Method overriding

Method overriding is a language feature in which a class can provide an implementation of a method that is already provided by one of its parent classes. The implementation in this class replaces (that is, overrides) the implementation in the parent class.

When you define a method with the same name as that of a parent class, that new method replaces the inherited definition. The new method must have the same return type and take the same number and type of parameters as the method you are overriding. Here’s an example:

@interface MyClass : NSObject {

}

- (int)myNumber;

@end

@implementation MyClass : NSObject {

}

- (int)myNumber {

    return 1;

}

@end

@interface MySubclass : MyClass {

}

- (int)myNumber;

@end

@implementation MySubclass

- (int)myNumber {

    return 2;

}

@end

If you create an instance of MyClass and send it a myNumber message, it returns 1. If you create an instance of MySubclass and send it a myNumber message, it returns 2.

Notice that the subclass’s method must have the same name and parameter list as the superclass's overridden method.

In addition to completely replacing an existing implementation, you might want to extend a superclass’s implementation. To do this, you can invoke the superclass’s implementation using the super keyword.

Within a method definition, super refers to the parent class (the superclass) of the current object. You send a message to super to execute the superclass’s implementation of a method. You often do this within a new implementation of the same method to extend its functionality. In the following example, the implementation of myNumber by MySubclass simply adds 1 to whatever value is returned by the implementation of MyClass.

@implementation MySubclass

- (int)myNumber {

    int subclassNumber = [super myNumber] + 1;

    return subclassNumber;

}

@end