Platform Types

Kotlin is nullsafe, but there are things to keep in mind when using Kotlin from Java. Kotlin has an implementation that accepts it as T! Type called Platform types. For example, it is as follows.

fun getString(key:String, defValue:String) = sharedPref.getString(key:String, defValue:String) // platform types. return value is T!
fun getString(key:String, defValue:String): String? = sharedPref.getString(key:String, defValue:String) // Declare it to be nullable

The above is a simplified method of SharedPreferences#getString. but SharedPreferences#getString returns @Nullable String. On the Kotlin side, since it is received as String:T!, there is a possibility of crashing at runtime.

By describing it as @Nullable on the Java side, platform types and warnings are displayed on the IDE side. in an environment where Kotlin and Java coexist, it is necessary to understand platform types. Let’s explicitly declare whether it is @Nullable in Java.