CSC152 2005F, Class 23: Review of Template Classes

Admin:
* Approximate schedule for this week
* Questions on homework?
* New homework: To be determined at the end of class

Overview
* What is a class?  Three perspectives.
* Parts of a template class.
* Extended example.

Programs in Java combine classes and objects
* You write some of the classes; others come with the language
* Classes serve three primary roles within programs:
  * One designated Main Class holds the main method.
    * Describes how the other classes fit together.
  * Some number of Utility Classes collect useful methods
    * The Math class provides sqrt, tan, atan, etc.
    * Most utility methods are called with
      ClassName.methodname(params)
      double d = Math.atan(Math.sqrt(2.0) + Math.tan(1.0));
  * Most classes are Template Classes, which describe how to build and use objects
  * Classes can serve more than one of these roles.

Template classes model groups of similar objects:
* Describe what data we will associate with each object
* Describe how the object does things
* Describe how to build new objects (and what data we need)

Describing data:
* Called "fields"
* Declarations look (almost) excactly like variable declarations
	TYPE NAME;
* "I can use this name as a value of that type within *the methods of this class."
  * Unlike variables, which are restricted to one method, fields are shared between methods
  * Within methods we should refer to the fields as "this.NAME".
    * Although we may just use "NAME"
    * We use "this" for clarity
      * To distinguish it from variables which potentially have the same name
      * To distinguish the fields in the current object from fields in other objects
        * We refer to those fields as otherObjectName.fieldName
  * The value associated with a field *can* (and often does) change throughout the executation of a program
    * The name and number of fields does not


The example begins: PiggyBank

* Another way to think about fields is that they provide the "characteristics" of the object; 
  * What distinguishes one object in a class from another object in the same class.

Classes are more interesting when they do things.  "Methods" describe how.

public return-type name(params)
{
  body
  return value-of-designated-type;
}

* return-type may be "void" for "I don't compute anything"
* params may be empty
* nonempty params is a sequence of "Type name" pairs separated by commas
* If you don't write "public", the method is usable only by other objects in the same class (or the same object)
  * N kinds of protection: public, private, "nothing", protected
  * Don't worry about this issue right now.

EXAMPLE CONTINUES