nonnullbydefault

Nonnull by default is a naming used to describe a convention in some programming environments where a module, package, or API treats all reference types as non-null unless explicitly marked otherwise. Under this approach, parameters, return values, and fields are assumed to be non-null by default, reducing the need for repetitive null checks and making API contracts clearer. Nullability can be overridden on a per-element basis by annotating specific parameters, return values, or fields as nullable or by using an optional type.

In practice, this default is often implemented in Java and related ecosystems through package-level annotations placed in a package’s information file, such as a package-info.java. The exact annotation class can vary by library or framework, and several naming schemes exist (for example, a general NonnullByDefault or more specific markers like ParametersAreNonnullByDefault). Libraries and tools may also provide separate defaults for return values, fields, or method parameters. Overriding annotations, such as @Nullable, are used when a particular API element should permit null.

The concept aligns with broader efforts to improve null-safety and API clarity. Standards efforts like JSpecify aim to standardize nullability annotations across Java, while popular frameworks and libraries offer their own default annotations (for instance, Spring’s NonNullApi or JetBrains’ annotations). In languages with intrinsic null-safety, such as Kotlin, non-null by default may be enforced by the language itself rather than by external annotations.

Adoption of nonnull-by-default can reduce null-related bugs and clarify API intentions, but it requires consistent application across a codebase and its dependencies to avoid gaps where null values are still possible.