Non-nullable type
A non-nullable type is a type whose values are statically constrained not to include the null value.
Kotlin in Action, 2E
A non-nullable type is a Kotlin type whose values are not permitted to be null by the type system.
- How it is used
- Establishes compile-time boundaries between required values and values that may be absent.
- How deep it goes
- Explains declarations, smart casts, subtyping, generics, delayed properties, and Java overrides without detailing generated checks further.
- Examples
- Uses string-length functions, email sending, generic processors, Java interface overrides, and JUnit-style service setup.
- What you can do after
- Apply it by declaring required values non-nullable and converting nullable values only after an explicit safe path.
Pro ASP.NET Core 7, Tenth Edition
A non-nullable variable is a variable whose type does not permit the special null value.
- How it is used
- Prevents null reference mistakes by making permitted null states explicit and turning violations into compiler warnings.
- How deep it goes
- Explains declarations, required initialization, defaults, and array nullability, including how element and container annotations differ.
- Examples
- Uses Product.Name, Product arrays, Kayak and Lifejacket objects, null elements, required initializers, and string.Empty defaults.
- What you can do after
- Apply nullable annotations, required members, and defaults to make C# object and collection null behavior compile-time explicit.
Read how Pro ASP.NET Core 7, Tenth Edition explains non-nullable type →
Related concepts
- nullable typecontrasts with · A nullable type is a type whose values may be either a value of the underlying type or an explicit representation of absence, such as null.
- !! operatorproduced by · The !! operator is Kotlin's non-null assertion operator, which treats a nullable expression as non-null and throws an exception if it is null.
- missing valuecontrasts with · A missing value is a marker or condition indicating that an expected data value or reference is absent, unavailable, unknown, or inapplicable.
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