Overriding
A subclass can provide its own custom implementation of an instance method, type method, instance property, type property, or subscript that it would otherwise inherit from a superclass. This is known as overriding.
Overriding by accident can cause unexpected behavior, and any overrides without the override keyword are diagnosed as an error when your code is compiled.
The override keyword also prompts the Swift compiler to check that your overriding class’s superclass (or one of its parents) has a declaration that matches the one you provided for the override. This check ensures that your overriding definition is correct.
Preventing Overrides
You can prevent a method, property, or subscript from being overridden by marking it as final. Do this by writing the final modifier before the method, property, or subscript’s introducer keyword (such as final var, final func, final class func, and final subscript).
Any attempt to override a final method, property, or subscript in a subclass is reported as a compile-time error. Methods, properties, or subscripts that you add to a class in an extension can also be marked as final within the extension’s definition.
You can mark an entire class as final by writing the final modifier before the class keyword in its class definition (final class). Any attempt to subclass a final class is reported as a compile-time error.