How to find prime factors of a number in java

package org.arpit.java2blog;

import java.util.ArrayList;
import java.util.List;

public class PrimeFactorsMain {

 // This method calculate prime factor and add it to primeFactor list
 public static List primeFactors(int number) {
  int n = number;
  List primeFactors = new ArrayList();
  for (int i = 2; i <= n; i++) {
   while (n % i == 0) {
    primeFactors.add(i);
    n /= i;
   }
  }
  return primeFactors;
 }

 public static void main(String[] args) {
  System.out.println("Primefactors of 15 are : ");
  System.out.printf("%s %n",primeFactors(15));

  System.out.println("Primefactors of 48 are :");
  System.out.printf("%s %n",primeFactors(48));

  System.out.println("Primefactors of 91");
  System.out.printf("%s %n",primeFactors(91));

 }
}

Run above program and you will get following output:

Primefactors of 15 are : 
[3, 5] 
Primefactors of 48 are :
[2, 2, 2, 2, 3] 
Primefactors of 91
[7, 13]

You must be wondering we are not checking whether loop variable i is prime or not But you don’t need to do this because in any loop, number has been already divided by 2 to i-1 so i can only be divisor if it is prime.

More optimized solution:

 // This method calculate prime factor and add it to primeFactor list
 public static List primeFactors(int number) {
 int n = number;
 List primeFactors = new ArrayList();
 for (int i = 2; i <= n/i; i++) { while (n % i == 0) { primeFactors.add(i); n /= i; } } if(n>1)
 primeFactors.add(n);
 return primeFactors;
 }