راهنمایی و توضیح یک کد

import java.security.SecureRandom;
import java.util.Random;

public class RandomString {
   public static final String SOURCES =
           "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890";

   public static void main(String[] args) {
       RandomString rs = new RandomString();
       System.out.println(rs.generateString(new Random(), SOURCES, 10));
       System.out.println(rs.generateString(new Random(), SOURCES, 10));
       System.out.println(rs.generateString(new SecureRandom(), SOURCES, 15));
       System.out.println(rs.generateString(new SecureRandom(), SOURCES, 15));
   }

   /**
    * Generate a random string.
    *
    * @param random the random number generator.
    * @param characters the characters for generating string.
    * @param length the length of the generated string.
    * @retu
    */
   public String generateString(Random random, String characters, int length) {
       char[] text = new char[length];
       for (int i = 0; i < length; i++) {
           text[i] = characters.charAt(random.nextInt(characters.length()));
       }
       retu new String(text);
   }
}