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Kotlin Android: How to pass ArrayList of Serializable objects through intent without unchecked cast warning?

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public void showThings(ArrayList<Thing> things) {
    Intent intent = new Intent(this, ThingActivity.class);
    intent.putExtra(THINGS, things);
    startActivity(intent);
Then in the ThingActivity I want to get the ArrayList<Thing>
class ThingActivity {
   var things: ArrayList<Thing>? = null
   override fun onCreate(savedInstanceState: Bundle?) {
       things = intent.extras.getSerializable(OtherActivity.THINGS) as? ArrayList<Thing>
Unfortunately, I can't seem to figure out how to cast to the appropriate type without triggering an "unchecked cast" warning. Is there a way to gracefully set to null if (somehow unexpectedly) the cast fails?
Appending ?: return null does not seem to work as I've seen suggested elsewhere
                In IntelliJ or Android Studio, Alt+Enter, left-arrow on the item, suppress for the method, or class, or some other level. Hides the warning
– Zoe stands with Ukraine
                Oct 12, 2018 at 16:26
                Unfortunately, there's no way with Bundle. This library makes use of generics and implements type-safe data passing between Fragments: github.com/Miha-x64/Flawless
– Miha_x64
                Oct 12, 2018 at 16:32
                Ideally I'd pass whatever test it's expecting me to pass, not a big fan of supressing warnings.
– ray
                Oct 12, 2018 at 16:34
                Do you have access to the source of Thing ? You could implement Parcelable on it and then use the parcelable-related methods of Intent instead of the serializable-related ones.
– Ben P.
                Oct 12, 2018 at 16:49
The unchecked cast warning is happening due to the way Java generics work at runtime. Because of type erasure, at runtime, the type of the list is just List, and not List<Thing>. That means that the cast is considered unsafe, even though it's quite possible for a human to look at the code and see that there's no problem.
While I agree with you that suppressing the warning isn't ideal, in this case I think it's fine.
The best solution, though, would be to implement the Parcelable interface on Thing. That way, when you want to pass a List<Thing> through an intent, you could write:
intent.putParcelableArrayListExtra(THINGS, things)
And when you want to read it back out:
things = intent.extras.getParcelableArrayListExtra(OtherActivity.THINGS)
Neither of these will cause a compiler warning.
                This is good info. I'll read up more about Parcelable it seems that it's recommended for passing custom objects through Intents anyway.
– ray
                Oct 12, 2018 at 18:28
As an alternative to Ben P's answer, you could use Gson.
Assuming that Things is simply a data class (holds a bunch of variables), this will work perfectly (this is also required by Ben P's answer).
Here's a way to implement it:
public void showThings(ArrayList<Thing> things) {
    String json = new Gson().toJson(things);
    Intent intent = new Intent(this, ThingActivity.class);
    intent.putExtra(THINGS, json);
    startActivity(intent);
Then you can get the ArrayList like this:
String json = intent.getStringExtra(THINGS);
TypeToken<ArrayList<Things>> token = new TypeToken<ArrayList<Things>>() {};
ArrayList<Things> things = new Gson().fromJson(json, token.getType());
val intent = Intent(this, SecondActivity::class.java)
val arrayGuide = ArrayList<Guide>()
intent.putParcelableArrayListExtra("arrayInfo",arrayGuide)
startActivity(intent)
Activity two:
if(intent != null){
  val arrayList = 
  this.intent.getParcelableArrayListExtra<Guide>("arrayInfo")
        
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