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?

---

• [1] https://github.com/ReactiveX/RxJava
• [2] https://github.com/ReactiveX/RxJavaFX/blob/0.x/src/main/java/rx/observables/JavaFxObservable.java

Answer 1

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. ;)