Listing Prime Factors in Java

Finding Prime Factors in Java

Finding prime factors is all about selecting those factors of a number that are prime.
The prime factors of 36 as an example, are:

2 X 2 X 3 X 3.

Prime Factor Java code

We'll go about the code in a simple way:

Step 1:

Starting with 2, find the first factor of the number of interest - this factor will definitely be a prime.
Store this factor away.

Step 2:

Divide number in question by found factor.

Step 3:

Using result from Step 2 as the new number in question (number whose prime factors we are trying to find), repeat Step 1.

Step 4:

Continue recursively until we arrive at 1.

Step 5:

We'll use the square-root of number range.

Create a new class file; File, New File.
Call it PrimeFactors.
Type out the adjoining Java code for listing prime factors.

All those steps in a few lines of code;
Now functions come in handy, don't they?

Note: You can comment out the Java code for the main class from the previous lesson if you have been following.

Java code for PrimeFactors class.

package arithmetic;

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

public class PrimeFactors {

    private final List<Integer> found_prime_factors;
    private final int find_my_factors;
    private String render_factors;
    private int i;

    public PrimeFactors(int val) {
        found_prime_factors = new ArrayList<>();
        find_my_factors = val;
        render_factors = "The prime factors of " + val + " are: \n";
        i = 2;
    }

    /**
    * Our function checks 'find_my_factors';
    * If it finds a factor, it records this factor,
    * then divides 'find_my_factors' by the found factor
    * and makes this the new 'find_my_factors'.
    * It continues recursively until all factors are found.
    * @param  - integer.
     */
    private int onlyPrimeFactors(int in_question) {
        int temp_limit;
        temp_limit = (int) Math.ceil(Math.sqrt(in_question));

        while (i <= temp_limit) {
            if (i != 1 && (in_question % i) == 0) { // avoid an infinite loop with the i != 1 check.
                found_prime_factors.add(i);
                return onlyPrimeFactors(in_question / i);
            }
            i++;
        }
        found_prime_factors.add(in_question);

        return 0;
    }

    /**
     * Renders the prime prime factors to screen.
     * @return - String value
     */
    public String getPrimeFactors() {
        onlyPrimeFactors(find_my_factors);

        //iterate through and retrieve members
        found_prime_factors.stream().forEach((factor) -> {
            render_factors += factor + " X ";
        });
        render_factors = render_factors.substring(0, render_factors.length()-3);

        return render_factors;
    }

}

Main class code.

package arithmetic;

public class Arithmetic {

    public static void main(String[] args) {
        System.out.println("Welcome to our demonstration sequels");
        System.out.println("Hope you enjoy (and follow) the lessons.");
        System.out.println("");

        /*
        * List Prime Factors.
         */
        PrimeFactors prime_factors = new PrimeFactors(48);
        System.out.println(prime_factors.getPrimeFactors());
    }

}

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