number.");
        else
            System.out.println(num + " is not a prime number.");
    }
}

This is what is referred to as the brute force method. It verifies divisibility of all the numbers between 2 and n-1. Inefficient, educational.

Optimized Prime Numbers in Java Program (Method 2)

Then I thought… it isn’t necessary to check all numbers up to n-1, to check up to n. That reduced my code run time by a lot.

public class PrimeCheckOptimized {
    public static void main(String[] args) {
        int num = 29;
        boolean isPrime = true;

        if (num <= 1) {
            isPrime = false;
        } else {
            for (int i = 2; i <= Math.sqrt(num); i++) {
                if (num % i == 0) {
                    isPrime = false;
                    break;
                }
            }
        }

        if (isPrime)
            System.out.println(num + " is a prime number.");
        else
            System.out.println(num + " is not a prime number.");
    }
}

This version made me realize the difference between smart code and code that writes more lines but not the right lines.

Prime Numbers in Java Program Using Recursion (Method 3)

Recursion was popular with some of my friends. It terrified me personally. However, after I wrote the program to compute prime numbers in Java with recursion, I found it so beautiful:

public class PrimeRecursion {
    static boolean isPrime(int num, int i) {
        if (num <= 2)
            return (num == 2) ? true : false;
        if (num % i == 0)
            return false;
        if (i * i > num)
            return true;
        return isPrime(num, i + 1);
    }

    public static void main(String[] args) {
        int num = 29;
        if (isPrime(num, 2))
            System.out.println(num + " is a prime number.");
        else
            System.out.println(num + " is not a prime number.");
    }
}

Generate Prime Numbers in Java Program (Method 4)

That is fine to check a single number, but what about a list of primes in the range [1, 100]? That’s when loops save the day:

public class PrimeList {
    public static void main(String[] args) {
        int limit = 100;

        for (int num = 2; num <= limit; num++) {
            boolean isPrime = true;

            for (int i = 2; i <= Math.sqrt(num); i++) {
                if (num % i == 0) {
                    isPrime = false;
                    break;
                }
            }

            if (isPrime)
                System.out.print(num + " ");
        }
    }
}

Typical errors that I committed when writing the Prime Number in Java Program.

• Losing the memory that 0 and 1 are not prime numbers.
• And there are only even numbers that are not prime (except 2!).
• Failure to deal with negative inputs.
• Infinite loops due to bad loop conditions.

And you are committing these errors now, it is all right. All coders do at some time.

Final Thoughts ❤️

The Java program on writing a prime numbers program taught me not only how to write a Java program, but also patience, optimization, and problem solving. I initially stumbled along with basic loops, then got to know recursion, and eventually understood advanced algorithms such as the Sieve of Eratosthenes.

This is one of the best places to begin when you are learning Java. It’s small, yet powerful. And remember: you write a prime number program with Java, not once only not twice, but a hundred and a thousand times, you are not writing code, but a set of habits you will carry you through the next hundred and a thousand lines of code.

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