Why does this Kotlin code still require ?. to access members?

Question

I have the following function in a Kotlin class:

private fun raiseResponse(ex: Exception, resObj: JSONObject) {
    if (_activity == null || _responseHandler == null) return

    _activity.runOnUiThread {
        _responseHandler.invoke(ex, resObj)
    }
}

Where both _activity and _responseHandler are defined as:

var _activity: Activity? = null
var _responseHandler: ((Exception, JSONObject) -> Unit)? = null

However, the line that calls runOnUiThread will only compile if I use ?. instead of ..

I've tried Google searching but I'm afraid my terminology is lacking. I assumed my null check on the first line of raiseResponse was enough to ensure that the ?. was unecessary?

hotkey · Accepted Answer · 2017-06-01T01:41:02.063

First, note that the check and the usage get the value of the property twice. Since _activity is a var, it can be reassigned (potentially, by another thread) between the check and the usage. It is not null-safe to use the property value, because it can be not the value that was checked, and the compiler requires you to check the value again.

^{In other words, the compiler cannot smart-cast _activity to a non-nullable type.}

You can fix this by storing the value in a local variable (it cannot be changed from outside, so the compiler can prove it's the same value):

val activity = _activity
if (activity == null || _responseHandler == null) return

activity.runOnUiThread {
    _responseHandler.invoke(ex, resObj)
}

Or, equivalently, use some idiom like ?.let { ... }:

_activity?.let { activity ->
    if (_responseHandler == null) return

    activity.runOnUiThread { // safe, because it is surely the same value
        _responseHandler.invoke(ex, resObj)
    }
} ?: return

Why does this Kotlin code still require ?. to access members?

1 Answers1