By default, types in method signatures that lack a null annotation are regarded as nullable. This annotation (with it default value true) indicates that types in method signatures that lack a null annotation are regarded as non-null. It is generally good practice to add @NotNullByDefault to all your package declarations (i.e. 'package-into.java' files), because in well-designed code (especially API methods), @NonNull is significantly more frequent than @Nullable.