03. Creational Patterns

Creational Patterns

A creational design pattern is any design pattern that concerns how objects in your program are created. These patterns can help you manage the creation of objects as your code becomes increasingly complex.

Singleton Pattern

When Are Singletons Useful?

You may want to use a singleton if:

• A class that has only one instance, but no clear owner.
• You want that instance to be available everywhere in your code.
• The instance is initialized only when it's first used (also known as lazy initialization).

Singleton Example

Demo Code

import java.util.Objects;

public final class Database {
    private static Database database;

    private Database() {}

    public static Database getInstance() {
        if (database == null) {
            database = new Database();
            database.connect("/usr/local/data/users.db");
        }
        return database;
    }

    // Connects to the remote database.
    private void connect(String url) {
        Objects.requireNonNull(url);
    }

    public static void main(String[] args) {
        Database a = Database.getInstance();
        Database b = Database.getInstance();

        System.out.println(a == b);
    }
}

By the way, you probably noticed that this Database class doesn't actually connect to a remote database. The purpose of the code is to demonstrate the singleton design pattern as simply as possible.