Kotlin 中的音乐 URI 空指针异常

Question

Greeting all I had been passed over several days over and still can't solve the problem, null pointer

exception error on setting music URI "var musicTrack = ContentUris.withAppendedId(MediaStore.Audio.Media.EXTERNAL_CONTENT_URI,curSong!!)"

enter image description here

 fun PlaySong(){
    mp!!.reset()
    var song = songlist?.get(songPosition!!)
    var curSong = song?.Id


    var musicTrack = ContentUris.withAppendedId(MediaStore.Audio.Media.EXTERNAL_CONTENT_URI,curSong!!)
    try {
        mp!!.setDataSource(applicationContext,musicTrack!!)
    }catch (e: Exception){
        Log.e("Music Service","Error On Setting Data Source",e)
    }
    mp!!.prepareAsync()
}

Answer 1

The stack trace is saying it's line 78 where the problem is happening. mp is probably null and you're trying to call a method on it, at a guess.

Using !! is a bad sign because it means that value could be null, but you're not handling that possibility and saying "trust me, it's never going to be null". That usually doesn't work out! I know this from experience...

Either make mp a non-nullable type if you know it's never going to be null, otherwise you need to null-check it before you do anything with it. Never use !! (the rare times you actually do need it, you'll know you need it and it's an exception to the rule)