Java Program to Check Whether a Number is Prime or Not

Prime numbers are special numbers with only two factors 1 and that number itself, they cannot be divided by any other number. Therefore, to check whether a given number is prime or not, first, we will divide the given number by all natural numbers less than or equal to it and then, we check the number of factors. If the count of factors is only two then, the given number is prime otherwise not. This way of checking prime numbers is called as factorization. Now, let's understand how we can identify if a given number is prime or not by implementing the mentioned logic.

Java Program to Check whether a number is Prime or not

Before jumping to the example programs of checking prime numbers, let's discuss the problem statement properly with the help of an example.

Instance

Input 1

1

Output

1 is not a prime number

Input 2

5

Output

5 is a prime number  

Example 1

The following example illustrates how to take a number as input from the user and check whether that number is prime or not.

Approach

• First, define a count variable that will store the number of factors of the given number.

• Then, create an instance of the Scanner class so that we can take input from the user.

• Next, take an if-else block to check whether the input is prime or not. If the input is 2, print it is prime otherwise move to the else block where we will check the number of factors using a for loop.

• Now, take another if-else block to check the number of factors. If it is 2 then, print prime otherwise not a prime number.

import java.util.*;
public class Example1 {
   public static void main(String[] args) {
      // initial count of factors
      int count = 0;
      // creating an instance of Scanner class
      Scanner sc = new Scanner(System.in);
      System.out.println("Enter a number to check prime number");
      // to take input from user
      int num = sc.nextInt();
      // to check prime number
      if(num == 2) {
         System.out.println(num + " is a prime number");
      } else {
         // checking number of factors
         for(int i = 1; i <= num; i++) {
            if(num % i == 0) {
               count++;
            }
         }
         // checking number of counts to print result
         if(count == 2) {
            System.out.println(num + " is a prime number");
         } else {
            System.out.println(num + " is not a prime number");
         }
      }
   }
}

Output 1

Enter a number to check prime number
13
13 is a prime number

Output 2

Enter a number to check prime number
56
56 is not a prime number

Example 2

In this Java program, instead of taking input from the user, we will initialize an integer variable to check if it is prime or not. We won't change the logic of program.

public class Example2 {
   public static void main(String[] args) {
      int num = 89;
      System.out.println("The given number is: " + num);
      // initial count of factors
      int count = 0;
      // to check prime number
      if(num == 2) {
         System.out.println(num + " is a prime number");
      } else {
         // checking number of factors
         for(int i = 1; i <= num; i++) {
            if(num % i == 0) {
               count++;
            }
         }
         // checking number of counts to print result
         if(count == 2) {
            System.out.println(num + " is a prime number");
         } else {
            System.out.println(num + " is not a prime number");
         }
      }
   }
}

Output

The given number is: 89
89 is a prime number

Conclusion

This article first explains what is the condition for a number to be called as prime and how could we check if a given number is prime or not. Then, we have written two different Java programs which use the if-else block and for loop to identify prime numbers.