How Does a Java Enum Implement the Singleton Pattern?

Implementing Singleton with an Enum: Unveiling the Mechanism

The concept of implementing the Singleton design pattern using an enum in Java raises questions regarding instantiation. To delve into the inner workings, let's examine the code snippet provided:

public enum MySingleton {
     INSTANCE;   
}

Initially, it's imperative to note that the above enum declaration creates an implicit empty constructor. However, for clarity's sake, let's make it explicit:

public enum MySingleton {
    INSTANCE;
    private MySingleton() {
        System.out.println("Here");
    }
}

Now, let's create a test class with a main() method to observe the behavior:

public static void main(String[] args) {
    System.out.println(MySingleton.INSTANCE);
}

Upon executing this code, you'll witness the following output:

Here
INSTANCE

The question now arises: how is MySingleton instantiated? The answer lies in the nature of enum fields. They act as compile-time constants but are simultaneously instances of the corresponding enum type. Their construction occurs when the enum type is referenced for the very first time, which happens within the main() method in our case.

Therefore, it's crucial to understand that enum fields are essentially instances of their enum type, providing a convenient mechanism to enforce the Singleton pattern without the need for explicit instantiation using new.

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