Singleton Pattern

Finally, let’s look at one other common creational pattern: the singleton pattern. The singleton pattern is a simple pattern that allows a program to enforce the limitation that there is only a single instance of a class in use within the entire program. So, when another class needs an instance of this class, instead of instantiating a new one, it will simply get a reference to the single existing object. This allows the entire program to share a single instance of an object, and that instance can be used to coordinate actions across the entire system.

¹

The UML diagram for the singleton pattern is super simple. The class implementing the singleton pattern simply defines a private constructor, making sure that no other class can construct it. Instead, it stores a static reference to a single instance of itself, and includes a get method to access that single instance.

Let’s look at how this could work in our ongoing example.

Singleton Factory

Let’s update our DeckFactory class to use the singleton pattern.

public class DeckFactory{
    // private static single reference
    private static DeckFactory instance = null;
    
    // private constructor
    private DeckFactory(){
        // do nothing
    }
    
    public static DeckFactory getInstance() {
        // only instantiate if it is called at least once
        if DeckFactory.instance == null{
            DeckFactory.instance = new DeckFactory();
        }
        return DeckFactory.instance;
    }

    public Deck getDeck(DeckType deck) {
        // existing code omitted
    }
}

class DeckFactory:

    # private static single reference
    _instance: DeckFactory = None
    
    # constructor that cannot be called
    def __init__(self) -> None:
        raise RuntimeError("Cannot Construct New Object!")
        
    @classmethod
    def get_instance(cls) -> DeckFactory:
        # only instantiate if it is called at least once
        if cls._instance is None:
            # call `__new__()` directly to bypass __init__
            cls._instance = cls.__new__(cls)
        return cls._instance

    def get_deck(self, deck: DeckType) -> Deck:
        # existing code omitted

class DeckFactory:

    # private static single reference
    _instance: DeckFactory = None

    # new method to construct the instance
    def __new__(cls) -> DeckFactory:
        if cls._instance is None:
            # call `__new__()` on the parent `Object` class
            cls._instance = super().__new__(cls)
        return cls._instance

    def get_deck(self, deck: DeckType) -> Deck:
        # existing code omitted

Using a Singleton

Now we can update our main method code to use our singleton DeckFactory instance instead of creating one when it is needed:

public class CardGame{

    public static void main(String[] args) {
        // ask user for input and store in `deckType`
        String deckType = "Standard 52";
        Deck cards = DeckFactory.getInstance().getDeck((DeckType.valueOf(deckType)));
        // game code goes here
    }
}

from typing import List


class CardGame:

    @staticmethod
    def main(args: List[str]) -> None:
        # ask user for input and store in `deck_type`
        deck_type: str = "Standard 52"
        cards: Deck = DeckFactory.get_instance().get_deck(DeckType(deck_type))
        # Python method described above means the code doesn't change!
        # cards: Deck = DeckFactory().get_deck(DeckType(deck_type))
        # game code goes here

Why would we want to do this? Let’s assume we’re writing software for a multiplayer game server. In that case, we may not want to instantiate a new copy of the DeckFactory class for each player. Instead, using the singleton pattern, we can guarantee that only one instance of the class exists in the entire system.

Likewise, if we need a system to assign unique numbers to objects, such as orders in a restaurant, we can create a singleton class that assigns those numbers across all of the point of sale systems in the entire store. This might be useful in your ongoing class project.