JavaAlgorithms/Maths/PerfectNumber.java

package Maths;

/**
 * In number theory, a perfect number is a positive integer that is equal to the sum of its positive
 * divisors, excluding the number itself. For instance, 6 has divisors 1, 2 and 3 (excluding
 * itself), and 1 + 2 + 3 = 6, so 6 is a perfect number.
 *
 * <p>link:https://en.wikipedia.org/wiki/Perfect_number
 */
public class PerfectNumber {
  public static void main(String[] args) {
    assert isPerfectNumber(6); /* 1 + 2 + 3 == 6 */
    assert !isPerfectNumber(8); /* 1 + 2 + 4 != 8 */
    assert isPerfectNumber(28); /* 1 + 2 + 4 + 7 + 14 == 28 */
  }

  /**
   * Check if {@code number} is perfect number or not
   *
   * @param number the number
   * @return {@code true} if {@code number} is perfect number, otherwise false
   */
  public static boolean isPerfectNumber(int number) {
    int sum = 0; /* sum of its positive divisors */
    for (int i = 1; i < number; ++i) {
      if (number % i == 0) {
        sum += i;
      }
    }
    return sum == number;
  }
}