i have error in android studio with kotlin

问题:

why this error?

09-12 16:36:31.502 1886-1886/com.getloction.nourmedhat.smartgate.getlocation E/AndroidRuntime: FATAL EXCEPTION: main Process: com.getloction.nourmedhat.smartgate.getlocation, PID: 1886 kotlin.TypeCastException: null cannot be cast to non-null type kotlin.String at com.getloction.nourmedhat.smartgate.getlocation.All_Station$onCreate$2.onDataChange(All_Station.kt:56) at com.google.android.gms.internal.to.zza(Unknown Source) at com.google.android.gms.internal.vj.zzHX(Unknown Source) at com.google.android.gms.internal.vp.run(Unknown Source) at android.os.Handler.handleCallback(Handler.java:751) at android.os.Handler.dispatchMessage(Handler.java:95) at android.os.Looper.loop(Looper.java:154) at android.app.ActivityThread.main(ActivityThread.java:6682) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1520) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1410)

回答1:

As commented by AlexTa you're getting a TypeCastException because you cast null to a non-null type.

In Kotlin types are "@NotNull" by default. So if you declare a variable like this:

var yourVar: String

you cannot assign null to it. If it should be capable of storing null (which it often shouldn't) you declare it like this:

var yourVar: String? = null