## What is prime number

A number is called as a prime if it has only two divisors. In other words, a number which is divided only by 1 and itself. i.e.; number of factors for a prime number should only be 2 (1 and itself).
For example, 2 is a prime number since it has only two factors 1 and 2. 6 is not a prime number since it has 4 factors 1, 2, 3, and 6.

## Prime number program

I've developed a sample Java program to identify given number is prime number or not. Logic is very simple, for every number 1 and itself are the definite factors. So, we should count the factors of a given number from 2 to n-1. If we find any factory between 2 and n-1, then the given number is not a prime number
`import java.io.*;/** * This class demonstrates the prime number. * @author SANTHOSH REDDY MANDADI * @version 1.0 * @since 19 August 2012 */class PrimeNumber{  public static void main(String args[]) throws Exception  {    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));    System.out.println("Please enter a number:");    int number = Integer.parseInt(br.readLine());    if(isPrimeNumber(number))    {      System.out.println(number+" is a prime number.");    }    else    {      System.out.println(number+" is not a prime number.");    }  }  public static boolean isPrimeNumber(int number)  {    int i;    for(i=2; i< number; i++)    {      if(number%i == 0)      {        break;      }    }    if(i == number)    {      return true;    }    return false;  }}`
`>java PrimeNumberPlease enter a number:2323 is a prime number.>java PrimeNumberPlease enter a number:66 is not a prime number.`
`import java.io.*;/** * This class lists all prime number upto the given number. * @author SANTHOSH REDDY MANDADI * @version 1.0 * @since 19 August 2012 */class PrimeNumbers{  public static void main(String args[]) throws Exception  {    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));    System.out.println("Please enter a number:");    int number = Integer.parseInt(br.readLine());    System.out.println("List of prime numbers between 1 and "+number+": ");    int count = 0;    for(int i=2; i<=number;i++)    {      if(isPrimeNumber(i))      {        System.out.printf("%10d", i);        count++;      }    }    System.out.println("\n\nThere are "+count+" prime numbers between 1 and "+number);  }  public static boolean isPrimeNumber(int number)  {    int i;    for(i=2; i< number; i++)    {      if(number%i == 0)      {        break;      }    }    if(i == number)    {      return true;    }    return false;  }}`
`> java PrimeNumbersPlease enter a number:100List of prime numbers between 1 and 100:         2         3         5         7        11        13        17        19        23        29        31        37        41        43        47        53        59        61        67        71        73        79        83        89        97There are 25 prime numbers between 1 and 100`