11.2 Subtype Polymorphism vs Explicit Higher Order Functions

We've seen how inheritance lets us reuse existing code in a superclass while implementing small modifications by overriding a superclass's methods or writing brand new methods in the subclass. Inheritance also makes it possible to design general data structures and methods using polymorphism.

Polymorphism, at its core, means 'many forms'. In Java, polymorphism refers to how objects can have many forms or types. In object-oriented programming, polymorphism relates to how an object can be regarded as an instance of its own class, an instance of its superclass, an instance of its superclass's superclass, and so on.

Consider a variable deque of static type Deque. A call to deque.addFirst() will be determined at the time of execution, depending on the run-time type, or dynamic type, of deque when addFirst is called. As we saw in the last chapter, Java picks which method to call using dynamic method selection.

Suppose we want to write a python program that prints a string representation of the larger of two objects. There are two approaches to this.

1. Explicit HoF Approach

def print_larger(x, y, compare, stringify):
    if compare(x, y):
        return stringify(x)
    return stringify(y)

2. Subtype Polymorphism Approach

def print_larger(x, y):
    if x.largerThan(y):
        return x.str()
    return y.str()

Using the explicit higher order function approach, you have a common way to print out the larger of two objects. In contrast, in the subtype polymorphism approach, the object itself makes the choices. The largerFunction that is called is dependent on what x and y actually are.