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

Question

I'm creating an Intent like so:

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

Answer 1

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.

Answer 2

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());

Answer 3

Activity one:

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")
}