Using Objects 1 of 22 Computer Programming Object Classes

Using Objects 1 of 22

Computer Programming Object Classes.


USING OBJECTS• What are Objects?

• Properties and Capabilities

• Classes and Instances

• Making Instances

• A Sample Program

objects rule the wor

ld

pray to the objects

the object is your t

rue

master

follow the path of t

he object

a Jedi’s strength fl

ows from

the object

the object is a road

leading over towards the

horizonobjects are everywhere

a world without objects is

nothingevery object is sacred

trust in the object

Obj ects


What are Software Objects?

• Building blocks of software systems– a program is a collection of interacting objects– objects cooperate to complete a task– to do this, they communicate by sending “messages” to each other

• Object model tangible things– school– car– ____________________________– ____________________________

• Objects model conceptual things– meeting

• Objects model processes– finding a path through a maze– sorting a deck of cards

• Objects have– capabilities: what they can do, how they behave– properties: features that describe them

What are some

objects in this

project3?


Object Capabilities: Actions

• Objects have capabilities that allow them to perform specific actions- objects are smart—they “know” how to do things- an object gets something done only if some other object tells it to use one of its

capabilities

• Also called behaviors

Capabilities can be:- constructors: establish initial state of object’s properties- commands: change object’s properties- queries: provide answers based on object’s properties

• Example: trash cans are capable of performing specific actions

- constructor: be created- commands: add trash, empty yourself- queries: reply whether lid is open or closed, or whether can is full

or empty


Object Properties: State

• Properties determine how an object acts- some properties may be constant, others variable- properties themselves are objects — they also can receive messages-

Properties can be:- attributes: things that help describe an object- - associations: things an object knows about, but are not parts of that object

• State: collection of all of an object’s properties; changes if any property changes- some don’t change, e.g., steering wheel of car- others do, e.g., car’s color

• Example: properties of trash cans

- attributes: color, material, smell- associations: a trash can can be associated with the room it’s in


Example: Tetris

• What are the game’s objects?

• What do those objects know how to do?

• What properties do they have?


Example: Tetris (cont.)

• What are the game’s objects?– piece, board

• Capabilities: What do those objects know how to do?– piece:

• be created• fall• rotate• stop at collision

– board:• be created• remove rows• check for end of game

• Properties: What attributes and components do they have?– piece:

• orientation• position• shape• color

– board:• size• rows


Classes and Instances

• Our current conception: each object corresponds directly to a particular real-life object, e.g., a specific atom or automobile

• Disadvantage: it’s much too impractical to work with objects this way

– there may be infinitely many objects (i.e., modeling all atoms in the universe)– may not want to describe each individual separately; they may have much in

common

• Classifying objects factors out commonality among sets of similar objects

– describe what is common just once– then “stamp out” any number of copies later

Rubber stamp (object class)

Imprints (object instances)


Object Classes

• Object class– a class is a category of object– defines capabilities and properties common among a set of individual

objects• all trash cans can open, close, empty their trash

• Classes implement capabilities as methods– a method is a sequence of statements in C++ or Java– objects cooperate by sending messages to others– each message “invokes a method”

• i.e., C++ executes the sequence of statements in the method in response to a message

• Classes implement properties as instance variables– slot of memory allocated to the object that can hold a potentially

changeable value


Object Instances

• Object instances are individual objects– made from class template– one class may represent any number of object instances– creating an object instance is called instantiating that object

• Shorthand:– class: object class– instance: object instance (not to be confused with instance variable)

• Different instances of, say, TrashCan class may have:– different color and position– different types of trash inside

• So their instance variables have different values


Object Instances (continued)

• Individual instances have individual identities– this allows other objects to send messages to given object– each instance is unique, even though they all have the same

capabilities– think of class of CS57 students

• A reference is the address in memory where its instance is stored– also called pointer

