Get the prime factors of a number - Java java.lang

Java examples for java.lang:int prime

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Description

Get the prime factors of a number

Demo Code


//package com.java2s;
import java.util.ArrayList;

public class Main {
    /**//w w  w .  ja v  a  2s  .  c  om
     * Get the prime factors of a number
     * @param number what number to factorize
     * @return a list of all prime factors
     */
    public static ArrayList<Integer> getPrimeFactors(int number) {
        ArrayList<Integer> factors = new ArrayList<Integer>();
        ArrayList<Integer> primeNumbers = getPrimeList(number);

        for (int i = 0; i < primeNumbers.size(); i++) {
            while (number % primeNumbers.get(i) == 0) {
                factors.add(primeNumbers.get(i));
                number = number / primeNumbers.get(i);
                i = 0;
            }
        }
        return factors;
    }

    /**
     * Get a list of all primes from 2 to maxCheck
     * @param maxCheck What maximum number to check
     * @return A list filled with prime numbers
     * @post all numbers in the list are prim
     * @post no numbers in the list are negative
     */
    public static ArrayList<Integer> getPrimeList(int maxCheck)
            throws IllegalArgumentException {
        ArrayList<Integer> primeList = new ArrayList<Integer>();

        if (maxCheck < 2) {
            throw new IllegalArgumentException("Smallest prime number is 2");
        }

        for (int i = 2; i < maxCheck; i++) {
            if (isPrime(i)) {
                primeList.add(i);
            }
        }

        return primeList;
    }

    /**
     * Check if an integer is Prime
     * @param input number to check
     * @return True if number is prime
     * @post Number is prime if returned true
     */
    public static boolean isPrime(int input) {
        return isPrime((long) input);
    }

    /**
     * check if a long is Prime
     * @param input input number to check
     * @return True if number is prime
     * @post Number is prime if returned true
     */
    public static boolean isPrime(long input) {
        if (input < 2) {
            return false;
        }
        // no number y above sqrtx can divide x (except x))
        long maxLimit = (long) Math.sqrt(input) + 1;
        for (int i = 2; i < maxLimit; i++) {
            if (input % i == 0) {
                return false;
            }
        }
        return true;
    }
}

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