You’ll use and encounter nullable types in several areas: Allowing null values in your code is considered a “bad smell.” (When I look at code that permits null values I think, “How quaint, this is like code from 1996.”) Places where you’ll use nullable types The ? operator says, “This instance of this type is allowed to contain a null value.”Īs mentioned in the previous chapter, you should write your code to never use null values. A nullable type is a variation of an existing type, and can contain a null value.Default Kotlin types cannot contain null values.I do this because they’re declared with null values, but at some point later in their lifetime they’ll presumably contain more useful values. Notice that no exceptions are thrown in the REPL when you declare a nullable type: > var s: String? = nullĪlso notice that I intentionally declare those variables as var fields. This simple addition to your type allows your variable to contain null values. You declare a type to be nullable by adding a question mark after it: var s: String? = null Similarly, variables that are instances of custom types can’t be null either: > class Person (var name: String)Įrror: null can not be a value of a non-null type Line_3.PersonĪ nullable type is a variation of a type that permits null values. Variables that are instances of standard Kotlin types can’t contain null values: val> val s: String = nullĮrror: null can not be a value of a non-null type StringĮrror: null can not be a value of a non-null type Int I use String in those statements, but the same things can be said about every other type, including Int, Float, Person, etc. The operations that can be performed on nullable types are very limited.A variable of the nullable type String? can be null.A variable of type String can never be null.

Nullable types are an approach in Kotlin to help you eliminate null values and null pointer exceptions (the dreaded NullPointerException).

Kotlin assign if not null