Question

I’m looking at this code of the correct way to do a singleton in java: What is an efficient way to implement a singleton pattern in Java?

I’m a little confused, how do you add a method to a enumeration?

 public enum Elvis {
       INSTANCE;
       private final String[] favoriteSongs =
           { "Hound Dog", "Heartbreak Hotel" };
       public void printFavorites() {
           System.out.println(Arrays.toString(favoriteSongs));
       }
   }

And the enumeration in the code above, doesn’t even make sense to me, you have the symbol:

INSTANCE;

how is that a correct line?

I’m coming from c#, and this syntax got me curious and I’m hoping someone can explain or make sense of the above.

I guess it means java has a more purer idea of an enumeration as it can have behaviour?

Answer 1

An enum in Java is actually a class. One that implicitly extends java.lang.Enum. The special syntax you see at INSTANCE; is a declaration of an enum constant. This is going to be used to make an instance of your enum class which can be referred to in code. As a matter of fact, you can even have a non-default constructor for your enum and use it in the constant declaration. Example:

public enum Test {

    INSTANCE("testing");

    private final String content;
    public Test(final String content) {
        super();
        this.content = content;
    }

}

Treating enums as true objects has certain advantages. Putting some logic into enums is convenient, but shouldn’t be abused either. The singleton pattern via enums is a nice solution in my opinion, though.