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
PrimeNumber.java
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;
  }
}
Output
>java PrimeNumber
Please enter a number:
23
23 is a prime number.

>java PrimeNumber
Please enter a number:
6
6 is not a prime number.

We can even extend above program to display prime numbers list between 1 and given number. We can reuse the method isPrimeNumber here as well. PrimeNumbers.java
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;
  }
}

Output
> java PrimeNumbers
Please enter a number:
100
List 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
        97

There are 25 prime numbers between 1 and 100