* Null Safety

In kotlin it's impossible to have a =null= value.
To help prevent issues with =null= values in your programs, Kotlin has null safety in place.
Null safety detects potential problems with =null= values at compile time, rather that at run time.

** Nullable types

Kotlin supports nullable types which allows the possibility for the declared type to have =null= values.
By default, a type is *not* allowed to accept =null= values.
Nullable types are declared by explicitly adding =?= after the type declaration.

Example

#+begin_src kotlin
  fun main() {
      var neverNull: String = "This can't be null"

      // Throws compiler error
      neverNull = null

      var nullable: String? = "you can keep a null here"

      nullable = null
  }
#+end_src

** Check for null values

You can check for the presence of =null= values within conditionals expressions.
In the following example, the =describeString()= function has an =if= statement that checks whether =maybeString= is *not* =null= and if its =length= is greater than zero

#+begin_src kotlin
  fun describeString(maybeString: String?): String {
      if (maybeString != null && maybeString.length > 0) {
	  return "String of length ${maybeString.length}"
      } else {
	  return "Empty or null string"
      }
  }

  var nullString: String? = null
  println(describeString(nullString))
#+end_src

#+RESULTS:
: Empty or null string

** Elvis operator

You can provide a default value to return if a =null= value is detected by using the *Elvis operator* =?:=

#+begin_src kotlin
  var nullString: String? = null
  println(nullString?.length ?: 0)
#+end_src

#+RESULTS:
: 0