Unusual “static” method declaration

Question

public class Card {

    public enum Rank { DEUCE, THREE, FOUR, FIVE, SIX,
        SEVEN, EIGHT, NINE, TEN, JACK, QUEEN, KING, ACE }

    public enum Suit { CLUBS, DIAMONDS, HEARTS, SPADES }

    private final Rank rank;
    private final Suit suit;
    private Card(Rank rank, Suit suit) {
        this.rank = rank;
        this.suit = suit;
    }

    public Rank rank() { return rank; }
    public Suit suit() { return suit; }
    public String toString() { return rank + " of " + suit; }

    private static final List<Card> protoDeck = new ArrayList<Card>();

    // Initialize prototype deck
    **static** {
        for (Suit suit : Suit.values())
            for (Rank rank : Rank.values())
                protoDeck.add(new Card(rank, suit));
    }

    public static ArrayList<Card> newDeck() {
        return new ArrayList<Card>(protoDeck); // Return copy of prototype deck
    }
}

I have a quick question. The code block that starts right after the static keyword declaration, what type of method is that ? I haven't ever seen that before. If anyone could enlighten me, that would be greatly appreciated. Thanks.

Answer 1

This is not a method, but a static Initializer block of a class. You can read more about it in the Java Language Specification .

The code within is executed once after loading the class.

Answer 2

As Grzegorz correctly points out, it's a static initializer block .

Here is another resource explaining the difference between class initialization and instantiation , as well as the order in which class variables and static initializer blocks are called.

A related concept is that of instance initializer blocks , which can be used together with anonymous classes for the handy double-brace initialization idiom:

private static final Set<String> VALID_CODES = new HashSet<String>() {{
    add("XZ13s");
    add("AB21/X");
    add("YYLEX");
    add("AR2D");
}};

Answer 3

It is also good to know initialization order, as I remember first in this order comes static variables, than static block. Also important point is time when this static block executes - at the first mention of corresponding class, not at the class instance creation.