Frequently Asked Java Program 11: Check Given Number is Prime Or Not

Introduction

In this tutorial, we will learn how to check if a given number is a prime number using Java. A prime number is a natural number greater than one that has no positive divisors other than 1 and itself. Understanding how to determine if a number is prime is fundamental in number theory and has practical applications in fields such as cryptography and computer science.

Step 1: Understand Prime Numbers

Before we write the code, let’s clarify what prime numbers are:

• A prime number is greater than 1.
• It has exactly two factors: 1 and the number itself.
• Examples of prime numbers include 2, 3, 5, 7, 11, and 13.
• Numbers like 4, 6, 8, and 28 are not prime because they have more than two factors.

Step 2: Set Up Your Java Environment

To write and execute Java code, ensure you have the following installed:

• Java Development Kit (JDK)
• An Integrated Development Environment (IDE) like IntelliJ IDEA, Eclipse, or VSCode.

Step 3: Write the Java Code to Check for Prime Numbers

Here is a simple Java program that checks whether a number is prime:

import java.util.Scanner;

public class PrimeCheck {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a number: ");
        int number = scanner.nextInt();
        
        if (isPrime(number)) {
            System.out.println(number + " is a prime number.");
        } else {
            System.out.println(number + " is not a prime number.");
        }
    }
    
    public static boolean isPrime(int num) {
        if (num <= 1) return false; // Not prime if less than or equal to 1
        for (int i = 2; i <= Math.sqrt(num); i++) { // Check up to the square root of num
            if (num % i == 0) {
                return false; // Not prime if divisible by any number
            }
        }
        return true; // Prime if no divisors found
    }
}

Code Explanation

• The program starts by importing the Scanner class for user input.
• It prompts the user to enter a number.
• The isPrime method checks if the number is prime:
  • It returns false if the number is less than or equal to 1.
  • It loops from 2 to the square root of the number, checking for divisibility.
  • If the number is divisible by any of these, it is not prime.

Step 4: Compile and Run Your Program

To compile and run the program:

1. Save the code in a file named PrimeCheck.java.
2. Open a terminal or command prompt.
3. Navigate to the directory where the file is saved.
4. Compile the code using:

   javac PrimeCheck.java
   

5. Run the program using:

   java PrimeCheck
   

Step 5: Test with Different Numbers

After running your program, test it with various inputs:

• Input 19 and check the output (should be prime).
• Input 28 and check the output (should not be prime).
• Explore edge cases like 0, 1, and negative numbers to see how the program responds.

Conclusion

In this tutorial, we learned how to determine if a number is prime using Java. We covered the definition of prime numbers, set up a Java environment, and wrote a program to check for primality. You can expand this program by adding features such as error handling or checking for multiple numbers in a single execution. Experiment with the code and enhance your programming skills!