Kotlin’s idiomatic approach to Null Safety Computer scientist Said: Tony Hoare I call it my billion-dollar mistake. It was the invention of the null reference in 1965 The objects introduces a fundamental problem with type system. For e.g If you declare a object as String, it doesn’t guarantee that the value is real or . nullable String null We normally skip checks based on our assumptions in control flow of code. But when we are wrong, the code crashes with . Java 8 introduced Optionals to deal with Nullable objects. But it has some flaws null Null Pointer Exception Why Java Optionals is not a greatway to handle nulls? Wrapping and unwrapping objects into Optional class makes code verbose. Also since it was introduced only in JDK8, you still need to deal with null values returned from older JDK methods. How Kotlin handles null safety? If you are designing a new language, How will you handle null references? Won’t you add a explicit type definitions for . That’s what Kotlin does. nullables By default all objects are , So you can’t store in them. But if a variable can hold null values, you have to explicitly append to indicate it’s . Then, Kotlin forces you to call methods on them using safe call operator non-nullable nulls ? nullable ?. How to replace Java Optionals in Kotlin? Anything you can do with Java Optionals can be done idiomatic way in Kotlin. For e.g: To do transformation can be replaced with safe call operation Optional.map(...) ?. To filter non from a collection, we can use method from Collection library. nulls filterNonNull() To execute a piece of code only if value is , You can use function in Kotlin. function is useful to execute a block of code when the value is . non-null let let non-null To assign a default value in case of null, you can use elvis operator. ?: Elvis operator can also be used to throw exception in case of null ?: The below code shows Java Optionals and their equivalents in Kotlin. How Kotlin ensures null safety when mixing with Java code? Kotlin interoperates nicely with Java methods that accepts objects as parameters. It does it using annotations and which you can import them from package. nullable @Nullable @NotNull org.jetbrains.annotations If a method param is not annotated, Kotlin compiler can’t prevent you from passing , so it can result in NPE if you pass null. null If a method param is annotated with , Kotlin compiler won’t allow you to pass value. @NotNull null If a method param is annotated with , Kotlin compiler will allow you to pass value, but it could result in NPE if not handled safely. @Nullable null The below example shows Java method with annotations and how it behaves when invoked from Kotlin. Always use and annotations when calling Java code from Kotlin. @Nullable @NotNull Do you really need to use nullable objects? You can avoid passing nullable type as method parameters by simply not invoking method if parameter value is . null While working with collections, you can replace nullable variables with empty collections. If you need to represent absence of an object. For e.g next node in Linked List or left node in tree. Instead of null references use Null Object Design Pattern That will leave you with only reason to use reference is returning value from a method in case of boundary conditions and errors. Even this can be avoided by using functional style . null Try Monad Don’t use nullable types unless it is absolutely needed References Book: Kotlin in Action
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