Brute Force password attack java could you explain how to work one method.

public void doItMulti()

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package bfpasswrd;

import java.util.ArrayList;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public class PasswordCracker
{

String passwordToCrack;
public boolean passwordFound;
int min;
int max;
StringBuilder crackedPassword;

public void prepare(String text)
{
    passwordToCrack = text;

    passwordFound = false;
    min = 32;
    max = 126;
    crackedPassword = new StringBuilder();
    crackedPassword.append((char) (min - 1));
}

public void result()
{
    System.out.println("Cracked Password is: " + crackedPassword.toString());
}

public void incrementString(StringBuilder text, int min, int max)
{
    text.setCharAt(0, (char) ((int) text.charAt(0) + 1));
    for (int i = 0; i < text.length(); i++)
    {
        if (text.charAt(i) > (char) max)
        {
            text.setCharAt(i, (char) min);
            if (text.length() == i + 1)
            {
                text.append((char) min);
            }
            else
            {
                text.setCharAt(i + 1, (char) ((int) text.charAt(i + 1) + 1));
            }
        }
    }
}

public void runMulti(String text)
{
    prepare(text);
    double time = System.nanoTime();
    doItMulti();
    time = System.nanoTime() - time;
    System.out.println(time / (1000000000));
    result();

}

public void runSingle(String text)
{
    prepare(text);
    double time = System.nanoTime();
    doItSingle();
    time = System.nanoTime() - time;
    System.out.println(time / (1000000000));
    result();
}

public void doItSingle()
{
    while (passwordFound == false)
    {
        incrementString(crackedPassword, min, max);
        passwordFound = crackedPassword.toString().equals(passwordToCrack);
    }
}

public void doItMulti()
{
    int cores = Runtime.getRuntime().availableProcessors();
    ArrayList<Future<?>> tasks = new ArrayList<Future<?>>(cores);
    ExecutorService executor = Executors.newFixedThreadPool(cores);
    final long step = 2000;
    for (long i = 0; i < Long.MAX_VALUE; i += step)
    {
        while(tasks.size() > cores)
        {
            for(int w = 0; w < tasks.size();w++)
            {
                if(tasks.get(w).isDone())
                {
                    tasks.remove(w);
                    break;
                }
            }
            try
            {
                Thread.sleep(0);
            }
            catch (InterruptedException e)
            {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
        {
            final long j = i;
            if (passwordFound == false)
            {
                tasks.add(executor.submit(new Runnable()
                {

                    public void run()
                    {
                        long border = j + step;
                        StringBuilder toCrack = new StringBuilder(10);
                        toCrack.append(constructString3(j, min, max));
                        for (long k = j; k < border; k++)
                        {
                            incrementString(toCrack, min, max);
                            boolean found = toCrack.toString().equals(passwordToCrack);
                            if (found)
                            {
                                crackedPassword = toCrack;
                                passwordFound = found;
                                break;
                            }
                        }
                    }
                }));
            }
            else
            {
                break;
            }
        }
    }
    executor.shutdownNow();
}

public String constructString3(long number, long min, long max)
{
    StringBuilder text = new StringBuilder();
    if (number > Long.MAX_VALUE - min)
    {
        number = Long.MAX_VALUE - min;
    }
    ArrayList<Long> vector = new ArrayList<Long>(10);
    vector.add(min - 1 + number);
    long range = max - min + 1;
    boolean nextLetter = false;
    for (int i = 0; i < vector.size(); i++)
    {
        long nextLetterCounter = 0;
        while (vector.get(i) > max)
        {
            nextLetter = true;
            long multiplicator = Math.abs(vector.get(i) / range);
            if ((vector.get(i) - (multiplicator * range)) < min)
            {
                multiplicator -= 1;
            }
            vector.set(i, vector.get(i) - (multiplicator * range));
            nextLetterCounter += multiplicator;
        }
        if (nextLetter)
        {
            vector.add((long) (min + nextLetterCounter - 1));
            nextLetter = false;
        }
        text.append((char) vector.get(i).intValue());
    }
    return text.toString();
}
}

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package bfpasswrd;


import java.util.Scanner;

public class Test
{
    public static void main(String[] args)
    {

    System.out.print("Type password to be cracked: ");
    @SuppressWarnings("resource")
	String input = new Scanner(System.in).nextLine();
    PasswordCracker cracker = new PasswordCracker();
    System.out.println("Multithreaded");
    cracker.runMulti(input);
    cracker = new PasswordCracker();
    System.out.println("Single threaded Original");
    cracker.runSingle(input);
    System.out.println("Finished...");
    }
}

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