How to Check if a Number is Prime Using Java – Simple Java Code with Full Explanation

Introduction

A prime number is a number that is greater than 1 and divisible only by 1 and itself. For example, 2, 3, 5, 7, 11, etc., are prime numbers.

In this blog, we will write a Java program to check whether a given number is prime or not. We will also explain the logic step-by-step so even beginners can understand.

---

🔍 What is a Prime Number?

A prime number is a natural number that has exactly two distinct factors:

1. 1

2. The number itself

For example:

• 5 is prime because it can only be divided by 1 and 5.

• 6 is not prime because it is divisible by 1, 2, 3, and 6.

---

🧠 Logic to check prime number in Java

Here’s how you can think about the problem:

1. A number less than or equal to 1 is not a prime.

2. If a number has a factor other than 1 and itself, it's not a prime.

3. You only need to check for divisibility up to the square root of the number. (This is for optimization, but we’ll use a simple loop for beginners.)

---

💡 Java Code to Check if a Number is Prime

import java.util.Scanner;

public class PrimeCheck {

    public static void main(String[] args) {

        // Step 1: Take input from the user
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a number: ");
        int number = scanner.nextInt();

        // Step 2: Call the isPrime function and store result
        boolean result = isPrime(number);

        // Step 3: Show the result to the user
        if (result) {
            System.out.println(number + " is a Prime Number.");
        } else {
            System.out.println(number + " is NOT a Prime Number.");
        }

        scanner.close();
    }

    // Function to check if the number is prime
    public static boolean isPrime(int num) {

        // Numbers less than or equal to 1 are not prime
        if (num <= 1) {
            return false;
        }

        // Check from 2 to num-1
        for (int i = 2; i < num; i++) {
            // If number is divisible by any i, then it's not prime
            if (num % i == 0) {
                return false;
            }
        }

        // If no divisor found, it's a prime number
        return true;
    }
}

🔍 Step-by-Step Explanation of the Code

✅ Step 1: Take Input

Scanner scanner = new Scanner(System.in);
System.out.print("Enter a number: ");
int number = scanner.nextInt();

We use Scanner class to get user input. The user enters a number which we want to check.

---

✅ Step 2: Call the isPrime() Function

boolean result = isPrime(number);

We pass the user input to the function isPrime() which returns true if the number is prime, otherwise false.

---

✅ Step 3: Print the Result

if (result) {
    System.out.println(number + " is a Prime Number.");
} else {
    System.out.println(number + " is NOT a Prime Number.");
}

Based on the result from isPrime(), we print the appropriate message.

---

✅ Step 4: The isPrime() Function

public static boolean isPrime(int num)

This is a custom function that checks if the given number is prime.

if (num <= 1) {
    return false;
}

Any number less than or equal to 1 is not prime, so we return false.

for (int i = 2; i < num; i++) {
    if (num % i == 0) {
        return false;
    }
}

We check for any divisor from 2 up to number-1. If the number is divisible by any of them, it is not prime.

return true;

If the loop completes and no divisor is found, it means the number is prime.

---

⚙️ Sample Output

Input:

Enter a number: 7

Output:

7 is a Prime Number.

📌 Bonus Tip: Optimized Version

You can also optimize the loop by checking up to √num (square root of number). This reduces execution time for large numbers.

for (int i = 2; i <= Math.sqrt(num); i++)

✅ Conclusion

Checking whether a number is prime in Java is a great beginner-level programming task. It helps improve understanding of:

• Loops

• Conditionals

• Functions

• Basic math logic

We hope this explanation helped you learn how to write and understand a prime number checker in Java.