Use of Private Constructor in OOPS

Use of Private Constructor in OOPS

DOTNET Programming

Oktober 05, 2017 Add Comment

If we set access specifier of a constructor to private, then that constructor can only be accessed inside the class. A private constructor is mainly used when you want to prevent the class instance from being created outside the class. 

This is mainly in the case of singleton class. Singleton classes are employed extensively in concepts like Networking and Database Connectivity. Using private constructor we can ensure that no more than one object can be created at a time. 

Example of Private Constructor in a Singleton Class

public class SingletonClass

{

    public static SingletonClass singletonClass;

    private SingletonClass() 

    {

    }

    public static SingletonClass getInstance() 

    {

        if(singletonClass == null) 

        {

            singletonClass = new SingletonClass();

        }

        return singletonClass;

    }

}

A class with private constructor cannot be inherited.

If we don’t want a class to be inherited, then we make the class constructor private. So, if we try to derive another class from this class then compiler will flash an error. Why compiler will flash an error? 

We know the order of execution of constructor in inheritance that when we create an object of a derived class then first constructor of the base call will be called and then constructor of derived class. Since base class constructor is private, hence, derived class will fail to access base class constructor.

We can also use sealed class to stop a class to be inherited. Sealed class provide more flexible and readable way to stop inheritance.

Diamond Problem in Multiple Inheritance in OOPS

Diamond Problem in Multiple Inheritance in OOPS

Delphi DOTNET Programming

Oktober 05, 2017 Add Comment

The diamond problem occurs when two super classes of a class have a common base class. 

Suppose there are four classes A, B, C and D. Class B and C inherit class A. Now class B and C contains one copy of all the functions and data members of class A. Class D is derived from class B and C. Now class D contains two copies of all the functions and data members of class A. One copy comes from class B and another copy comes from class C.

Let’s say class A has a function with name display(). So class D have two display() functions as I have explained above. If we call display() function using class D object then ambiguity occurs because compiler gets confused that whether it should call display() that came from class B or from class C. If you will compile above program then it will show error.

This kind of problem is called diamond problem as a diamond structure is formed (see the image).

That is why major programming languages like C#, Java and Delphi don't have multiple inheritance because it can lead to diamond problem and rather than providing some complex way to solve it, there are better ways through which we can achieve the same result as multiple inheritance. We can use interfaces to resolve this problem.

C++ supports multiple inheritance.

Notice that the above problem with multiple class inheritance can also come with only three classes where all of them has at least one common method.

Because of this problem we can not extend two classes for implementing multiple inheritance and to resolve this problem of multiple inheritance in object oriented programming we use interfaces for implementing the functionality of multiple inheritance. 

As we know we do not define a function but only declare that function in an interface. So if we use interfaces we can extend one class and one or more interfaces or we can implement more than one interfaces at a time to use the functionality of multiple inheritance and we can escape from diamond problem.

How to declare Static/Class variables, properties, functions and procedures in Delphi?

How to declare Static/Class variables, properties, functions and procedures in Delphi?

Delphi

September 28, 2017 Add Comment

In Delphi, we do have static variables, properties, functions and procedures, but instead of the word "static" we refer the word "class". In this tutorial, I have shown the syntax of declaring the static variables, properties, functions and procedures in Delphi. We don't have to use "static" keyword while declaring the static variables and properties, just use "class" keyword. While declaring static functions and procedures, we have to use both "class" and "static" keyword.

Declare a static/class variable in Delphi

type

  TStaticDemoClass = class(TObject)

  public

    class var StaticVar: integer;

  end;

Declare a static/class property in Delphi

type

  TStaticDemoClass = class(TObject)

  private

    class var FStaticVar: integer;

  public

    class property StaticVar: integer read FStaticVar write FStaticVar;

  end;

Now we can use above StaticVar variable/property anywhere in the unit like this:

procedure TForm1.Button1Click(Sender: TObject);

var a:integer;

begin

  TStaticDemoClass.StaticVar := 1;

end;

We can also use static/class functions and procedures in Delphi. Below example, I have copied from documentation:

type

  TMyClass = class

    strict private

      class var FX: Integer;

    strict protected  

      //Note: accessors for class properties must be declared class static.

      class function GetX: Integer; static;

      class procedure SetX(val: Integer); static;

    public

      class property X: Integer read GetX write SetX;

      class procedure StatProc(s: String); static;

  end;

TMyClass.X := 17;

TMyClass.StatProc('Hello');

You need to use "static" keyword after function / procedure declaration. Classes can have static class methods -- i.e. methods that can be called from a class type. Class static methods can be accessed without an object reference. Unlike ordinary class methods, class static methods have no Self parameter at all. They also cannot access any instance members. They still have access to class fields, class properties, and class methods. Also unlike class methods, class static methods cannot be declared virtual.

