Extend a class

Use extends to create a subclass, and super to refer to the superclass:

class Television {
  void turnOn() {
    _illuminateDisplay();
    _activateIrSensor();
  }
  // ···
}

class SmartTelevision extends Television {
  void turnOn() {
    super.turnOn();
    _bootNetworkInterface();
    _initializeMemory();
    _upgradeApps();
  }
  // ···
}

For another usage of extends, see the discussion of parameterized types on the Generics page.

Subclasses can override instance methods (including operators), getters, and setters. You can use the @override annotation to indicate that you are intentionally overriding a member:

class Television {
  // ···
  set contrast(int value) {...}
}

class SmartTelevision extends Television {
  @override
  set contrast(num value) {...}
  // ···
}

An overriding method declaration must match the method (or methods) that it overrides in several ways:

• The return type must be the same type as (or a subtype of) the overridden method's return type.
• Parameter types must be the same type as (or a supertype of) the overridden method's parameter types. In the preceding example, the contrast setter of SmartTelevision changes the parameter type from int to a supertype, num.
• If the overridden method accepts n positional parameters, then the overriding method must also accept n positional parameters.
• A generic method can't override a non-generic one, and a non-generic method can't override a generic one.

Sometimes you might want to narrow the type of a method parameter or an instance variable. This violates the normal rules, and it's similar to a downcast in that it can cause a type error at runtime. Still, narrowing the type is possible if the code can guarantee that a type error won't occur. In this case, you can use the covariant keyword in a parameter declaration. For details, see the Dart language specification.

To detect or react whenever code attempts to use a non-existent method or instance variable, you can override noSuchMethod():

class A {
  // Unless you override noSuchMethod, using a
  // non-existent member results in a NoSuchMethodError.
  @override
  void noSuchMethod(Invocation invocation) {
    print('You tried to use a non-existent member: '
        '${invocation.memberName}');
  }
}

You can't invoke an unimplemented method unless one of the following is true:

• The receiver has the static type dynamic.

• The receiver has a static type that defines the unimplemented method (abstract is OK), and the dynamic type of the receiver has an implementation of noSuchMethod() that's different from the one in class Object.

For more information, see the informal noSuchMethod forwarding specification.