but instead it prints "null" in case lat is indeed null. If println() did not accept null values, this call would fail, is declared as String? and Kotlin knows it may be null. The second "null" is because the value itself of lat Otherwise, it assumes nullableLocation is safe The first "null" is because the ? operator will check if May print "null", "null", or the lat String value. 100% sure it'll print the corrisponding String value Fine, may print "null" or the value, if present.
Val nullableLocation: Location? = getLocation() val location: Location = getSafeLocation() In this case you can safely stick with ?, knowing that this operator is your friend if you are unsure about whether what you're referencing will be null. Sometimes it is not even up to you to receive a null or non-null value. However, a certain variable being null is something you developer may not know. We just don't know what it will be, and the Kotlin environment strictly makes sure that you are handling the eventuality of that value being null, as the type of our references variable is defined as nullable. is nullable, thus the Kotlin Compiler infers that accessing a nullable property can lead to NPEs. This happens because in the first case, the object before ?. Println(location.lat) // Compile-time error. Otherwise, null is retuned from the statement. If you use it in a call chain, it is checking that your code chain goes onto the next element just if the previous element is not null. data class User(val lat: String?, val lon: String) Let's also say Location class has a property called lat, which is a nullable String, and a lon non-nullable String. In Kotlin, this is represented as val location: Location? Suppose you have a class, called Location, which we will declare in a nullable variable.
We could explain the full theory behind those two operators, but I believe an example is really all you need. The eventuality of having null values is one of these, and Kotlin, as we'll see, handles this in a really smart manner. Of course, targeting the JVM brings many challenges to a programming language. In Kotlin, you have to be very specific about what you want to do with null values, because the language is designed to be null-safe out of the box. It depends on the behavior you want to achieve. Here which null safety operator I should use? You have hit one of the best (and most useful) points while coding in Kotlin. (And about rethinking a little bit your code), otherwise, use ?. If mLastLocation is never null, feel safe about using !!. operator is meant to be used only when you are sure that the previous chain operation result is not null. Use it when you are unsure about the chain nullability.
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