VCL Hierarchy in Delphi: Types of Controls in Delphi: TWinControls and TGraphicControls

VCL Hierarchy in Delphi: Types of Controls in Delphi: TWinControls and TGraphicControls

Delphi

September 26, 2017 Add Comment

In VCL (Visual Component Library) hierarchy, you will find TObject at the top, then TPersistent, then TComponent and then TControl.

TObject >> TPersistent >> TComponent >>TControl

VCL Hierarchy in Delphi

TObject is the base class from which all classes descend.

TPersistent class descends directly from TObject. The special characteristic of TPersistent is that it is an abstract class that defines the methods that allow it to be streamed.

TComponent is the base class from which all components descend. Non-visual components are descendants of TComponent. For example: TTimer

TControl is the base class from which all the visual components descend. For example: TEdit, TListBox, TComboBox etc.

Again, there are two types of controls (TControl): Window Controls (TWinControl) and Graphic Controls (TGraphicControl)

Following is the difference between TWinControl and TGraphicControl in Delphi:

TWinControls

TWinControls can receive input focus and they can be parents to other controls.

Example: TEdit, TListBox and TComboBox

TGraphicControl

TGraphicControls differ from TWinControls in that they cannot receive input focus. Also, they cannot be parents to other controls.

Example: TLabel

TGraphicControls are used when you want to display something on the form.

Two key advantages of TGraphicControls over TWinControls:

1. TGraphicControls don't use up system resources since they don't require a window handle.

2. They are a bit faster at drawing than their TWinControl counterparts since their painting is not subject to the windows message dispatching system. Instead, they piggyback on their parent's paint process.

For more details, please refer to documentation.

What is the difference between Singleton Class and Static Class? Which one to use and when?

What is the difference between Singleton Class and Static Class? Which one to use and when?

DOTNET Programming

September 26, 2017 Add Comment

Almost both singleton class and static class serve the same purpose which creates confusion among developers and architects about which one to use. When to use singleton class and when to use static class. In this article I have tried to differentiate between the two. Following is the list of differences between a singleton class and a static class:

1. Singleton classes are more inclined to the object-oriented concepts as compared to the static classes. With Singleton classes, you can use inheritance and polymorphism to extend a base class, implement an interface and capable of providing different implementations. While a static class cannot inherit their instance members. So, Singleton classes are more flexible than static classes.

2. Singleton classes can be passed as an object to other methods while a static class allows only static methods and you cannot pass static class as parameter.

3. If your requirement is to maintain the state, then singleton class is the better choice than static class, because maintaining the state with static classes is very cumbersome and leads to subtle bugs.

4. Singleton classes can be lazy loaded if it’s a heavy object, but static class doesn't have such advantages and always eagerly loaded. Static class is generally initialized when it is first loaded and it will lead to potential class loader issues.

5. Static classes are hard to mock and consequently hard to test than singletons, which are easy to mock and thus easy to test. It’s easier to write unit tests for singleton than static classes, because you can pass mock object whenever singleton is expected, e.g. into constructor or as method arguments.

6. Singleton classes provide more flexibility. Consider the possibility that your application has a global shared object and tomorrow you decide that you must make more than one instance of the class. If you use a singleton class then all you should do is make the constructor public. This is not so simple with static classes.

7. Many Dependency Injection framework manages singleton quite well e.g. Spring, which makes using them very easy.

8. Singleton objects are stored in Heap, but static objects are stored in Stack.

9. Static class provides better performance than singleton class, because static methods are bonded on compile time.

10. Singleton classes maintain single object across the application life cycle, so the scope is at application level. The static does not have any object pointer, so the scope is at App Domain level. Moreover, both should be implemented to be thread-safe.

Security and Privacy issues with IoT (Internet of Things) - Top barriers to IoT success

Security and Privacy issues with IoT (Internet of Things) - Top barriers to IoT success

IoT

September 14, 2017 Add Comment

IoT is one of the biggest breakthroughs in the history of the tech industry. It will soon be an inherent part of every aspect of our lives. Security and Privacy are the most significant challenge for the IoT. Security and Privacy issues are acting as top barriers to IoT success. So, addressing underlying security and privacy concerns is very crucial. 

As more and mored devices are being added to the IoT everyday, privacy and security concerns are becoming vital. 

