How will you handle null references if you are designing a new language?

Kotlin’s idiomatic approach to Null Safety

Computer scientist Tony Hoare Said:

I call it my billion-dollar mistake. It was the invention of the null reference in 1965

The nullable 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 String or null.

We normally skip null checks based on our assumptions in control flow of code. But when we are wrong, the code crashes with Null Pointer Exception. Java 8 introduced Optionals to deal with Nullable objects. But it has some flaws

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 nullables. That’s what Kotlin does.

By default all objects are non-nullable, So you can’t store nulls in them. But if a variable can hold null values, you have to explicitly append ? to indicate it’s nullable. Then, Kotlin forces you to call methods on them using safe call operator ?.

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 Optional.map(...) can be replaced with safe call operation ?.
• To filter non nulls from a collection, we can use filterNonNull() method from Collection library.
• To execute a piece of code only if value is non-null , You can use let function in Kotlin. let function is useful to execute a block of code when the value is 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 nullable objects as parameters. It does it using annotations @Nullable and @NotNull which you can import them from org.jetbrains.annotations package.

1. If a method param is not annotated, Kotlin compiler can’t prevent you from passing null, so it can result in NPE if you pass null.
2. If a method param is annotated with @NotNull , Kotlin compiler won’t allow you to pass null value.
3. If a method param is annotated with @Nullable , Kotlin compiler will allow you to pass null value, but it could result in NPE if not handled safely.

The below example shows Java method with annotations and how it behaves when invoked from Kotlin.

Always use @Nullable and @NotNull annotations when calling Java code from Kotlin.

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 null reference is returning value from a method in case of boundary conditions and errors. Even this can be avoided by using functional style Try Monad.

Don’t use nullable types unless it is absolutely needed

References

Book: Kotlin in Action