Hi guys. May I know what's the difference between ...

# android

[Ayden]

Hi guys.
May I know what's the difference between findViewById() and findViewById<>()?

[gildor]

findViewById<>() is not valid syntax in Kotlin, are you asking about Java?

[gildor]

this called diamond operator in Java,
https://www.javaworld.com/article/2074080/core-java/jdk-7--the-diamond-operator.html

[Ayden]

I'm writing it in Kotlin.

[gildor]

You cannot write findViewById<>() in Kotlin

[Ayden]

recyclerView = findViewById(R.id.vibe_recycler_view).apply{} Not valid

[gildor]

Only something like findViewById<SomeView>()

[gildor]

Exactly

[Ayden]

recyclerView = findViewById<RecyclerView>(R.id.vibe_recycler_view).apply{} Valid.

[gildor]

Because findViewById can returns different type

[gildor]

in some cases Kotlin can understand what do you expect, sometimes not, depends on case

[gildor]

in this case you explicitly specify that you want to cast result to RecyclerView

[Ayden]

I place <RecyclerView> just to tell the compiler i am confirm this findViewByID is RecyclerView?

[gildor]

Yes, because compiler dont’ know type which you want

[gildor]

see

[gildor]

val recyclerView: RecyclerView = findViewById()

is valid

[gildor]

val recyclerView = findViewById<RecyclerView>()

also valid

[Ayden]

My case is

private lateinit var recyclerView: RecyclerView

recyclerView = findViewById<RecyclerView>(R.id.vibe_recycler_view)

[gildor]

but this is not:

val recyclerView = findViewById()

because compiler doesn’t know type that you want, function findViewById is parameterized

[Ayden]

I see.

[Ayden]

But I already told Kotlin that the recyclerView is RecyclerView

[Ayden]

But why inside of the function I still need to indicate again to tell compiler this findViewById is RecyclerView?

[gildor]

I see you problem

[gildor]

Because findViewById retuns nullable value

[gildor]

this is your problem

[gildor]

findViewById just returns null if cannot find view

[gildor]

use requireViewById instead

[Ayden]

I tried requireViewById also require to pass in explicitly argument.

[gildor]

Works for me

[gildor]

could you show full code snippet?

[gildor]

Yes, because this is just utility method in your case

[Ayden]

You mean the .apply?

[gildor]

no, requireViewById

[Ayden]

So it does not matter for me to use requireViewById or findViewById right?

[gildor]

you can write own extension function

[gildor]

no, you need non-nullable RecyclerView

[Ayden]

💡 I get it now.

[gildor]

You can write something like this to have requireViewById without passing additional param

fun <T : View> Activity.requireViewById(id: Int) = findViewById<T>(id) ?: error("View with id $id not found")

[Ayden]

Let me try it out.

[Ayden]

Thanks @gildor!

[Ayden]

Much appreciated for your time.