An Introduction to Object Oriented Programming

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Object Oriented Programming

Use a responsibility-driven approach
- Start by thinking about things and the way they are used
- Describe things in terms of their responsibilities
Focus on understanding real-world concepts
- A car carries people and things between locations
- A music system organizes and plays songs
Think about data and processes together

The OOP Community

A Warning:
- Some people/textbooks "lump" various concepts together in order to simplify the discussion
- Some people/textbooks) "lump" concepts together becuase they actually do not understand the differences/distinctions/subtleties
What About JMU?
- The faculty at JMU (generally) agree on what these concepts mean and how they should be used
- You should feel free to disagree with the JMU "school of thought" if you can argue your case well
- While you are here, you should follow the JMU "school of thought" (especially on graded assignments)

Key Concepts: Classes and Objects

Class:
- A definition of a set written in terms of the characteristics of the elements (rather than by listing the elements)
Object:
- A particular element (or instance) of the set

Key Concepts: Encapsulation

The Concept:
- The process of combining the attributes and behaviors/activities/functionalities of an entity in a program unit
The History:
- In the "early days", this wasn't done at all.
- Abstract Data Types (ADTs) popularized this idea

Key Concepts: Abstraction

The Concept:
- Focusing on the important characteristics for the problem/question at hand and ignore other characteristics
A Car Class for a Parking Application:
A Car Class for an Ordering Application:

Key Concepts: Information/Data Hiding

The Concept:
- Hide the internal details of a component from all other components
Consider a Weight Class:
- If we were to give a Weight class to someone they could use it improperly (e.g., assign negative values to ounces or pounds; assign 16 or greater to ounces)
A Related Term:
- An Application Programmer's Interface (API) is the visible/accessible portion of a class

Key Concepts: Characteristics of Classes

A Reminder:
- A class is a definition of a set using the characteristics of the elements
What About Characteristics of the Set?
- Example: The jackpot for progressive slot machines
- They are often called static characteristics

Final Thoughts

Where We Are:
- We've discussed some of the concepts and principles that define the OOP paradigm
What's Left?
- Inheritance
- Polymorphism
- Interfaces
- Abstract Classes
- Packaging