Thread safety, "array of singletons".

From:
"Daniel Pitts" <googlegroupie@coloraura.com>
Newsgroups:
comp.lang.java.programmer
Date:
25 Mar 2007 14:26:01 -0700
Message-ID:
<1174857961.405686.28380@o5g2000hsb.googlegroups.com>
I have a "Card" class which has objects that represent cards out of a
standard poker deck. I statically initialize the array.

My basic question is: Are there circumstances in which the following
code will not produce "==" comparable Card objects? These objects may
be passed through RMI or otherwise serialized.

<sscce>
import java.io.InvalidObjectException;
import java.io.ObjectStreamException;

public final class Card {
  static final long serialVersionUID = 1L;

  public enum Suit {
    DIAMOND,
    CLUB,
    HEART,
    SPADE;
  }

  public static final int SUITS = Suit.values().length;
  public static final int CARDS_PER_SUIT = 13;
  public static final int NUM_CARDS = SUITS * CARDS_PER_SUIT;
  private static final Card[] cards = new Card[NUM_CARDS];
  private static final String[] numbers = new String[NUM_CARDS];

  public static final int JACK_ORDINAL = 10;
  private static final int QUEEN_ORDINAL = JACK_ORDINAL + 1;
  private static final int KING_ORDINAL = QUEEN_ORDINAL + 1;

  static {
    numbers[0] = "A";
    for (int i = 1; i < JACK_ORDINAL; ++i) {
      numbers[i] = String.valueOf(i + 1);
    }
    numbers[JACK_ORDINAL] = "J";
    numbers[QUEEN_ORDINAL] = "Q";
    numbers[KING_ORDINAL] = "K";
    for (int i = 0; i < NUM_CARDS; ++i) {
      cards[i] = new Card(i);
    }
  }

  public final transient Suit suit;
  public final transient String number;
  public final transient int value;
  public final int ordinal;

  private Card(int ordinal) {
    this.ordinal = ordinal;
    final int suitOrdinal = ordinal / CARDS_PER_SUIT;
    final int cardOrdinal = ordinal % CARDS_PER_SUIT;
    value = cardOrdinal == 0 ? KING_ORDINAL : cardOrdinal - 1;
    suit = Suit.values()[suitOrdinal];
    number = numbers[cardOrdinal];
  }

  protected final Object clone() throws CloneNotSupportedException {
    throw new CloneNotSupportedException();
  }

  protected final Object readResolve() throws ObjectStreamException {
    if (ordinal < 0 || NUM_CARDS <= ordinal) {
      throw new InvalidObjectException("Not a valid card.");
    }
    return forOrdinal(ordinal);
  }

  public static Card forOrdinal(int ordinal) {
    return cards[ordinal];
  }
}
</sscce>

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