Singleton pattern

Purpose

The singleton pattern ensures that at most one instance of a class can be created.

For example, we might have a SmartHome class containing things like the name of the smart home, a list of devices, and so on. We would only want one of these objects to exist at any one time, otherwise we could have conflicting settings.

Implementation

In Java, the Singleton pattern is typically implemented by making the constructor private and providing a static method to access the single instance:

public class SmartHome {
    private static SmartHome instance;
    private String homeName;
    private int securityMode;

    private SmartHome(String name, int mode) {
        this.homeName = name;
        this.securityMode = mode;
    }

    public static SmartHome getInstance(String name, int mode) {
        if (instance == null) {
            instance = new SmartHome(name, mode);
        } else {
            System.out.println("A smart home already exists.");
        }
        return instance;
    }

    // Additional methods...
}

Now, only one instance of SmartHome can be created, and subsequent calls to getInstance will return the existing instance.

java

public class Main {
    public static void main(String[] args) {
        // Create an instance
        SmartHome smartHome = SmartHome.getInstance("Mojo Dojo", 2);
        System.out.println(smartHome.getHomeName());

        // Retrieve the instance again
        SmartHome sameHome = SmartHome.getInstance("Casa House", 1);
        System.out.println(sameHome.getHomeName());
        System.out.println(smartHome == sameHome); // true

        // Attempt to create a new instance directly (will cause a compile-time error)
        SmartHome secondHome = new SmartHome("Another Home", 3);
        // Error: constructor is private
    }
}

output

SmartHome { homeName: 'Mojo Dojo', securityMode: 2 }
SmartHome { homeName: 'Mojo Dojo', securityMode: 2 }
true
file:///home/shai/Repos/corndel/smart-home/models/SmartHome.js:6
      throw new Error('A settings object already exists.')
            ^

Error: A settings object already exists.