类型不匹配。 必需：数组 <Uri> ！ 找到：数组 <Uri?>

Question

The ? and !! syntax is very confusing in kotlin! I'm declaring a value callback as a private member of my activity:

private lateinit var mFilePathCallback: ValueCallback<Array<Uri>>

And I assign it on onShowFileChooser on onCreate method like so:

    override fun onShowFileChooser(
    webView: WebView?,
    filePathCallback: ValueCallback<Array<Uri>>?,
    fileChooserParams: FileChooserParams?
): Boolean {
    mFilePathCallback = filePathCallback!! //Assigned here
    val intent = Intent(Intent.ACTION_GET_CONTENT)
    intent.type = "image/*"
    val PICKFILE_REQUEST_CODE = 100
    startActivityForResult(intent, PICKFILE_REQUEST_CODE)
    return true
}

But when I try to use it in onActivityResult like this:

override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
    super.onActivityResult(requestCode, resultCode, data)

    if(data != null && resultCode == Activity.RESULT_OK) {
        val resultsArray = arrayOfNulls<Uri>(1)
        resultsArray[0] = data.data
        mFilePathCallback.onReceiveValue(resultsArray)
        Log.d("ACTIVITY RESULT", data.data.toString())
    } else {
        Log.d("ACTIVITY RESULT", "Cannot get file path.")
    }
}

I get the following error in onRecieveValue function call: Type mismatch. Required: Array<Uri>! Found: Array<Uri?> Type mismatch. Required: Array<Uri>! Found: Array<Uri?> Type mismatch. Required: Array<Uri>! Found: Array<Uri?> This is so confusing!

Answer 1

1. arrayOfNulls<Uri>(1) returns Array<Uri?> because its elements can be null and are null to start with.

2. Setting an element with resultsArray[0] = data.data doesn't change the type (why would it?).

3. So you are passing an Array<Uri?> to mFilePathCallback.onReceiveValue which expects an Array<Uri> (an array whose elements are all Uri which can't be null).

Instead of the cast as in your own answer, just create the array of the correct type to start with:

val uri = data.data
if (uri != null) {
    val resultsArray = arrayOf<Uri>(uri)
    mFilePathCallback.onReceiveValue(resultsArray)
    Log.d("ACTIVITY RESULT", uri.toString()) // why toString?
} else {
    // your original code doesn't cover this case
    // so decide what to do here
}

Note that if you just do

val resultsArray = arrayOf<Uri>(data.data)

you should ideally get a warning because data.data can be null, but it doesn't seem to be properly marked as @Nullable .

Answer 2

Kotlin is very strict on types. As Alexey Romanov already mentioned , when you create an array with arrayOfNulls , that creates an Array<Uri?> .

A nullable type can contain the type itself, but a non-null type cannot contain null. Conversion from Array<Uri> to Array<Uri>? is fine for an instance. However, due to variance in generics, you can't pass Array<Uri> as an argument of the type Array<Uri?> . More on that here .

You need an Array<Uri>! . The ! is a special syntax you can't use, but that indicates a platform call to a method with either a return type or argument with an undefined nullability type. What's important to keep in mind here is that you can pass a non-null type to it, or a nullable one for that matter. ! just means the compiler doesn't know, so it doesn't strictly enforce null-safety in the same way as it usually would.

For an instance, this method:

public void someMethod(String str) {}

Will have the str parameter interpreted as a String! in Kotlin. Add @Nullable and it's String? , or @NonNull and it's String .

The key point, as outlined in the previous link, the method is interpreted as a potentially nullable array of non-null URIs:

Array   <Uri>                            !
^ Type  ^ Generic type (here: non-null)  ^ signifies the type (NOT the generic type) is potentially nullable (not normally useable)

Anyway, the main issue here is that you're trying to pass Array<Uri> to Array<Uri?> - as outlined in the post I linked earlier, if you want to support that, you need to add out Uri? as the parameter. The method being a part of the stdlib means you can't do that.

If you're completely sure you'll never get a null type in the array after it's populated, you can, as you outlined in your answer, cast it. That being said, I'd highly recommend you use the safe cast operator in combination with let to avoid a crash if you get null. In case you didn't check, that can happen :

 Returns URI The URI of the data this intent is targeting or null. 

You can go with something like Alexey Romanov mentioned - Run an if-else statement to check if data is null, and put it directly into an array using arrayOf if it isn't null. That being said, you should still handle getData() returning null, even if you go with the as approach.

Answer 3

The ? and !! syntax is very confusing in kotlin!

They are not Confusion both Symbol have different Meaning Kotlin.

To Explain Both Symbol :

Lets Take :

Symbol ? and it known as Safe Nullable cast operator

This Symbol work as Nullable. Means when you try to assign Value in Kotlin

val myString : String? = null

and Difference

val myString : String = null // This will give Error So you have to put string with null value (e.g.[""]).

and The Other Symbol !! and it is called as Null Safety :

It means Not Nullable.

When you pass have to Pass Value according to it's datatype. Means if it asks for string you have to pass String.

But when it receives null value or wrong response.

Then it will give you

NullPointerException

If you have any query then just comment it down i'll clear for you.

Answer 4

我要做的就是将结果数组转换为Array<Uri> ：

mFilePathCallback.onReceiveValue(resultsArray as Array<Uri>)