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Creational patterns

The five creational patterns of the GoF catalogue: Singleton, Factory Method, Abstract Factory, Builder and Prototype. Their intent, participants, a code example and their limits.

  • 25 cards
  • 4 topics
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The outline, topic by topic

Each topic is weighted the way the exam weighs it.

  1. Concepts
  2. Questions
  3. Sort and link
  4. Understand

A few cards from the deck

  • Singleton: what is it?

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    Creational pattern that guarantees a class has a single instance and provides a global access point to it.

    The constructor is private and the class keeps its only instance in a static field. The global access point is also its flaw: it hides a dependency and gets in the way of tests.

  • This Singleton lets two instances be created, even in a program without concurrency. Why?

    ```java
    public class Config {
    private static Config instance;
    public Config() { }
    public static Config getInstance() {
    if (instance == null) {
    instance = new Config();
    }
    return instance;
    }
    }
    `

    • The constructor is public: new Config() creates a second instance
    • The instance field is static: it is overwritten on every call to getInstance
    • getInstance is static: each calling class gets its own copy
    • The condition instance == null creates an object on every call
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    The constructor is public: new Config() creates a second instance

    Nothing stops other code from calling the public constructor: uniqueness is no longer guaranteed. It must be declared private. The static field exists only once for the whole class, the static getInstance returns that same object to every caller, and the condition only creates the object on the first call.

  • For a class to have only one instance, its constructor is … and it keeps the instance in a … field.

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    For a class to have only one instance, its constructor is private and it keeps the instance in a static field.

    With a public constructor, any code could call new and create a second instance: the uniqueness guarantee would collapse.

  • Factory Method: what is it?

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    Creational pattern that defines a creation method in a parent class and lets the subclasses decide which concrete class to instantiate.

    The creator's code calls the factory method and only knows the product's type, never its concrete class. Adding a product means adding a subclass.

  • With an Abstract Factory, why can't the application show a light button next to a dark menu?

    • Each widget checks, when created, the theme of the widgets already made
    • All its widgets come from the same concrete factory, chosen only once
    • Light and dark widgets inherit from the same concrete class
    • The client tests the current theme before every call to new
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    All its widgets come from the same concrete factory, chosen only once

    The client never calls new on a widget: it asks a single factory, light or dark, for createButton() and createMenu(). The family's consistency comes from that one choice, not from scattered checks.

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