nonoverridable

Nonoverridable is a term used in object-oriented programming to describe a class member, typically a method, that cannot be overridden by subclasses. Overriding allows a subclass to replace or extend the behavior of a base class method, while nonoverridable members lock in the base class implementation.

Languages implement nonoverridability with specific keywords or modifiers. In Java, a method declared as final cannot be overridden by any subclass. In C++, a virtual function can be declared final to prevent further overrides in derived classes. In C#, a sealed modifier on an overridden method stops further overriding in subclass hierarchies. In Swift and Kotlin, final is used to prevent overriding, with Swift applying it to methods and classes, and Kotlin making non-open members nonoverridable by default unless marked open.

The use of nonoverridable members is typically motivated by design and safety considerations. It helps preserve invariants, guarantees consistent behavior, and can simplify reasoning about a class’s interface. Nonoverridability may also aid performance optimizations, as the compiler or runtime can make stronger assumptions about call targets. However, it reduces extensibility and can complicate testing or mocking, since subclass-based customization becomes impossible or limited.

In practice, developers choose nonoverridable methods when preserving core behavior, ensuring security properties, or enforcing a stable external API is more important than allowing every subclass to alter implementation. Alternative patterns, such as the use of well-defined override points or composition, can provide extensibility without sacrificing the benefits of nonoverridability. See also final, sealed, override, and virtual.