CHAPTER 13

Object Oriented Programming

Objectives

Design class definitions Implement data hiding and

encapsulation Use accessor and mutator methods Implement an initializing constructor Use public, and private access modes Create and reference a two-dimensional

array

Introduction

An abstraction is an idea with varying amounts of definition

In computer science, abstractions are class definitions Examples of system-defined class

definitions: Button, TextBox, Label, Form Objects are created from class definitions

(abstractions) Objects are implementations of the

abstraction

Introduction (continued)

Introduction (continued)

System namespace(System)

Drawing class library(System::Drawing)

Math class library(System::Math)

Drawing::GraphicsDrawing::BrushDrawing::FontDrawing::Pen

.

.

.

Math::Sin()Math::Cos()Math::Sqrt()Math::PI

.

.

.

methods

constant

object class defs

Introduction (continued)

OOP Example

Consider a program in which Frog objects are created and made to hop across the screen

The Frog class definition is an abstraction The Frogs seen on the interface are

objects (instances of the class) Objects exhibit all the properties and

methods defined by the class they are derived from

OOP Example (continued)

OOP Example (continued)

The Frog Class Definition

A Frog class definition needs Class variables (the Frog icons) Instance variables

X and y coordinates of the Frog The assigned icon

Constructor

The Frog Class Definition (continued) A Frog class definition needs

Instance methods Operations that Frog objects can perform Example: showIcon()

Returns the Frog’s assigned icon to the client program so that it can be displayed

The Frog Class Definition (continued)

Instantiation and Use

Frog objects can be instantiated using gcnew

Instantiation and Use (continued)

Initializing Constructors

Constructors are used to instantiate objects

Initializing constructors use parameters to construct objects with specific properties that can be passed in as actual arguments

Initializing Constructors (continued) The code for an initializing constructor

(from Frog.h) Parameters xcoord and ycoord are

assigned to instance variables x and y

Initializing Constructors (continued) The default constructor assigns every Frog

object it makes the same x and y coordinates

Initializing constructors may be preferable to default constructors because the client can specify the location for each Frog object created

To prevent the client from using the default constructor, it should be placed in the private portion of the Frog class definition

Data Hiding

Data hiding is the practice of making data members inaccessible by designating them as private

Data members that are private can only be accessed by class methods and not by the client

Data Hiding (continued)

In this example, the client constructs a Frog with x and y locations -50, -75 and later assigns other negative coordinate values

This could lead to problems, since negative coordinates are off of the interface

Data Hiding (continued)

To make it impossible for the client to place invalid values into instance variables like x and y, these data members can be “hidden” by placing them in the private portion of the class definition Invalid assignments by the client are no longer

allowed Unfortunately, valid assignments are not allowed

either public methods are provided to the client to

allow it to change values stored in private data members

Data Hiding (continued)

Hidden data members are not visible to the client

The good news: Invalid assignments are no longer possible

The bad news Valid assignments are not allowed either

public methods are provided to the client to allow it to change values stored in private data members

Accessor and Mutator Methods (continued) Accessor and mutator methods are public methods that provide access to private data members

An accessor method is a public method that returns the value stored in a private data member

Accessor and Mutator Methods (continued) In this example, public methods getX()

and getY() are available to the client, allowing it to retrieve the data stored in private data members x and y

Accessor and Mutator Methods (continued) The definitions of public methods getX() and getY() are contained in the class definition

Accessor and Mutator Methods (continued) A mutator method is a public method

that changes the value stored in a private data member

In this example, setX() is a public mutator method used to change the private x coordinates of each of four Frog objects

Accessor and Mutator Methods (continued) Mutator methods can be used to screen

data before it is assigned to a private data member, as shown in the class definition of setX()

Utility Methods

Utility methods are instance methods that perform operations other than those performed by constructors, destructors, accessors, and mutators. (Example: verifyX() )

Utility Methods (continued)

A Frog class definition might want a public method that the client can call when they want a frog to hop

Complete Frog Class Definition private data members

x, y, icon private methods

Default constructor (Frog()) Utility methods

verifyX() verifyY()

Complete Frog Class Definition (continued) public methods

Initializing constructor (Frog(int, int)) Accessor methods

getX() getY() showIcon()

Complete Frog Class Definition (continued) public methods

Mutator methods setX() – used to assign a value to x setY() - used to assign a value to y setLeaping() used to assign the leaping icon setSitting() used to assign the sitting icon

Utility method hop() – used to add 25 to the x coordinate

Complete Frog Class Definition (continued)

Complete Frog Class Definition (continued)

Client Code

The client needs to instantiate four Frog objects and position them vertically along a starting line

Constant STARTX stores the starting x coordinate postion

Constants START1, START2, START3 and START4 store the starting y coordinate positions

Client Code (continued)

Using the initializing constructor, each new Frog can be positioned in a different location

Client Code (continued)

To draw a single Frog you first construct a Rectangle object Upper left corner x coordinate is Frog->getX()

Upper left corner y coordinate is Frog->getY()

Width and height are both 25 pixels The DrawIcon() method displays an

icon in a Rectangle The icon is Frog1->showIcon()

Client Code (continued)

Client Code (continued)

To reset the interface all frogs are repositioned back to their starting X location and all frog icons are set to the sitting frog icon

Client Code (continued)

A random number is used to select one of the four frogs. The chosen frog’s icon is set to the leaping icon

Summary

Class definitions are abstractions Objects are instances of the class Initializing constructors are useful if the

objects they construct need to have unique values assigned to their attributes

public data members and methods are visible to the client

private data members and methods are not visible to the client

Summary (continued)

Data hiding is used to restrict client access to data members

Accessor methods are public methods that can be used to retrieve a value stored in a private data member

Mutator methods are public methods that can be used to assign new values to a private data member

Summary (continued)

Utility methods are carry out operations that constructors, destructors, accessors and mutators do not perform