How do I store instances of an `enum class` implementing an interface in a `TreeMap` such a way that...

# getting-started

[Andrew Ebling]

How do I store instances of an enum class implementing an interface in a TreeMap such a way that I can set/get set them without throwing a ClassCastException?

So far I have:

interface MyEnumInterface {}

enum class MyEnumClass: MyEnumInterface { ... }

var myTreeMap: TreeMap<MyEnumInterface, (() -> (Unit))> = TreeMap()

however when I call this method, passing an instance of `MyEnumClass`:

fun add(howToExecute: () -> (Unit), myEnum: MyEnumInterface) {
   myTreeMap[myEnum] = howToExecute // ClassCastException here
}

... I get a ClassCastException.

Why does this happen? Why doesn’t it happen when the instance is passed to the add() method?

What’s the most appropriate resolution please?

[Andrew Ebling]

Is this happening because the compiler is only generating code specific to the interface? Can I add some kind of type constraint to the method signature to say “something which implements MyEnumInterface”?

[diesieben07]

I can't reproduce this with the code you've shared: https://pl.kotl.in/7c_jbm5dL
Can you provide more details?

[diesieben07]

One thing that I can imagine happening is that if you use an implementation of MyEnumInterface that is not also implementing Comparable (which enums do by default). TreeMap requires either the keys to implement Comparable or otherwise be given an external Comparator

[Andrew Ebling]

yes - that’s exactly what I’m doing @diesieben07 and I’m at a loss to explain why it works in the playground but not in my production code

[Andrew Ebling]

this is in an Android app - could that make any difference?

[diesieben07]

I am not too familiar with Android. Can you share what line exactly throws the ClassCastException as well as the message? Does it happen inside TreeMap code?

[Andrew Ebling]

at java.lang.Enum.compareTo(Enum.java:186)
at java.lang.Enum.compareTo(Enum.java:61)
at java.util.TreeMap.put(TreeMap.java:569)
at com.mycompany.MyObject.add(MyObject.kt:42)

[diesieben07]

So you must have more than one entry in the map. Is it all the same enum?

[diesieben07]

Because if not, obviously they cannot be ordered within each other.

[Andrew Ebling]

ah that will be it

[diesieben07]

then you must provide an external Comparator that knows how to compare all possible keys

[Andrew Ebling]

many thanks for your help! 👏

[kqr]

@diesieben07 nice