Impact of Polymorphism on Program Enhancements

OOPSLA97 Workshop:
Resources for Early Object Design Education

TITLE:

Impact of Polymorphism on Program Enhancements

RESOURCE CONTACT:

Ed C. Epp

Mathematics and Computer Science Department
University of Portland
5000 N. Willamette Blvd.
Portland, OR 97203

epp@uofport.edu

GOAL:

• Concepts - to understand inheritance, polymorphism, dynamic binding
• Design - to understand how inheritance affects the relationships between objects in a hierarchical system
• Implementation - ability to write code that implements inheritance
• Maintenance - ability to modify existing code that utilizes inheritance

AUDIENCE:

End of first semester or Beginning of second semester of a CS1/2 sequence.

PREREQUISITES:

• Understanding of object use
• Ability to implement classes
• Elementary notion of inheritance desirable

MOTIVATION:

This exercise is designed to show that enhancing a system utilizing inheritance makes some problems easier to understand, implement, and enhance.

APPLICABILITY:

Inheritance is a difficult idea to understand, and therefore it is often difficult to motivate students to incorporate inheritance in their programs. Ability to use inheritance requires an understanding of concept as well as experience. Concepts are readily introduced in lecture. The experience is gained in a laboratory setting.

STRUCTURE:

Students are given two systems to enhance: one using inheritance and the other not. They record the effort required to modify each system. Since, the function of this activity is to demonstrate the utility of inheritance, a software system is chosen that benefits from inheritance.

CONSEQUENCES:

The implementation using inheritance is expected to be easiest to modify. Some students may believe it is easier to understand because there is less code. Other students may believe it is more difficult to understand because inheritance, polymorphism, and dynamic binding are new ideas.

IMPLEMENTATION:

1. Software components
   
   This exercise focuses on two implementations of a drawing program. Diagrams are created in a display by selecting one of several drawing tools from a tool palette, for example: rectangle, line, or star. Once the tool is selected, a component is drawn using point and drag.
   
   One of the implementations does not use inheritance. It utilizes a list of glyphs. Each glyph has a tag field that specifies whether it is a rectangle, line, or star. The paint method iterates through the list and, based on the components tag, calls the appropriate paint for rectangle, line, or star.
   
   The second implementation uses inheritance. It utilizes a glyph class which is inherited by rectangle, line, and star. Polymorphism is used to store all three types in a single list. Dynamic binding is used to call paint without needing to determine what kind of glyph is being used.
   
   
2. Activity
   
   Students are required to add an oval glyph to each implementation of draw. They must record each modification made: where the modifications were made, as well as the code that achieved the modification.
   
   
3. Analysis
   
   Students rate the effort required for each modification by assigning x points for each new statement and y points for each method (or other location) requiring modifications. In addition, students provide subjective observations about the process of enhancing these programs. These figures, along with observations, are used evaluate which design is easiest to maintain.

REFERENCES:

Patterns in Computer Science Using Java
by Ed. C. Epp

To be published by McGraw Hill.