Class 16: Basics of Object-Oriented Programming

Held: Monday, February 27, 2006

Summary: Today we begin our exploration of object-oriented programming.

Related Pages:

• EBoard.
• Responses to the two main questions.

Assignments

• Read (and comment on) Chapters 1-3 of Beyond Java.

Notes:

• Expect a semantics assignment sometime soon.
• Today we transition from functional to object-oriented programming, and from theoretical eradings to more practical ones.

Overview:

• What is object-oriented programming?
• Three key issues: Encapsulation, Inheritance, Polymorphism.
• Why OOP?
• A biased history.
• Polymorphism vs. Overloading.

What is object-oriented programming?

• As most of you suggested, object-oriented programming (and, similarly, object-oriented programming) is a strategy that emphasizes objects.
• Objects group data and functionality. That is, an object stores information and provides methods that use or modify that information.

Three key issues: Encapsulation, Inheritance, Polymorphism

• To most object-oriented programmers, object-oriented programming is more than just the idea of objects. It also incorporates three related ideas:
  • Objects encapsulate data.
  • An object (or class) may inherit methods and fields from other objects (or classes)
  • Objects can be used polymorphically in place of each other.

Why OOP?

• Why not?
• Strives to be a silver bullet for software design.
• More reuse with inheritance and polymorphism.
• Well-designed objects can be reused in many contexts.
• Natural for some problems (e.g., modeling).
• Natural for modern application design (i.e., user interfaces and event-driven programming).

A Biased History

Sam's grounding quote (emphasis mine) from Nygaard and Dahl

The principle extensions which convert ___ to ___ provide the ability to:
1. Declare a class
2. Generate objects of the declared class
3. Name the generated objects
4. Form a hierarchical structure of class declarations

• Sketchpad.
• Simula and Simula 67: Modeling.
• Smalltalk (and its variants): Purity; Language as environment.
• CLOS (early 1980's): Objects in LISP
• Many years of oop as primarily research topic.
• C++: Add some object-oriented features to C.
• Java: Do C++ right.

Polymorphism vs. Overloading.

• Both polymorphism and overloading permit a method to behave differently based on its parameters.
• In overloading, you must write different versions of the method for each set of parameters.
• In polymorphism, you write one method, and it behaves differently by using methods of different objects.
  • Of course, those different objects needed to implement different versions of the same called method (or methods).
• Consider the simple example of square roots.

Back to Scheme Semantics (3). On to Java as Object-Oriented Language.

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Disclaimer: I usually create these pages on the fly, which means that I rarely proofread them and they may contain bad grammar and incorrect details. It also means that I tend to update them regularly (see the history for more details). Feel free to contact me with any suggestions for changes.

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Samuel A. Rebelsky, rebelsky@grinnell.edu