Java Program To Check A Number Is Prime or Not

By Saruque Ahamed Mollick

In this post, we are going to learn, a number entered by the user is a prime number or not by using Java program.

What is a prime number?

The number which is not divisible by any number other than 1 and the number itself, is known as Prime Number.
Example: suppose a number is 13. 13 is divisible by only 1 and 13 so it’s a prime number.
11, it is divisible by 1 and 11 only so it is also a prime number.
But no 14, it is divisible by 1,2,7,14 so it is not a prime number.

The numbers less than 1 are all non-prime numbers.

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Java Code to check a number is prime or not:

import java.util.Scanner;
public class Codespeedy {
    static int p;
    public static void main(String []args) {
    Scanner input=new Scanner(System.in);
     System.out.println("Enter a Number:");
    int n=input.nextInt();
    if(n<1){
        System.out.println(n+" is not a prime number");
    }
    else
    {
       for(int i=2;i<n;i++){
           if(n%i==0){
               System.out.println(n+" is not prime number");
               p=1;
               break;
              
           }
           
              
                    }
       if(p!=1){
            System.out.println(n+" is a prime number");
               
           
       }
       
    }
}
}

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Output:

run:
Enter a Number:
13
13 is a prime number
BUILD SUCCESSFUL (total time: 2 seconds)

Explanation and algorithm of this program:

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Step 1: take a static int p, p is defined “static” because we want to access it globally.
Step 2: take a variable integer type n, and take input from the user, store it into n.
Step 3: Apply condition, If the number is less than 1 then it’s obviously  Not A Prime Number.

Step 4: If the condition is false then it will run a loop starting from 2 and ending with n-1. because we know that every number is divisible by 1 and the number itself. So we will check if any other number is available there in between 1 to n which can divide the number, i,e (n%i==0)

Step 5: If we find a number like that which can divide our number then we will print n is not a prime number.
And we will assign 1 to “p”.

Step 6: If no number is found between 1 to n which can divide our number then we will simply print it’s a prime number. In order to that, we just make a condition after our for loop. If p is not equal to 1 then it is a prime number.( because  for prime number we have assigned p=1)

break;

is used to end our loop. otherwise, the loop will keep printing same line.