Learning Kotlin: Nullable Types

Robert MacLean

25 June 2018

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The next Koan looks at how Kotlin handles nulls, and it does it wonderfully; Null is explicitly opt-in. For example, in C# you can assign null to a string variable but in Kotlin unless you say you want to support nulls, which you do by adding a trailing question mark to the class, you cannot.

Their example in this Koan is a nice example:


fun test() {
    val s: String = "this variable cannot store null references"
    val q: String? = null
if (q != null) q.length      // you have to check to dereference
val i: Int? = q?.length      // null
val j: Int = q?.length ?: 0  // 0

}

Let us dig into the Koan, where we get given the following Java code:


public void sendMessageToClient(@Nullable Client client, @Nullable String message, @NotNull Mailer mailer) {
    if (client == null || message == null) return;
PersonalInfo personalInfo = client.getPersonalInfo();
if (personalInfo == null) return;

String email = personalInfo.getEmail();
if (email == null) return;

mailer.sendMessage(email, message);

}

and we need are going to rewrite it using the Nullable language features of Kotlin, which looks like:


fun sendMessageToClient(client: Client?, message: String?, mailer: Mailer) {
    val email = client?.personalInfo?.email
    if (email == null || message == null) return
mailer.sendMessage(email, message)

}

The big changes from Java:

The @NotNull attribute for mailer is no longer needed
The @Null attribute for the other parameters becomes the question mark
We do not need to pre-check client before calling the parameter, as you can use the null safe operator ?. to ensure you only check before we call the method.
Unfortunately, the null safe operator in Kotlin doesn’t support calling methods on null objects as C# currently does with its Elvis operator

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