Check type of ArrayList in Kotlin

Question

Kotlin provides Array.isArrayOf() for checking if an array is of a certain type.

It's used like this

if(object.isArrayOf<String>())

And defined like this

/**
 * Checks if array can contain element of type [T].
 */
@Suppress("REIFIED_TYPE_PARAMETER_NO_INLINE")
public fun <reified T : Any> Array<*>.isArrayOf(): Boolean =
    T::class.java.isAssignableFrom(this::class.java.componentType)

But it's only for Array. I need to check ArrayList.

I thought to change the signature like so.

@Suppress("REIFIED_TYPE_PARAMETER_NO_INLINE")
public fun <reified T : Any> ArrayList<*>.isArrayListOf(): Boolean =
    T::class.java.isAssignableFrom(this::class.java.componentType)

but class.java.componentType is specific to Array

How can I check what type of ArrayList I have?

I should clarify, I only care if its one of 3 types, so I don't need a completely open-ended way of checking.

Why are you checking using ArrayList? It's not the base implementation — Zoe - Save the data dump
– Zoe - Save the data dump, Commented Nov 29, 2018 at 19:15
I need to direct it into the appropriate Bundle method. bundle.putStringArrayList(), bundle.putIntegerArrayList(), ect — lbenedetto
– lbenedetto, Commented Nov 29, 2018 at 19:16
Lists in general rely on generics. Because of type erasure, you can't check the generic type it has. — Zoe - Save the data dump
– Zoe - Save the data dump, Commented Nov 29, 2018 at 19:17

Kevin Meneses Palta · Accepted Answer · 2020-09-18 23:03:33Z

8

If you want to check the type of a list you can do:

when (list.firstOrNull()) {
   is String -> { /*do something*/ }
   is Int -> { /*do another thing*/ }
   else -> { /*do something else*/ }
}

And if you need to use the list of a certain type you can use:

list.filterInstance</*the type you need*/>()

Hope this works for you.

edited Sep 18, 2020 at 23:03

answered Sep 18, 2020 at 21:19

Kevin Meneses Palta

951 silver badge5 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Alexey Romanov · Accepted Answer · 2018-11-29 19:18:28Z

4

You can't. Arrays are the only generic type for which this is possible (because they aren't really generic in the same sense, Kotlin just hides it).

The only thing you can do is look at its contents, but of course

that won't work for empty lists;
if a list contains e.g. a String, it could be ArrayList<String>, ArrayList<CharSequence>, ArrayList<Any>, etc.

For this purpose:

I need to direct it into the appropriate Bundle method. bundle.putStringArrayList(), bundle.putIntegerArrayList(), ect

neither should be a problem, I believe.

answered Nov 29, 2018 at 19:18

Alexey Romanov

172k38 gold badges329 silver badges509 bronze badges

6 Comments

lbenedetto Over a year ago

Explain how that isn't a problem? I have an object and I don't know if its an ArrayList of Parcelable or Integers. I can't just pass one into the other. I know they all just end up in a HashMap<String, Object> anyway, but I still need to call the right method.

lbenedetto Over a year ago

Also, if it's an ArrayList of something that you can't put in a Bundle, then it would fail.

Alexey Romanov Over a year ago

As I said, check the contents. If the first element is e.g. an Integer, cast to ArrayList<Int> etc. If the list is empty, then it doesn't matter what you cast it to, as all put* methods will just store 0 length.

Alexey Romanov Over a year ago

Actually, after checking, since Bundle.put* methods don't look at the contents at all and just put the argument into a map, you can ignore them as well and just cast to ArrayList<String>. This could change in a future version, I guess.

lbenedetto Over a year ago

Yeah it puts them in a map, but at some point it writes that map out into a string of some kind so it can be stored on disk. And whatever is doing THAT probably cares that it's one of the valid types.

|

Rod_Algonquin · Accepted Answer · 2018-11-29 19:23:57Z

0

If the list is of one type then you can convert the list to array using: toTypedArray() and after you can then check the type using: isArrayOf

But this would be inefficient since you are converting the list to array, better if you can just directly guess or retrieved the first item of the list.

answered Nov 29, 2018 at 19:23

Rod_Algonquin

26.2k7 gold badges54 silver badges63 bronze badges

2 Comments

lbenedetto Over a year ago

I thought of retrieving the first item and checking, but the list might be empty

Alexey Romanov Over a year ago

And the bigger problem is that toTypedArray needs to know the type of the collection in the first place. If you have an ArrayList<*>, you'll just get Array<Any>.

Collectives™ on Stack Overflow

Check type of ArrayList in Kotlin

3 Answers 3

Comments

6 Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

6 Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related