In Kotlin, nullability is a part of the type system. By default, any given type T is non-nullable. If you append a "?" to the type, it
becomes nullable: T?.
When accessing properties or functions of a nullable type, you need to handle the case when the target is null. However, while
accessing a non-nullable type, it is redundant to test for null, as the compiler statically ensures that the value can never be
null. So all the nullability checks on the non-nullable types are considered code smells.
On the other hand, performing a null-check on a value that is always null is equally as redundant.
Here is an example of a non-nullable variable. s is of a type String and cannot be null.
val s: String = ""
Here is an example of a nullable variable. Nullable variables are declared by using the ?.
val s: String? = null
Explicit null checks are comparing a result to null using == or != operators. In Kotlin, there are various
other means of implicitly or explicitly performing a null check or assertion, including the following:
- Safe call operator
?.
- Elvis operator
?:
- Not-null assertion operator
!!
-
requireNotNull and checkNotNull functions
