Class 29: Getting Started with Object-Oriented Programming

Held: Monday, 10 March 2003

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

Assignments:

• Read Introduction to J2: Objects in Java and Discussion for J2: Objects in Java.

Notes:

• Are there questions on exam 2?
• I thought you might have fun doing the muddy children problem, an interesting application of recursion (or induction) to problem solving,
• I wrote much of Experiments in Java five years ago (ouch). Some of my views of Java programming have changed slightly. Java has, too.

Overview:

• An introduction to object-oriented programming.

An Introduction to Object-Oriented Programming

• As you might guess, object-oriented languages concern themselves with objects.
  • You might have a sense of what a real world object is.
  • In computerese, an object is a collection of information (sometimes called attributes or fields) and operations that the object can perform (called methods).
• For example, we might say that the textbook for our class
  • has title Java Structures (attribute)
  • is published in hardback (attribute)
  • was written by Dwayne Bailey (attribute)
  • contains text that you can extract (the text is an attribute, the extracting of the text might be considered a method)
• We often categorize objects into classes. A class specifies common aspects of a set of objects. These aspects are often generic attributes and specifications of methods (E.g., ``all objects in this class have a color, a size, and can draw themselves''.)
  • Almost every book has a title
  • Almost every book has one or more authors
  • Almost every book has text that you can extract
• Most object-oriented languages support inheritance, in which classes can be based upon other classes. For example, we might say that library books are a subclass of books and inherit all the attributes of books. Library books also extend books.
  • Since books have titles, library books also have titles
  • Since books have authors, library books also have authors
  • Library books also have loan records: information on who has the book.
• Some notes on subclasses:
  • A subclass inherits the attributes and methods of its parent class.
  • The parent class is often called a superclass.
• Most object-oriented languages support polymorphism, in which a member of a subclass can be used in place of a member of a superclass.
  • If we know how to read a book, we know how to read a library book.
• Many object-oriented languages are event-driven. That is, you can specify when this event happens, call this method of this object.
• A key aspect of object-orientation (and good program design in any language) is information hiding. In general, objects should know what other objects do, but not how they do it.
  • For example, you might know that a Menu object can draw menus on the screen, but it's not important to you how it does it.
  • This property is also called encapsulation.
• Polymorphism, inheritance, and encapsulation are three key aspects of object-oriented programming.