Use of adding functions/fields to enum cases in Java other than overriding?

By accident I just discovered that the Java 1.8 compiler allows the following syntax:

enum AnimalType {
    DOG {
        @Override
        public String toString() {
            return "I am a dog";
        }
    },
    CAT {
        @Override
        public String toString() {
            return "I am a cat";
        }

        public void doCatThings() {
            // ...
        }
    },
}

Overriding toString() individually works perfectly fine. A call on an AnimalType.DOG results in the String “I am a dog”.

Apart from this though, I couldn’t find any information on what this enum case customization could be used for. Note the other public method for the case CAT. When defining any other public method or field, it seems like it can’t be accessed from the outside anyway.

So what’s the deal about this? Is this just something that is technically correct syntax but pointless?

Answer

toString is inherited from Object, but behavior is still polymorphic: DOG and CAT “print themselves” differently. Now your own custom methods (like, sound() in the following example) will make much more sense if you ‘ll use an abstract method at the level of AnimalType enum definition:

enum AnimalType {
    DOG {
        @Override
        public String toString() {
            return "I am a dog";
        }

        @Override
        public void sound() {
           System.out.println("Meowww");
        }
     
    },
    CAT {
        @Override
        public String toString() {
            return "I am a cat";
        }

        @Override
        public void sound() {
           System.out.println("Woof, I'm a barking dog");
        }
    };

    abstract void sound();
}

Now you can add polymorphic custom behavior to the enum and use it without known the actual animal:

public class SampleClass {

    public static void doSound(AnimalType animal) {
       ...
       animal.sound();
    }
}

// usage:

SampleClass.doSound(AnimalType.DOG);

The actual usages can vary, out of my head, you can implement finite state machine, parsing if enums are tokens, calculations if enums are geometric figures (like calculate volume of 3d figure) and what not. Its a tool for java programmers like many others. Use it wisely 🙂