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Tutorial - WRITING A CLASS LIBRARY

1 * CLASS is TYPE (2/16)
2 * Writing Class in style of OOP (2/17)
3 * Whai is Enumerate? (2/18)
4 * Method Declaration of Class(2/19)
5 * Method of Calculation sample (2/20)
6 * Testing Class you had created. (sample of TVAT from 2/20) (2/21)
7 * Instance and CLASS (2/22)
8 * Save Time by indicate and simplify Instance.(2/24)
9 * Internal Data Storage (Property). (2/26-28)

1 * CLASS is TYPE (2/16)
In Object Pascal Type begin with "T". In Delphi Class almost all begin with "T" and originated from Class name TObject

Example TVAT = Class(TObject).

Writing Class is a real backbone for programmer. Let's say if you are programming an accounting program, you should write about class related to finance, costing and so on. You have to write about Class within Encapsulate between Type and End;

Based on below sample we are dealing with VAT=Valued Added Tax which could be different based on country, or government policy.

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2 * Writing Class in style of OOP (2/17)
OOP is a way you write a CLASS with carefully thought out and include "DATA" and "METHOD". You can always modify or add features in this Class later on as well. Creating a Class for a programmer is like an architect creating a blue print before building a house.

Continuing with our TVat, we should consider all necessary factors to complete our "Class" here.

Unit UVAT; Interface Type .......TVatType = (ExcludeVAT, IncludeVAT) <enumerate of VatType include or exclude> Type .......TVat = Class ..........Amount : Extended; <variable store value to be calculate for Vat> ..........VatType : TVatType; ..........VatAmt : Extended; <variable store value for Vat actual value, could be 6%, 10% or ...> End; implementation end.

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3 * What is Enumerate?(2/18)
A set of commend word is a part of program called "Enumerate".

ExclueVAT in math is Ordinary Cardinal or in brief Ordinal value =0
IncludeVAT is an Ordinal value =1

We normally use Enumerate when we have stable conditions or factors in our program such as

So based this explanation, we cab refer back to out sample why TVatType is a enumerate.

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4 * Method declaration of Class (2/19)
A method declaration is a "Pseudo" CLASS which is a plan of how to do it, and you need to implement your plan by real action by writing "Method" of how things actually get done.

Sample Below Color match indicate Class connection with Method

Unit UVAT; Interface Type .......TVatType = (ExcludeVAT, IncludeVAT) Type ......TVat = Class ..........Amount : Extended; ..........VatType : TVatType; ..........VatAmt : Extended; Procedure CalculatedVAT (Value:Extended); <this is method declaration> End; implementation Procedure TVAT.CalculatedVAT (Value:Extended); <implement method which belong to TVAT> .....Begin .....End; end.

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5 * Method of calculation sample (2/20)
Before calculation, you have to consider all factors and conditions of the calculation formula first. For example your client ask you to write a program with option of include and exclude tax in calculation possible, then see sample below of our first method of our Class TVAT.

Procedure TVAT.CalculatedVAT (Value:Extended); .....Begin ..........If VatType = ExcludeVat then ..............VatAmt := Value * 0.07 <exclude VAT multiple by 7> ..........else ..............VatAmt :=Value * (107/100); <Include VAT multiple by 107 and divide by 100> ..............Amount : = Value; .....End; end.

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6 * Testing Class you had created. (sample of TVAT from 2/20) (2/21)
Assume you are another programmer and another programmer is using TVAT , go to Main Menu choose File/New Application. In order to enable our new program to see UVAT we just created, we should add UVAT in Uses section.

This is what you will see (color indicate relationship)

users .....Windows, Messages, SysUtils, .....Classes, Graphics, Controls, .....Forms, Dialogsm, UVAT ; type .....TForm1 = Class (TForm) .....private ............. {private declarations} .....public ............. {public declarations} .....end; var ..........Form1 : TForm1; ..........V1 : TVAT; implementaion

Note : V1 is an Instance refer to Class name TVAT

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7 * Instance and CLASS (2/22)
In the old time programmer need to deal with INIT and DESTRUCT variable which is a "pointer" which point to the "Object" in the memory and could make simple program turn into a nightmare.

The new age of programmer use "INSTANCE" to call existing CLASS to make life a little easier. Class itself is an ENTITY, so we need to instantiated CLASS and turn it into OBJECT, then this Object is then buried itself in the Hard disk (Stream) waiting to do what we tell it to do. When we terminate or exit program, this object will disappear and give back our hard disk resource.

Each instance cost you 4 bytes. If you handle Class, Instance and your object correctly, you program will NOT be a fat large program.

Back to our sample V1 that refer to Class TVAT, we are now need to instantiated, create or initialized whatever you want to call it by using CONSTRUCTOR METHOD "Create".

Doing this ...................... V1 : = TVAT.Create ;

 

V1 : = TVAT.Create; V1.CalculateVAT(35350.00); ........... ........... ??? V1.VATamt V1.Free;

After Sentence Create; V1 is born. If you try to use V1 before create, it will caused Memory Error GFP. After V1 is created, then we can call Class Method name "CalculateVAT and send value 35350.00 to be calculate VatAmt within Method name CalculateVAT. When finished with this object we will FREE instance, but this instance will be existing here for next use.

The above may looks normal, but programmer should indicate TYPE of vat to avoid error, therefore it should be.

 

V1 : = TVAT.Create; V1.VatType := ExcludeVAT; V1.CalculateVAT(35350.00); ........... ........... ??? V1.VATamt V1.Free;

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8 * Save Time by indicate and simplify Instance.(2/24)
Back to UVAT for small changes by indicating "Instance", so that programmer or you do not have to indicate "Instance" everytime you want to calculate VAT.

Do this changes below

Var ........VAT : TVAT; <add instance VAT refer o TVAT Class> implememtation Procedure TVAT.CalculatedVAT (Value:Extended); .....Begin ..........If VatType = ExcludeVat then ..............VatAmt := Value * 0.07 ..........else ..............VatAmt :=Value * (107/100); ..............Amount : = Value; .....End; Initialization <this will be done before start program> .............VAT := TVAT.Create; .............VAT.VatType := ExcludeVAT; Finalization <this will be done before exit program> .............VAT .Free; end.

Note : Now you don't have to call V1,but call out instance VAT and followed by Method CalculatedVAT right away.

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9 * Internal Data Storage (Property). (2/26-28)
If you writing program with another programmer and he has to call "Method" name "CalculatdVat" , or if he could remember name of "Method" then it could be frustrating ,isn't it? Indicating "Property" value will make thing easier. You can do this by adding "F" infront Amount and become FAmount. Next time when you call FAmount , it will automatically called Method name CalculatedVat. To do this you have to transfer it to Private Section. Technically, we called this "Internal data Storage".

Sample of TVAT again

Type ........TVAT = Class ..... .....private ............. FAmount : Extended ; ............. FVatType : TVatType; ..............FVatAmt : : Extended ; .....public ............. Procedure CalculatedVat (Value:Extended); ..............Property Amount: Extended Read FAmount Write CalculatedVat; ..............Property VatAmt :Extended Read FVAtAmt; .....End; Var ........VAT : TVAT; implememtation Procedure TVAT.CalculatedVAT (Value:Extended); .....Begin ..........If FVatType = ExcludeVat then .............. ..............FVatAmt := Value * 0.07 ..........else ..............FVatAmt :=Value * (107/100); ..............FAmount : = Value; .....End;

Note : FvatAmt is in private so nobody will be able to see it, so allowing internal reading by adding property in public (see RED)

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