Flow of control in java - multiple classes

Question

I am a beginner to java and have been going through oracle's java tutorial. In those tutorials I came across a question involving multiple classes. The class called Deck uses class called Cards. I want to know if the class Deck ever uses the main method in class Cards. Does the line of flow ever go through the main method of Cards.

public class Card {
private final int rank;
private final int suit;

// Kinds of suits
public final static int DIAMONDS = 1;
public final static int CLUBS    = 2;
public final static int HEARTS   = 3;
public final static int SPADES   = 4;

// Kinds of ranks
public final static int ACE   = 1;
public final static int DEUCE = 2;
public final static int THREE = 3;
public final static int FOUR  = 4;
public final static int FIVE  = 5;
public final static int SIX   = 6;
public final static int SEVEN = 7;
public final static int EIGHT = 8;
public final static int NINE  = 9;
public final static int TEN   = 10;
public final static int JACK  = 11;
public final static int QUEEN = 12;
public final static int KING  = 13;

public Card(int rank, int suit) {
    assert isValidRank(rank);
    assert isValidSuit(suit);
    this.rank = rank;
    this.suit = suit;
}

public int getSuit() {
    return suit;
}

public int getRank() {
    return rank;
}

public static boolean isValidRank(int rank) {
    return ACE <= rank && rank <= KING;
}

public static boolean isValidSuit(int suit) {
    return DIAMONDS <= suit && suit <= SPADES;
}

public static String rankToString(int rank) {
    switch (rank) {
    case ACE:
        return "Ace";
    case DEUCE:
        return "Deuce";
    case THREE:
        return "Three";
    case FOUR:
        return "Four";
    case FIVE:
        return "Five";
    case SIX:
        return "Six";
    case SEVEN:
        return "Seven";
    case EIGHT:
        return "Eight";
    case NINE:
        return "Nine";
    case TEN:
        return "Ten";
    case JACK:
        return "Jack";
    case QUEEN:
        return "Queen";
    case KING:
        return "King";
    default:
        //Handle an illegal argument.  There are generally two
        //ways to handle invalid arguments, throwing an exception
        //(see the section on Handling Exceptions) or return null
        return null;
    }    
}

public static String suitToString(int suit) {
    switch (suit) {
    case DIAMONDS:
        return "Diamonds";
    case CLUBS:
        return "Clubs";
    case HEARTS:
        return "Hearts";
    case SPADES:
        return "Spades";
    default:
        return null;
    }    
}

public static void main(String[] args) {

    // must run program with -ea flag (java -ea ..) to
    // use assert statements
    assert rankToString(ACE) == "Ace";
    assert rankToString(DEUCE) == "Deuce";
    assert rankToString(THREE) == "Three";
    assert rankToString(FOUR) == "Four";
    assert rankToString(FIVE) == "Five";
    assert rankToString(SIX) == "Six";
    assert rankToString(SEVEN) == "Seven";
    assert rankToString(EIGHT) == "Eight";
    assert rankToString(NINE) == "Nine";
    assert rankToString(TEN) == "Ten";
    assert rankToString(JACK) == "Jack";
    assert rankToString(QUEEN) == "Queen";
    assert rankToString(KING) == "King";

    assert suitToString(DIAMONDS) == "Diamonds";
    assert suitToString(CLUBS) == "Clubs";
    assert suitToString(HEARTS) == "Hearts";
    assert suitToString(SPADES) == "Spades";

}
}

import java.util.*;

public class Deck {

public static int numSuits = 4;
public static int numRanks = 13;
public static int numCards = numSuits * numRanks;

private Card[][] cards;

public Deck() {
    cards = new Card[numSuits][numRanks];
    for (int suit = Card.DIAMONDS; suit <= Card.SPADES; suit++) {
        for (int rank = Card.ACE; rank <= Card.KING; rank++) {
            cards[suit-1][rank-1] = new Card(rank, suit);
        }
    }
}

public Card getCard(int suit, int rank) {
    return cards[suit-1][rank-1];
}
}

Answer 1

It's very typical, however I'd say a very bad, custom to have the main method in all the possible tutorials and examples. And yes, it confuses beginners.

Testing code should always be separated from the tested code. In a "normal" code for the good design. In a "school" code for the didactical reason, so the learners are not confused and see the good design even in the school code :)

Answer 2

No, it does not. The main method implemented in this example is intended for testing purposes (note the asserts).