Implementing Interface anonymously in Kotlin results in "has no constructors" error

Question

I'm trying to use a SurfaceView in Android to hold a Camera preview. The documentation tells me that I need to call startPreview in the surfaceCreated callback for the surface holder. I'm trying to set the callback like so

this.surface!!.holder!!.addCallback(SurfaceHolder.Callback() {
    fun surfaceChanged(holder: SurfaceHolder, format: Int, 
                       width: Int, height: Int) {

    }

    fun surfaceCreated(holder: SurfaceHolder) {

    }

    fun surfaceDestroyed(holder: SurfaceHolder) {

    }
})

However, I get the error:

SurfaceHolder.Callback has no constructors.

I'm confused why this doesn't work when something like this does:

Thread(Runnable() {
    fun run() {
        ...        
    }
})

Your second case is wrong as well, you are not actually overriding the `run()` method in Runnable but passing in a lambda that contains a nested `run()` function that is never called. Both examples are invalid. — Jayson Minard, Jun 02 '17 at 22:43
Also for the run example for Runnable, you don't need to specify the interface either and can just use SAM conversion and pass in a lambda `Thread { ... }` — Jayson Minard, Jun 02 '17 at 22:46

Paulo Mattos · Accepted Answer · 2017-07-19T01:56:54.997

10

To create an object of an anonymous subclass you need to use the object: expression:

this.surface!!.holder!!.addCallback(object: SurfaceHolder.Callback {
    override fun surfaceChanged(holder: SurfaceHolder, format: Int, 
                                width: Int, height: Int) {
        ...        
    }

    override fun surfaceCreated(holder: SurfaceHolder) {
        ...
    }

    override fun surfaceDestroyed(holder: SurfaceHolder) {
        ... 
    }
})

and don't forget to use the override keyword per overridden method as well ;)

edited Jul 19 '17 at 01:56

answered Jun 02 '17 at 21:02

Paulo Mattos

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Please add a note about using an `object` expression as well, which was also part of your fix. And link to documentation https://kotlinlang.org/docs/reference/object-declarations.html#object-expressions – Jayson Minard Jun 02 '17 at 22:51
@JaysonMinard Yeah, I forgot to mention that error as well... good catch ;) – Paulo Mattos Jun 02 '17 at 23:46
If `SurfaceHolder.Callback` is interface, then no `()` after. – Jayson Minard Jun 03 '17 at 06:49
Cheers for this, hunted for awhile. shame you can't lamda these though – Joe Maher Jul 19 '17 at 00:26

Implementing Interface anonymously in Kotlin results in "has no constructors" error

1 Answers1