CODEWITHSUNDEEP
syntaxenumskotlin
Julian A.
Julian A.

Reputation: 11450

Is there a way to nest an enum within a data class in Kotlin?

Is there a way to nest an enum within a data class in Kotlin? 

data class D(val a:Any) {
    enum class E {F,G}
    ...
}

Or to declare it inline within a function? 

fun foo() {
    enum class E {F,G}
    doSomething()
}

I can't find documentation on the rules for where enums are allowed to be declared.

Upvotes: 13

Views: 19365

Answers (2)

guenhter
guenhter

Reputation: 12167

Yes, you can nest the enum in a data class, but not in a function:

data class Outer(val a: InnerEnum) {
    enum class InnerEnum { A, B }
}

fun foo() {
    val o = Outer(Outer.InnerEnum.A)
    println(o) // --> Outer(a=A)
}

Upvotes: 18

holi-java
holi-java

Reputation: 30676

There is no kotlin specification for the syntax at present. if you want to find the specification you can see JLS, because Kotlin is based on Java, so some specifications also appropriate in Kotlin.

A nested enum type is implicitly static. It is permitted for the declaration of a nested enum type to redundantly specify the static modifier.

This implies that it is impossible to declare an enum type in the body of an inner class (§8.1.3), because an inner class cannot have static members except for constant variables.

and, all local classes are inner classes.

So the enum class can be declared anywhere except the local function scope and inner classes.

If you don't sure where can define a type you can try to prompt the scope in turn: local > class > top, then the kotlin compiler will give you the correct compiler error message to you. for example:

IF you define a const val in the local function, the compiler report the error as below:

fun local() {
    const val foo="bar"
    // ^--- the modifier `const` is not applicable to `local variable`  
}

IF you define a const val in the common class/interface, the compiler will report the error as below:

interface Foo {
  const val foo = "bar"
  //^--- `const val` only allowed on top-level or objects.
}

Upvotes: 6

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