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You're missing the default value parameter

@dovahkiin98 What do you mean? Where?

@gabrielhuff when you use intent.getStringExtra("extra", "") you provide a default value, it's not always 0 or "" or false, sometimes it's different.
I would suggest this implementation :

inline fun <T> Activity.extra(key: String, defaultValue: T) = lazy {
    when(T::class.java) {
        Int::class.java -> intent.getIntExtra(key, defaultValue) as T
        Float::class.java -> intent.getFloatExtra(key, defaultValue) as T
...
    }
}

val intExtra: Int by extra("intExtra", 2)

@dovahkiin98 The current implementation allows you to define default values. The only difference is that we are getting them from a function instead of a raw value:

val intExtra by extra("intExtra") { 2 }

Note that you can omit the default value function. If so, the implementation will throw an IllegalArgumentException if no extra was passed.

Delegates are an expensive hammer to use for this. I don't think we've decided whether their use is appropriate for simple things like this.

IMHO delegates (in this case, specifically lazy instantiation) are perfect for this. Consider a scenario in which you need to access the same extra on different scopes. What would be the most appropriate way to do it?

A lateinit property is an order of magnitude less expensive.

I have no idea what "same extra on different scopes" means.

By different scopes I mean different methods / contexts. We don't want to read the same extra from the input intent more than once (not because of performance - just because the call might be kinda ugly):

override fun onCreate(state: Bundle?) {
  ...
  val extra = intent.getParcelableExtra("my_key") as MyCustomExtra
}

fun onSomethingElse() {
  val extra = intent.getParcelableExtra("my_key") as MyCustomExtra // Eww
}

But I get what you're saying. Performance-wise, lateinit would be ideal. However, the property would have to be a var and would require an additional line of code somewhere else for instantiation.