CODEWITHSUNDEEP
typeskotlinmismatch
Yan Rad
Yan Rad

Reputation: 23

Kotlin type mismatch !! and?

I just recently started studying kotlin and when I changed Webrtc release to a newer one(1.0.22920) encountered following problem:

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

Here is the part of code where the error occurs:

val rtcConfig = PeerConnection.RTCConfiguration(iceServers)
peerConnection = peerConnectionFactory.createPeerConnection(rtcConfig, getPeerConnectionMediaConstraints(), videoPeerConnectionListener)

Most likely this is due to the fact that in the Webrtc library createPeerConnection became @Nullable:

@Nullable
      public PeerConnection createPeerConnection(RTCConfiguration rtcConfig, MediaConstraints constraints, Observer observer) {
        long nativeObserver = PeerConnection.createNativePeerConnectionObserver(observer);
        if (nativeObserver == 0L) {
            return null;
        } else {
            long nativePeerConnection = nativeCreatePeerConnection(this.nativeFactory, rtcConfig, constraints, nativeObserver);
            return nativePeerConnection == 0L ? null : new PeerConnection(nativePeerConnection);
        }
    }

Attempt to put ? and !! in different places did not work.

I think that only my poor knowledge of kotlin separates me from solving the problem, can you help me?

Upvotes: 2

Views: 10743

Answers (2)

Zoe - Save the data dump
Zoe - Save the data dump

Reputation: 28288

Since the method is nullable, this means it has a chance of returning null. This means you cannot declare it as a non-null PeerConnection. You have three choices here:

  1. Declare the var as PeerConnection? - this means you accept nullable values
  2. Use non-null assertion with !!. At the end of the call to createPeerConnection, add !!. This throws an exception if it's null.
  3. Use the Elvis operator - add ?: [something to do here if it returns null without brackets] after the call to createPeerConnection.

If you need PeerConnection to be non-null, I recommend you pick the last option. It's also a recommended option if you're checking for null, as:

var someNullableVar = getPossibleNull()
if(someNullableVar == null) someNullableVar = whatever();

is the same as:

var nonNullVar = getPossibleNull() ?: whatever()

You can also return and stop continued execution, but if you want to throw an exception, use !! instead of the Elvis operator.

If you pick option #1 though, note that this means you'll have to do null checks on calls later.

Upvotes: 2

andras
andras

Reputation: 3655

Most probably because earlier you have declare the variable as not nullable:

var peerConnection: PeerConnection

And that means you cannot assign a @Nullablevalue to that variable. Change that to:

var peerConnection: PeerConnection?

Or you can force the value being returned to be non-null, (which I do not recommend) in which case:

peerConnection = peerConnectionFactory.createPeerConnection(rtcConfig, getPeerConnectionMediaConstraints(), videoPeerConnectionListener)!!

Notice the !! at the end.

Upvotes: 3

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