SimpleDecoder constructor is incompatible with Kotlin (nullness)

[robin2046]

I am using Exoplayer in my Kotlin app. When I try to make my own decoder which extends the SimpleDecoder class (coming from the ExoPlayer core library), I got build error.

The dependencies of ExoPlayer I am using:

implementation "com.google.android.exoplayer:exoplayer-core:2.12.1"
implementation "com.google.android.exoplayer:exoplayer-common:2.12.1"

The constructor of SimpleDecoder is as following:

public abstract class SimpleDecoder<I extends DecoderInputBuffer, O extends OutputBuffer, E extends DecoderException>
implements Decoder<I, O, E> {
 /**
  * @param inputBuffers An array of nulls that will be used to store references to input buffers.
  * @param outputBuffers An array of nulls that will be used to store references to output buffers.
  */
  protected SimpleDecoder(I[] inputBuffers, O[] outputBuffers)
}

And in my TestDecoder.kt I was trying to passing array of nulls to the super class:

class TestDecoder : SimpleDecoder<DecoderInputBuffer, SimpleOutputBuffer, DecoderException>(arrayOfNulls(16), arrayOfNulls(16)) 

And the error is:

TestDecoder.kt: (8, 93): Type mismatch: inferred type is Array<DecoderInputBuffer?> but Array<(out) DecoderInputBuffer> was expected

However, if I create a bridge class in Java inside my project, like following SimpleDecoderBridge.java:

public abstract class SimpleDecoderBridge<I extends DecoderInputBuffer, O extends OutputBuffer, E extends DecoderException> extends SimpleDecoder<I, O, E> {
    protected SimpleDecoderBridge(I[] inputBuffers, O[] outputBuffers) {
        super(inputBuffers, outputBuffers);
    }
}

Then make my TestDecoder extends from this bridge class, there is no build error.

class TestDecoder : SimpleDecoderBridge<DecoderInputBuffer, SimpleOutputBuffer, DecoderException>(arrayOfNulls(16), arrayOfNulls(16))

Could anyone here point out what is going on here? Why does the error happen and why does it not happen later? Am I doing it correctly(passing arrayOfNulls(size) to super) in Kotlin? If not, what is the correct way to do so? Thanks a lot.

[icbaker]

[I'm not a Kotlin expert, you may get better help asking this question in a more Kotlin-focussed context like Stack Overflow]

This error looks like it's related to Kotlin's handling of covariance and contravariance:
https://kotlinlang.org/docs/tutorials/kotlin-for-py/generics.html#declaration-site-covariance-and-contravariance

TestDecoder.kt: (8, 93): Type mismatch: inferred type is Array<DecoderInputBuffer?> but Array<(out) DecoderInputBuffer> was expected

I don't have an easily available setup to test Kotlin, so I don't know if this will work, but can you try explicitly listing the type parameter in your call to arrayOfNulls()? (based on my reading of this: https://kotlinlang.org/docs/reference/inline-functions.html#reified-type-parameters)

class TestDecoder : 
    SimpleDecoder<DecoderInputBuffer, SimpleOutputBuffer, DecoderException>(
        arrayOfNulls<out DecoderInputBuffer>(16), 
        arrayOfNulls(16)) 

I don't think this question is strictly related to ExoPlayer, I'll leave it open for a while in case anyone else has suggestions.

[robin2046]

Thanks for the reply.
I tried to do as you suggested:

class TestDecoder() : 
    SimpleDecoder<DecoderInputBuffer,
        SimpleOutputBuffer,
        MediaCodecRenderer.DecoderException>(
    arrayOfNulls<out DecoderInputBuffer>(16),
    arrayOfNulls<out SimpleOutputBuffer>(16)
)

Still get the same error.

I also tried to ask the same question in StackOverflow before open a question here, but there was not even a comment.

[ojw28]

I don't know if this is correct, but it does at least compile:

abstract class TestDecoder : SimpleDecoder<DecoderInputBuffer, SimpleOutputBuffer, DecoderException>(
  arrayOfNulls<DecoderInputBuffer>(16) as Array<out DecoderInputBuffer>,
  arrayOfNulls<SimpleOutputBuffer>(16) as Array<out SimpleOutputBuffer>)

[robin2046]

Thanks for the suggestion.
Despite of the warning "Unchecked cast: Array<DecoderInputBuffer?> to Array", it does compile without error. May be I need to file a issue to Kotlin team? But I don't know if this is really an issue. Because as you see if I declare a bridge class in Java extending from the SimpleDecoder class, this issue will not happen. Is there by any chance that this is related to any special build/compile/package flags/methods/tweaks used by the released ExoPlayer library?

[icbaker]

Ah I have another theory.

Is there by any chance that this is related to any special build/compile/package flags/methods/tweaks used by the released ExoPlayer library?

We have package-info.java files which mark everything as non-null by default, which includes forcing all collections/arrays to contain no nulls. This is partly to help with Kotlin inter-op (where otherwise everything from Java is maybe-null).

We use @Nullable (and similar) annotations to explicitly mark types that should be allowed to be, or contain, nulls. It looks like we're missing annotations on the constructor parameters in SimpleDecoder (since they're explicitly documented as being an array of nulls). I'll work on fixing that.

(Sorry for the wrong diagnosis above, I didn't spot the crucial ? inside the <> and got distracted with the out keyword instead)

The reason your bridge Java class works (I think) is that it's presumably not in a package with the same package-info.java file, so it allows nulls from Kotlin (and nothing is checking where your Java class passes up to SimpleDecoder).

[icbaker]

Ah it's been pointed out to me that this is a duplicate of #6792 - and correctly annotating SimpleDecoder isn't easy/possible at the moment.

I'm going to close this - please follow #6792 and in the meantime you'll need to keep casting I'm afraid.

[robin2046]

Thanks a lot for the info. It is OK for me to use casting. But the more important thing is to get to know the root cause. Appreciate it.