Enum constants are instantiated when an enum is referenced for the first time. Therefore, that allows to implement Singleton software design pattern with a single-element enum.

public enum Attendant {

    INSTANCE;

    private Attendant() {
        // perform some initialization routine
    }

    public void sayHello() {
        System.out.println("Hello!");
    }
}

public class Main {

    public static void main(String... args) {
        Attendant.INSTANCE.sayHello();// instantiated at this point
    }
}

According to “Effective Java” book by Joshua Bloch, a single-element enum is the best way to implement a singleton. This approach has following advantages:

• thread safety
• guarantee of single instantiation
• out-of-the-box serialization

And as shown in the section implements interface this singleton might also implement one or more interfaces.