nullability

Nullability refers to whether a value, such as a reference or a primitive, can be absent or represent a lack of value using a special null value. It is a core concept in programming languages that models optional data and helps manage potential missing values. Distinctions between nullable and non-nullable types help detect and prevent operations on missing values, such as dereferencing null, which can cause runtime errors.

In statically typed languages, nullability is encoded in the type system. C# uses nullable value types (T?) and nullable reference types in newer versions, Kotlin marks nullable types with a question mark (T?), and TypeScript distinguishes possibly null or undefined values under strict null checks. Java historically allowed null for most reference types, with Optional introduced to express absence more explicitly. Swift uses Optional types with nil, and other languages employ similar patterns. In dynamically typed languages, nullability is often checked at runtime and can be more permissive.

Common techniques to manage nullability include using nullable types or Maybe/Option wrappers, providing default or empty values, and performing explicit null checks. Safe navigation or optional chaining operators (such as a?.b) help access members only when the value is non-null. Non-null assertion operators or force unwraps can bypass the type system but carry a risk of runtime errors if misused.

Benefits of explicit nullability include reducing null reference errors and clarifying where values may be missing. Trade-offs include added complexity and potential for fragmented code. Best practices emphasize enabling strict nullability checks, preferring non-null-by-default designs, and using explicit wrappers or defaults rather than relying on nulls. See also: null safety, optional types, and related language features.