Concluding comments

Pedagogically, the HTMLgen problem is not only intellectually exciting, it is very motivating, because it is perceived by students as practical. The HTMLgen.py class library is well written and easy to read. It is a significant piece of open source software that students can benefit not only from using, but also from studying critically.

This paper shows that Java's and Python's underlying object models are very different; it is useful for students to compare these differences and see how they impact their solutions to problems such as HTMLgen.

There are other differences between Java's and Python's object orientation styles. Some examples include:

• In both languages the first argument of an instance method must be a reference to the instance, in Java this argument is implicit and must not be declared; in Python it must be explicitly declared. This was briefly mentioned in Section 2.

• Java forces constructor chaining (often implicitly),
  meaning that a constructor call is guaranteed to call the constructors of all superclasses. In contrast, if a Python constructor wishes to call its ancestors' constructors, it must do so explicitly. This was demonstrated in Section 7. To wit, in line 28, Href's __init__ needs to call HTMLElement's __init__ explicitly. The comparison of the implicit versus explicit instance reference and constructor chaining deepens students' understanding of such concepts.

• The above example also shows that a Python constructor is just a method (line 28), while in constrast, a Java constructor is not.

• Java allows a class to subclass from only one class and any number of interfaces; a Python class can subclass from any number of classes. Python's multiple inheritance is easy to understand and use.

• Finally, a Python class instance can dynamically add or delete a method, instance variable or superclass; Java cannot.

It is beyond this paper's scope to make an exhaustive comparison.