This article is created to cover a program in Java that checks whether a number entered by the user is a prime number or not. I've used the following two ways to do the job:

• Using the "for" loop, check the prime number.
• Using the "while" loop, check the prime number.

Note: A prime number is a number that can only be divisible by 1 and the number itself. For example, 2, 3, 5, 7, 11, etc.

Check the prime number in Java using the for loop

The question is, should I write a Java program to check the prime number or not use a for loop? The user must receive the number during program execution. The program given below has its answer:

import java.util.Scanner;

public class CodesCracker
{
   public static void main(String[] args)
   {
      int num, i, count=0;
      Scanner s = new Scanner(System.in);
      
      System.out.print("Enter a Number: ");
      num = s.nextInt();
      
      for(i=2; i<num; i++)
      {
         if(num%i == 0)
         {
            count++;
            break;
         }
      }
      
      if(count==0)
         System.out.println("\nIt is a Prime Number.");
      else
         System.out.println("\nIt is not a Prime Number.");
   }
}

The snapshot given below shows the sample run of the above Java program with user input at 19:

Since the number 19 can only be divisible by 1 and the number itself (19) without leaving any remainder, therefore, 19 is a prime number.

Check the prime number in Java using the while loop

This program is created using the while loop. Also, the program is modified in a way that it will print the number along with a message saying whether it is prime or not. Let's have a look at the program given below:

import java.util.Scanner;

public class CodesCracker
{
   public static void main(String[] args)
   {
      int num, i=2, count=0;
      Scanner s = new Scanner(System.in);
      
      System.out.print("Enter a Number: ");
      num = s.nextInt();
      
      while(i<num)
      {
         if(num%i == 0)
         {
            count++;
            break;
         }
         i++;
      }
      
      if(count==0)
         System.out.println("\n" +num+ " is a Prime Number.");
      else
         System.out.println("\n" +num+ " is not a Prime Number.");
   }
}

Here is its sample run with user input, 12:

12 is not a prime number because, except for 1, the numbers 2, 3, 4, and 6 also divide 12.

Same Program in Other Languages

• C Check Prime or Not
• C++ Check Prime or Not
• Python Check Prime or Not

Java Online Test

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