Class 06: Classes and Objects

Held Friday, September 3, 1999

Overview

Today, we will consider how one writes and uses classes and objects in Java. Most of this consideration will be through a laboratory.

Notes

• Don't forget that you're supposed to read the discussion section of each lab before coming to class.
• We've decided to write an IMAP/MES mail client as the class project.
  • You can find some information on IMAP at http://www.imap.org.
• Yes, we have class on Monday (even though it's labor day).

Contents

• Classes in Java
  • About Methods
• Lab: Classes and Objects
  • Reflection

Handouts

• Lab J2
• About IMAP

Summary

• Due:
  • Lab J2
• Assigned:
  • Lab J3 (for Monday)
  • Think about the parts of a mail client.

Classes in Java

• As we've seen, object-oriented programming involves not just objects, but also classes that provide generic information about groups of similar objects.
  • The kinds of attributes these objects will have (e.g., every book has a title). These are usually called fields.
  • The facilities these objects will have. These are usually called methods.
• In Java, most of your programming will center around classes.
• As you've seen, each class will go in its own file.
• What goes in a class?
• A class definition begins with the keywords public and class class and is followed by the name of the class and the body of the class, surrounded in curly braces.
  • Later, you'll see some variations.
• What does in the body of the class?
  • The methods.
  • The fields.
  • Not much else.

About Methods

• We've talked about much of this already, but it doesn't hurt to review.
• Methods provide functionality for classes and objects.
  • They are like the functions you know from Scheme.
• Unlike Scheme functions, Java methods must also be typed. This means that you must specify what kind of information each method takes as parameters, and what kind of information each method returns.
  • A print method might take something to print as a parameter and return nothing.
  • An average method might take numeric values to average as parameters and return a number.
• Hence each method has
  • a name (so that we can tell which method we're using),
  • a return type (what kind of value it returns),
  • a parameter list (giving names and types of parameters),
  • a body (describing what it does), and
  • a number of optional components that we won't yet concern ourselves with.
• There are two kinds of methods:
  • object methods require an instantiated object to work;
  • class methods do not require objects.
• Object methods can refer to all the attributes of the object, as well as to other methods of the object.
  • For example, a Printer object might provide a print method for printing to the associated printer.
• Class methods can only refer to class attributes and class methods (they can, of course, refer to object methods in conjunction with an object of the same class).
  • For example, a average method would not need to knowk anything about a particular object (other than its parameters).
• We will almost exclusively use object methods.

Lab: Classes and Objects

Do sections J2.1, J2.2A, J2.3, J2.4, J2.5, and J2.7 of Lab J2. If you cannot finish all the sections during class, please complete them over the weekend.

Reflection

• What were some of the interesting things you learned from this lab?
• What was the most challenging part of this lab?