Singleton

Motivation: Some classes must have exactly one instance (e.g., a file system, a database connection pool, a window manager).

Intent: Ensure a class has only one instance and provide a global point of access to it.

Here, we enforce a single source of truth.

object DatabaseConnection {
    fun query(sql: String): List<String> {
        // ...
        return listOf()
    }
}
 
// Usage---only one instance ever exists
val result = DatabaseConnection.query("SELECT * FROM users")

Then we pass around this single connection.

class Singleton {
    // keep the constructor private to prevent other classes from instantiating it
    private Singleton() { /* ... */ }
 
    private static Singleton instance = null;
    public static Singleton getInstance() {
        if (instance == null) { instance = new Singleton(); }
        return instance;
    }
}