Kotlin Tour

Nullability

Nullable types

https://kotlinlang.org/docs/reference/null-safety.html#nullable-types-and-non-null-types

All types in Kotlin are non nullables (they don't include the null value).
To do so the type must be explicitly marked as nullable with the ? sign.

Safe calls

https://kotlinlang.org/docs/reference/null-safety.html#safe-calls

Calling a function on a nullable value might raise a NullPointerException.
Thanksfully Kotlin has a safe call operator : ?.

Elvis

https://kotlinlang.org/docs/reference/null-safety.html#elvis-operator

What if we don't want to stay in the dangerous nullable world and escape from it ?
We can thank Elvis for that and use the operator ?: allowing to specify a non-null value to use in case the value is null.

I ❤️ (K)NPE

https://kotlinlang.org/docs/reference/null-safety.html#the--operator

Finally if really we want to raise a NullPointerException we can use the !! operator.

Safecast

https://kotlinlang.org/docs/reference/null-safety.html#safe-casts

In Kotlin the keyword as is used to cast an object into a specific type. If the cast fail a ClassCastException is raised, as? can be used to return a null value if the cast failed.

Smart casts

https://kotlinlang.org/docs/reference/typecasts.html

Using the operators is and !is we can check if an object is of a given type. What's really nice is that with smart casts the compiler infer the type of our object after these operators.