Properties

instancei

instancea

instanceb


Memory Revealed

• Every instance is stored in computer’s memory– memory is a set of consecutively numbered storage locations, each

containing a byte– each instance is stored in a series of contiguous bytes starting at a

given location

• An instance is identified and referenced by unique address that refers to its starting location– address looks like 0xeff8a9f4 (hexadecimal notation, base 16)– just like postal address represents actual home

memory addressof instance 1

memory address of instance 2

memory addressof instance 3

0x000000000x000000010x00000002

0x00000080

( vertical representation )

( horizontal representation of memory )


Messages for Object Communication

• No instance is an island — must communicate with others to accomplish task

– properties allow them to know about other objects

• Instances send messages to one another to invoke capabilities (i.e., to execute a task)

– a method is code that implements messages– “call a method” shorthand for “invoke capability”

• Each message requires:– sender: object initiating action– receiver: instance whose method is being called– message name: name of method being called– optionally parameters: extra info needed by method to operate. (Parameters

are also called arguments)• Same as in functions (input or output parameters)

• Receiver can (but does not need to) reply– we’ll discuss return types in detail in a few lectures


Views of a Class• Objects separate interface from implementation

– object is “black box,” hiding internal workings and parts– interface protects implementation from misuse

• Interface: public view– allows instances to cooperate with one another without knowing too many details– like a contract: consists of a list of capabilities and documentation for how they can be

used

• Implementation: private view– properties that help capabilities complete their tasks

Public Capability Public Capability



Private Properties

Note: private properties shown schematically as literally contained in an object. In reality, they are actually stored elsewhere in memory and referenced by their addresses


Object Oriented Programming vs Procedural Programming

Striker Code

Ball Code

Mouse Code

3 Days

3 Days

3 Days



Mouse ClassDraw

SetMouseCoordinates

GetMouseCoordinates

Striker ClassDraw ( );MoveStrikerLeft ( );MoveStrikerRight ( );SetCoordiantes ( int Row, int Col);GetCoordinates ( );

Ball ClassDraw ( );SetCoordiantes ( int Row, int Col);GetCoordinates ( );

Objects Integration Code



• With Object-Oriented-Programming total software development cost reduce as compared to procedural programming.

• Classes can be reusable.

• Easy to maintain (We can easily add additional features).

• Easy to test or debug.


Mouse Class Definition (Mouse.h)

class Mouse{ public:

void Mouse ( ); void ShowMouse ( ); void HideMouse ( );

void SetMouseCoordinates (int row, int col); void ChangeMouseCoordinates ( );

int GetMouseCol ( ); int GetMouseRow (); int GetMouseButton ( );

private:

int RowCord, ColCord, int ButtonStatus; union REGS in, out;

};


Mouse Class Implementation (Mouse.cpp)

void Mouse::Mouse ( ){

RowCord = 12; ColCord = 39;

ButtonStatus = 0;}

void Mouse::ShowMouse ( ){ in.x.ax=1;

int86(51,&in,&out);}

void Mouse::HideMouse ( ){ in.x.ax=2;

int86(51,&in,&out);}



void Mouse::SetMouseCoordinates (int row, int col ){ in.x.ax = 4;

in.x.cx = row*8; in.x.dx = col*8; int86(51,&in,&out);

}

void Mouse::ChangeMouseCoordinates (){ in.x.ax = 3; int86(51,&in,&out);

ButtonStatus = out.x.bx; RowCord = out.x.cx/8; ColCord = out.x.dx/8;

}



int Mouse::GetMouseRow ( ){ return (RowCord);}int Mouse::GetMouseCol ( ){ return (ColCord);}

void Mouse::GetMouseButton (){ return (ButtonStatus);}


Calling Mouse Class (Myprogram.cpp)

#include “Mouse.h”#include “Mouse.cpp”void main (void){

Mouse mouseObject; mouseObject.ShowMouse ( );

mouseObject.SetMouseCoordinates (12, 39); while (1)

{mouseObject.ChnageMouseCoordinates ();gotoxy (12, 39);printf(“%d…%d”, GetMouseRow (),

GetMouseCol () );if ( GetMouseButton ( ) == 2 )

break;}

}

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Using Objects 1 of 22 Computer Programming Object Classes