Arrays.sort

우리는 만약 int[] a = {1,3,2} ; 라고 주어졌다면 Arrays.sort(a) ; 를 하면 되는 것으로 알고 있습니다. 그런데 AudioBook 의 경우에는 사용자가 정의한 객체이기 때문에 sort 의 기준이 명확하지 않아 compareTo 메서드를 사용해야하는 것으로 알고 있어요. 그런데 여기서 AudioBook 객체가 Comparable class 를 반드시 implements 해야하는 이유는 그래야 Arrays.sort() 를 이용할 수 있기 때문인가요? 만약 implements 없이 그냥 사용하면 Arrays.sort() 에서는 compareTo 를 활용해야한다는 인식 자체를 못하는 건가요?

아래에는 제가 질문을 하게끔 만든 코드입니다.

public class Books
{
    //-----------------------------------------------------------------
    //  Creates a AudioBookCollection object and adds some books to it. Prints
    //  reports on the status of the collection.
    //-----------------------------------------------------------------
    public static void main (String[] args)
    {
        AudioBookCollection library = new AudioBookCollection();
      
        library.addBook("The Kaiju Preservation Society",
          "John Scalzi", "Will Wheaton", 25.99, 482);
        library.addBook("Revenger",
          "Alastair Reynolds", "Clare Corbett", 22.94, 880);
        library.addBook("Klara and the Sun:A Novel",
          "Kazuo Ishiguro", "Sura Siu", 24.50, 616);
        library.addBook("Endurance: Shackleton's Incredible Voyage",
          "Alfred Lansing", "Simon Prebble", 21.83, 620);
        library.addBook("Barbarian Days: A Surfing Life",
          "William Finnegan", "William Finnegan", 21.99, 1088);
     
        System.out.println(library);

        library.sort();

        System.out.println(library);



    }
}

import java.text.NumberFormat;
import java.util.Locale;
import java.util.Arrays;


public class AudioBookCollection
{
    private AudioBook[] collection;
    private int count;
    private double totalCost;

    /*-----------------------------------------------------------------
    * Constructor: Creates an initially empty collection.
    *----------------------------------------------------------------*/
    public AudioBookCollection ()
    {
        collection = new AudioBook[100];
        count = 0;
        totalCost = 0.0;
    }

    /*-----------------------------------------------------------------
    *  Adds an audio book to the collection, increasing the size of the
    *  collection if necessary.
    *----------------------------------------------------------------*/
    public void addBook (String title, String author, String readBy,
                         double cost, int minutes)
    {
        if (count == collection.length)
            this.increaseSize();

        collection[count] = new AudioBook(title, author, readBy, cost, minutes);
        totalCost += cost;
        count++;
    }

    /*-----------------------------------------------------------------
    *  Returns a report describing the Book collection.
    *----------------------------------------------------------------*/
    public String toString()
    {
        Locale usa = new Locale("en", "US");
        NumberFormat fmt= NumberFormat.getCurrencyInstance(usa);

        String report = "~~~~~~~~~~~~~~Wow Cool~~~~~~~~~~~~~~~~~~~~~~~\n";
        report += "My Audio Book Collection\n\n";

        report += "Number of Books: " + count + "\n";
        report += "Total cost: " + fmt.format(totalCost) + "\n";
        report += "Average cost: " + fmt.format(totalCost/count);

        report += "\n\nBook List:\n\n";

        for (int i = 0; i < count; i++)
            report += collection[i] + "\n";

        return report;
    }

        public void sort()
    {	
	    Arrays.sort(collection,0,count);
    } 

    //-----------------------------------------------------------------
    //  Increases the capacity of the collection by creating a
    //  larger array and copying the existing collection into it.
    //-----------------------------------------------------------------
    private void increaseSize ()
    {
        AudioBook[] temp = new AudioBook[collection.length*2];

        for (int i = 0; i < collection.length; i++)
            temp[i] = collection[i];
      
        collection = temp;  
      
    }


}

import java.text.NumberFormat;
import java.util.Locale;

public class AudioBook implements Comparable<AudioBook>
{
    private String title, author, readBy;
    private double cost;
    private int minutes;

    //-----------------------------------------------------------------
    //  Creates a new audio book with the specified information.
    //-----------------------------------------------------------------
    public AudioBook (String title, String author, String readBy, double cost,  int minutes)
    {
        this.title = title;
        this.author = author;
        this.readBy = readBy;
        this.cost = cost;
        this.minutes = minutes;
    }

    //-----------------------------------------------------------------
    //  Returns a string description of this audio book.
    //-----------------------------------------------------------------	

    public String toString()
    {
        // for formatting money
        NumberFormat fmt= NumberFormat.getCurrencyInstance(Locale.US);
        // instead of using the ready-made Locale.US we could also create our own
        // Locale usa = new Locale("en","US")
        // try looking up country locale codes on the web!
      
        // for formatting strings
        String description;
        description = String.format(fmt.format(cost) + "\t" + minutes + "\t"
          + "%-20s" + "\t" + "%-50s",author,title);

        return description;
    }

    // compare based on book length
    public int compareTo(AudioBook other)
    {
        int answer = 0;
	    if (this.minutes<other.minutes){
            answer = -1;
        }
        if (this.minutes>other.minutes){
            answer = 1;
        }
        return answer;
    } 

}