kotlin safe conversion from string to enum

Question

I need to convert strings to Enum values, but want a function which returns null if the string is not an enum.

enum class Colors{
   Red, Green, Blue

}

I can used Colors.valueOf(testString) provided testString is value, but there will be an exception if it is not valid, and I want a null in that case.

Because I want to this often, an extension function would be ideal. But the extension needs to operate on the class Colors, and not an object of type Colors.

Anyone know how to write such an extension? Ideally one that is generic for any enum class.

It is simple to write a top level function, but I am seeking one that acts as the standard 'method' does

// instead of 
val willGetAnException = Colors.valueOf("Yellow") // standard existing fun
val willGetNull = Colors.valueOrNullOf("Orange")  // new fun i seek

And ideally one that is generic and works for any enum

lenooh · Accepted Answer · 2023-03-17 14:51:16Z

37

You don't want an extension since they must be invoked on an existing object. You want a top-level function. There is a built in one You can use:

/**
 * Returns an enum entry with specified name.
 */
@SinceKotlin("1.1")
public inline fun <reified T : Enum<T>> enumValueOf(name: String): T

You can call it by inferring the type, or explicitly:

val a : MyEnumClass = enumValueOf("A")
val b = enumValueOf<MyEnumClass>("B")

However this method is not nullable: it throws java.lang.IllegalArgumentException on unknown values.

But it's easy to mimick its behavior and have it work for nullable enums with a top level function:

inline fun <reified T : Enum<*>> enumValueOrNull(name: String): T? =
    T::class.java.enumConstants.firstOrNull { it.name == name }

edited Mar 17, 2023 at 14:51

lenooh

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answered Aug 3, 2018 at 0:34

Pawel

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3 Comments

innov8 Over a year ago

Thanks... that is about the point i got to as well..... if i make that function a method of the companion object of the class (and take out the generic) I can call it the way i wish.

Pawel Over a year ago

If you want it to be limited to single enum class you could have added method in companion object that wraps valueOf with a try-catch block.

innov8 Over a year ago

Thanks... that is about the point i got to as well. A top level function which is not what i want. Sort of one step further ..... if i make that function a method of the companion object of the enum class (and take out the generic) I can call it the way i wish. But how to have a generic method apply to the companion object of all enum classes is what i was hoping for,

Roberto · Accepted Answer · 2020-08-21 12:13:40Z

32

Colors.values().find { it.name == "Yellow" }

answered Aug 21, 2020 at 12:13

Roberto

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4 Comments

con Over a year ago

a verbal explanation is often helpful

Emanuel Moecklin Over a year ago

This is a very simple and elegant solution without the need for extension functions. You can also define a default value using the Elvis operator: Colors.values().find { it.name == "Yellow" } ?: Colors.Red

Mick Sear Over a year ago

Best solution IMHO. If you want to default it then just do val colour = Colors.values().find { it.name == "Yellow" } ?: Colors.UNKNOWN or something like that.

Mark Kahn Over a year ago

Similar to @MickSear, I use Colors.values().firstOrNull { it.name == "Yellow" }?: Colors.UNKNOWN

Luís Gonçalves · Accepted Answer · 2021-02-09 06:09:58Z

19

You can use something like this :

inline fun <reified T : Enum<T>> String.asEnumOrDefault(defaultValue: T? = null): T? =
    enumValues<T>().firstOrNull { it.name.equals(this, ignoreCase = true) } ?: defaultValue

Then: "Yellow".asEnumOrDefault(Colors.Green)

Or, if you it can't be infered: "Yellow".asEnumOrDefault<Colors>()

edited Feb 9, 2021 at 6:09

Luís Gonçalves

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answered Dec 11, 2019 at 20:03

Fab

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Comments

Tatsuya Fujisaki · Accepted Answer · 2023-07-12 02:02:45Z

3

enum class Colors {
    BLACK, WHITE, UNKNOWN;

    companion object {
        // Verbose for illustrative purposes
        fun fromOrdinal(ordinal: Int): Colors = entries[ordinal]
        fun fromOrdinalOrNull(ordinal: Int): Colors? = entries.getOrNull(ordinal)
        fun fromOrdinalOrDefault(ordinal: Int): Colors = entries.getOrElse(ordinal) { UNKNOWN }
        fun fromName(name: String): Colors = valueOf(name.uppercase())
        fun fromNameOrNull(name: String): Colors? = entries.find { it.name == name.uppercase() }
        fun fromNameOrDefault(name: String): Colors = entries.find { it.name == name.uppercase() } ?: UNKNOWN
    }
}

edited Jul 12, 2023 at 2:02

answered Aug 10, 2022 at 16:29

Tatsuya Fujisaki

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Comments

Braian Coronel · Accepted Answer · 2021-05-13 05:18:36Z

1

The Kotlin API does not work by simply using <reified T: Enum<T>>. It throws an exception of the type InvocationTargetException. So I pass directly to type: Class<T> by parameter.

private fun <T> enumValueOf(type: Class<T>, enum: String) : T {
    return type.enumConstants.first { it.toString() == enum }
}

Using

if (type.isEnum) enumValueOf(#Field.type, value as String)

answered May 13, 2021 at 5:18

Braian Coronel

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Comments

ymkjp · Accepted Answer · 2020-06-10 06:31:46Z

Given the fact it's not easy to access the Enum value safely in Kotlin, I published a library enum-or-null-kt to provide a collection of shorthand functions which makes you can write code like below:

class Example {
  enum class Direction(val az: Int) {
    NORTH(0),
    EAST(90),
    SOUTH(180),
    WEST(240)
  }

  fun printAz01(name: String = "EAST") {
    val direction = enumValueOrNull<Direction>(name) ?: Direction.EAST
    println("az01=${direction.az}")
  }

  fun printAz02(name: String = "EAST") {
    val direction = name.toEnumOrNull<Direction>() ?: Direction.EAST
    println("az02=${direction.az}")
  }

  fun printName01(az: Int = 0) {
    val direction = enumValueOrNull<Direction> {
      it.az == az
    } ?: Direction.NORTH
    println("name03=${direction.name}")
  }

  fun printName02(ordinal: Int = 0) {
    val direction = enumValueOrNull<Direction> {
      it.ordinal == ordinal
    } ?: Direction.NORTH
    println("name03=${direction.name}")
  }
}

With it, not only can you access the Enum value with names, but also you can pass an arbitrary higher-order function as a predicate clause. That is convenient when you need to deal with a custom conversion such as JPA attribute converters.

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