Class 04: Lab: Objects and Methods

Held Wednesday, August 30, 2000

Summary

Today we move more toward the object-oriented side of Java by considering details of classes and objects. Today's class involves both discussion and hands-on work.

Notes

• Homework 1 is now ready in draft form. Be sure to bring questions on Friday.
• We will continue lab J3 on Friday.
• I should be in my office from about 1:30 to about 4:30 today.
• I was a litle behind in getting today's stuff ready. I apologize.

Overview

• The three components of classes: constructors, fields, methods
• The format of a class
• The format of methods
• Constructors

Building Classes

• Now that we've written some simple programs, we're ready to consider more details about the construction of classes.
• There are three key parts to each class:
  • The fields that categorize the attributes of objects in the class.
  • The methods that describe and implement the capabilities of objects in the class.
  • The constructors that allow us to construct new objects in the class.
• We'll consider each in turn as we attempt to build a simple Fraction class.
• We begin by identifying potential attributes, methods, and constructors.
  • (Intentionally left out of the outline.)

The Class Declaration

• You must enclose the fields, methods, and constructors of a class in a class declaration.
• Each class gets its own file (at least in CS152).
• The typical structure of a class is as follows.

  import UsefulClass;
  /**
   * A helpful introductory comment that describes the class.
   */
  public class ClassName
  {
    // Field declarations
  
    // Constructors
  
    // Methods
  
  } // class ClassName
  

Fields

• There are three parts to a field declaration. In reverse order, they are:
  • The name of the field.
  • The type of the field. (The kinds of values it can take on.)
  • The word protected
• For example,

    protected String name;
    

• You may have noted that field declarations look a lot like variable declarations. How do they differ?
  • Field declarations are part of a class; variable declarations are part of a method definition.
  • Fields are available ``anywhere'' in the class; variables are available only in the method they are declared in.
  • Fields have protection levels; variables do not.
  • Variables may be initialized when they are declared; fields typically are not.
  • When you reference fields, you should preface them with this.
• What should the fields for our class look like?

Methods

• The things you called functions in Scheme we call methods in Java.
• There are typically 6 parts to a method declaration.
  • A comment that describes the purpose of the method.
  • The protection level of the method (typically public).
  • The type of value the method returns. You can use void when a method doesn't return anything.
  • The name of the method.
  • The parameters of the method.
  • The body of the method.
• For example

    public String getName()
    {
      return this.name;
    } // getName()
    public void setName(String newName)
    {
      this.name = newName;
    } // setName(String)
  

• We tend to classify methods as observers and modifiers.
  • Observers extract a value from the object without changing the object.
  • Modifiers change the object.

Constructors

• Constructors are special methods used to create new objects within the class. They are much like methods except that you don't include the type.
• Whenever you build a new object in a class, you call a constructor.

Scheme vs. Java

Just so you know what you're getting into, I've put together a small chart comparing Scheme to Java.

Lab: J2

Preparation

Are there questions on the lab?

Work

Start working on lab J2.

Reflection

There will be no reflection today. Prepare to reflect on Friday.