Type mismatch: inferred type is BluetoothDevice? but BluetoothDevice was expected

Question

I know that this is a very simple error but I can't quite comprehend the nuances of kotlin enough to solve it just yet.

I'm getting the error:

Type mismatch: inferred type is BluetoothDevice? but BluetoothDevice was expected

When trying to pass a BluetoothDevice in as parcelable

class ScanDevice(var device: BluetoothDevice, var rssi: Int): Parcelable {
   ...
    constructor(parcel: Parcel) : this(
        parcel.readParcelable(BluetoothDevice::class.java.classLoader), <--- here
        parcel.readInt()
    )
    ...

It's technically just a warning and not actually an error but I'm not sure what is happening here.

score 2 · Accepted Answer · answered Apr 10 '19 at 17:17

The documentation for Parcel.readerParcelable() says:

Returns the newly created Parcelable, or null if a null object has been written.

This means that, if we were living in Kotlin world, the return type of Parcel.readParcelable() would be T? and not T (i.e. it would be nullable).

Since you are in control of the writing, you know that it won't return null. That means that you're safe to convert the result to a non-nullable type by using the !! operator:

parcel.readParcelable(BluetoothDevice::class.java.classLoader)!!

Ask an easy question and you get an easy answer. I'll make it harder next time! Thanks! — luckyging3r, Apr 10 '19 at 17:25

score 1 · Answer 2 · answered Apr 10 '19 at 17:16

This is because Parcel.readParcelable is nullable. If the Parcel had previously had a null value written to it at that position, null would be returned. In your case, if you can guarantee that you are never writing null to the Parcel, you can simply assert that the value is not null:

constructor(parcel: Parcel) : this(
    parcel.readParcelable(BluetoothDevice::class.java.classLoader)!!,
    parcel.readInt()
)

This would crash at runtime if a null value were written to the Parcel.

score 1 · Answer 3 · answered Apr 10 '19 at 18:23

1

Instead of using !! it would probably be better to to perform a null safe call like this.

parcel?.let{
    it.readParcelable(BluetoothDevice::class.java.classLoader) 
    it.readInt()
}

This only executes the code when parcel is not null. Only use !! when you are 100% sure that variable/object is not null.

answered Apr 10 '19 at 18:23

Korbinian

23
5

I like that too – luckyging3r Apr 10 '19 at 19:29

Type mismatch: inferred type is BluetoothDevice? but BluetoothDevice was expected

3 Answers3