prime number program in java

Write a program to check given number is prime number or not.

Prime number

class PrimeNumberDemo {
  public static void main(String[] args) {  
    int number = 23;
    boolean isPrime = true;
    for (int i = 2; i < number/2; i++) {
     if (number % i == 0) {
      isPrime = false;
      break;
     }
    }  
    if (isPrime) {
     System.out.println("Number is Prime number: "+ number);
    } else {
     System.out.println("Number is not Prime number: " + number);
    }  
  }
}

Output: 

Number is Prime number: 23

Note : any number 'n' is prime number if it is not divisible by number from 2 to n-1.
Example : 29 number is not divisible by any number from 2 to 28.

But also note that if number is not divisible by 2 to n-1, then it is also not divisible by from 2 to n/2.
Example : 29 is not divisible by any number from 2 to 14. so we applied condition i < number/2



Write a program to display all prime number between 1 to 100.

Prime number between 1 to 100

class PrimeNumberDemo {
  public static void main(String[] args) {



   for (int number = 2; number <= 100; number++) {
      boolean isPrime = true;
      for (int i = 2; i < number/2; i++) {
       if (number % i == 0) {
        isPrime = false;
        break;
       }
      }  

      if (isPrime) {
       System.out.print(number + " ");
      }

   }


  }
}

Output: 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97

Note : we have started loop from 2 and not from 1, as 1 is not prime number.