Programming Fundamentals › Object-Oriented Programming

Method Overriding

A subclass replacing a parent class's method.

Also known as: overriding, override

Method overriding is when a subclass provides its own version of a method that the parent class already defines. Calls on the subclass use the new version.

class Notifier:
    def send(self, msg):
        print("log:", msg)

class EmailNotifier(Notifier):
    def send(self, msg):                 # overrides Notifier.send
        print("email:", msg)

n = EmailNotifier()
n.send("hi")                             # email: hi

The parent’s version still exists, and you can call it:

class LoggingEmail(EmailNotifier):
    def send(self, msg):
        super().send(msg)                # parent behavior, plus more
        audit(msg)

Why it matters

It’s the mechanism behind polymorphism. Code written against Notifier works with any subclass, and each runs its own send:

def alert(notifier: Notifier):
    notifier.send("disk full")           # whichever kind it really is

Rules to follow

• Keep the same meaning. The override must still do what callers of the parent expect (same inputs, a compatible result, no surprising errors), or substituting the subclass breaks things (Liskov substitution).
• Keep the same signature. Different parameters make it a different method, or an overload.
• Mark intent where the language allows: @Override in Java, override in C# and TypeScript, so a typo can’t create a new method silently.
• Call super when the parent’s work is still needed.
• Parents should say what’s meant to be overridden, or use an abstract class to require it.

Prefer composition when you override a lot (composition over inheritance).