Circular reference in Kotlin Enum

Question

How can I create an enum class with circular references?

Simple example (taken from this Java question):

enum class Hand(val beats: Hand) {
    ROCK(SCISSORS), // Enum entry 'SCISSORS' is uninitialized here
    PAPER(ROCK),
    SCISSORS(PAPER);
}

Answer 1

As reassignment is forbidden for val properties, this problem is generally hard to solve and often indicates problems in your data model. For a discussion in broader context refer to this question/answer(s).

However, this simple example can be solved using a val property with custom getter (thus without a backing field). Using when, the getter can be defined in a very readable way:

enum class Hand {
    ROCK,
    PAPER,
    SCISSORS;

    val beats: Hand
        get() = when (this) {
            ROCK -> SCISSORS
            PAPER -> ROCK
            SCISSORS -> PAPER
        }
}

An alternative solution (analogue to the answer by Eugen Pechanec) is to use sealed classes. Due to their less constrained concept, the implementation is¹ slightly simpler and more readable compared to an enum with overridden properties.

sealed class Hand {
    abstract val beats: Hand

    object ROCK: Hand() {
        override val beats = SCISSORS
    }

    object PAPER: Hand() {
        override val beats = ROCK
    }

    object SCISSORS: Hand() {
        override val beats = PAPER
    }
}

---

¹personal opinion

Disclaimer: I have no information on how these solutions work in conjunction with classic Java.

Answer 2

An alternative to mhoff's answer without flow control statements:

enum class Hand {
    ROCK {
        override val beats: Hand
            get() = SCISSORS
    },
    PAPER {
        override val beats: Hand
            get() = ROCK
    },
    SCISSORS {
        override val beats: Hand
            get() = PAPER
    };

    abstract val beats: Hand
}