Java enum with no instances

Question

In RXJava [1] there is an enum [2] defined as

public enum JavaFxObservable {
    ; // no instances


    public static void staticMethod() {
        // ...
    }
}

What's the purpose this technique using a enum with no instances? Why not use a standard class?

Not sure really, but i think it might be clean way to prevent from extending or creating instance — user902383, Sep 30 '14 at 10:56
This "class" is just a repository of static functions (and/or constants). An enum cannot be extended, is a final class. The alternative would be to make a final or abstract class with a private constructor. Which is more verbose. BTW enum is also used for a singleton, with concurrency safe initialisation. — Joop Eggen, Sep 30 '14 at 10:58
"Why not use a standard class?" can you give an example of how such a standard class would be better/simpler? — Peter Lawrey, Sep 30 '14 at 10:58
Saves you from writing a `private` constructor and marking the class `final`. e.g. This can be used to create helper classes which only contain static methods without being instantiable (And it also reduces the amount boilerplate code that you'd normally write) — Matt, Sep 30 '14 at 10:58

Peter Lawrey · Accepted Answer · 2014-09-30T11:02:59.763

What's the purpose this technique using a enum with no instances?

You are defining, in the simplest way, that this is a class which has no instances, i.e. it is utility class.

Why not use a standard class?

An enum is a class. It is also final with a private constructor. You could write

public final class JavaFxObservable {
    private JavaFxObservable() {
        throw new Error("Cannot create an instance of JavaFxObservable");
    }
}

But this is more verbose and error prone. e.g. I have seen this in real code

public final class Util {
    private Util() {
    }

    static void someMethod() { }

    static class DoesSomething {
         void method() {
             // WAT!? we can still create this utility class
             // when we wrote more code, but it's not as good.
             new Util().someMethod(); 
         }
    }
}

The comments are mine. ;)

Java enum with no instances

1 Answers1