nonnullability

Nonnullability (also called non-nullability) refers to the property that a value of a given type cannot be null. In languages that support non-nullable types, the type system distinguishes between values that can be absent or null and those that cannot, and it enforces this distinction through compile-time checks or runtime guards. Non-nullability is a key strategy in null safety, aimed at preventing null reference errors.

Implementation approaches vary. In statically typed languages, non-nullable types are often the default, with explicit syntax or annotations to indicate nullable alternatives. For example, Kotlin treats all types as non-null by default and requires a question-mark notation to permit nullability; Swift uses non-optional types and uses optionals to represent possible absence; TypeScript can enable strict null checks to enforce non-nullability across the codebase. In Rust, references are non-null by design and absence is modeled with the Option type. Java and other languages commonly provide annotations such as @NonNull or @Nullable to guide tooling and runtime checks, though enforcement may be weaker without dedicated language support.

Benefits of nonnullability include earlier error detection, improved code readability, and safer APIs. It also simplifies reasoning about ownership and state, especially in concurrent contexts. Challenges include interoperability with legacy code that uses null, API design complexity, and potential verbosity or friction during migration. Some systems trade strict non-null checking for flexibility by offering optional values or runtime guards.

Overall, nonnullability is a central concept in modern type systems and API design, aligning software behavior with explicit intent about the possible absence of values.