Security issue with IoT devices

Security is one of the main concerns of the IoT ("Internet of Things"). With billions of devices connected to the internet, it is needless to say that to protect your connected devices from malware penetrations is a real challenge. Security risks of IoT devices cannot be ignored whether it is from hackers or corporations. Increasing the number of connected devices increases the opportunity to exploit security vulnerabilities.

When IoT devices are connected to the cloud or data centers via internet, they get under the risk of being accessible to the outside world. Poorly designed devices can easily expose user data to theft. 

There are many reasons behind the state of insecurity in IoT. 

Companies do not update their devices enough or at all. This means that an IoT device which was safe when you first bought it can become unsafe as hackers discover new vulnerabilities.

The fundamental security weakness of the Internet of Things is that it increases the number of devices on the internet. Earlier, most of us had to only worry about protecting our computers, then, we had to worry about protecting our smartphones as well, and now after IoT, we have to worry about protecting our car, our home appliances, our wearables, and many other IoT devices.

Because there are so many devices that can be hacked, that means that hackers can accomplish more. You may have heard about how hackers could potentially remotely control cars and remotely accelerate or decelerate the car. But hackers could use even seemingly unimportant devices like baby monitors or your thermostat to uncover private information or just ruin your day. The point is that we have to think about what a hacker could do with a device if he can break through its security. 

In this way, our normal day to day life and health can become the target of IoT hack attacks. Urban infrastructure can also become a target for hackers.

Privacy issue with IoT devices

Corporations which create and distribute interconnected devices could also use these devices to obtain personal data, particularly dangerous when used for money transfers. Manufacturers can steal the information about your habits and misuse it. 

For example, consider how some companies are distributing Fitbits to their employees so that they can track their health and thus get lower health insurance premiums. Even if we ignore the worrying idea of workers’ health being monitored by corporations around the clock, there is the question of what corporations can do with the data they have gathered. Companies can attempt to send or even sell gathered data to other companies, which raises issues regarding our individual privacy rights.

Vision features and voice recognition are now being integrated into smart TVs. These features can listen continuously to conversations or look for activity and transmit data selectively to cloud services for processing. These cloud services may sometimes even include third parties. The collection of all this information faces a number of regulatory and legal challenges.

The collection of private information exposes legal and regulatory challenges facing data protection and privacy law. There are a wide range of regulatory and legal questions surrounding the IoT, which need thoughtful consideration. Legal issues with IoT devices include crossborder data flow, conflict between law enforcement surveillance and civil rights, data retention and destruction policies, and legal liability for unintended uses, security breaches or privacy lapses. 

In short, Security and Privacy issues are the biggest issues which consumers, businesses and governments need to consider before using IoT connected devices.

Real World Examples of IoT (Internet of Things) - How will IoT change our lives?

Real World Examples of IoT (Internet of Things) - How will IoT change our lives?

IoT

September 12, 2017 Add Comment

IoT (Internet of Things) is going to change our lives to a large extent. In the coming years, we will realize the real potential of IoT. Below are some real world examples of IoT.

Lets consider IoT enabled Alarm Clock or say Smart Alarm.

Imagine you wake up at 7am every day to go to work. Your alarm clock does the job of waking you just fine. That is, until something goes wrong. Your train is cancelled and you have to drive to work instead. The only problem is that it takes longer to drive, and you would have needed to get up at 6.45am to avoid being late. Oh, and it’s pouring with rain, so you’ll need to drive slower than usual. 

A connected or IoT-enabled alarm clock would reset itself based on all these factors, to ensure you got to work on time. It could recognize that your usual train is cancelled, calculate the driving distance and travel time for your alternative route to work, check the weather and factor in slower travelling speed because of heavy rain, and calculate when it needs to wake you up so you’re not late. 

If it’s super-smart, if might even sync with your IoT-enabled coffee maker, to ensure your morning caffeine’s ready to go when you get up.

Now consider IoT enabled car or say connected cars.

Having been woken by your smart alarm, you’re now driving to work. On comes the engine light. You’d rather not head straight to the garage, but what if it’s something urgent? In a connected car, the sensor that triggered the check engine light would communicate with others in the car. A component called the diagnostic bus collects data from these sensors and passes it to a gateway in the car, which sends the most relevant information to the manufacturer’s platform. The manufacturer can use data from the car to offer you an appointment to get the part fixed, send you directions to the nearest dealer, and make sure the correct replacement part is ordered so it’s ready for you when you show up.

You are on your way to a meeting; your car could have access to your calendar and already know the best route to take. If the traffic is heavy your car might send a text to the other party notifying them that you will be late. 

Now consider IoT enabled homes or say smart homes.

Take an instance where you need to monitor your home or child when you are away. A simple solution would be to fix an IP camera and monitor its feed using a web or mobile application. You can even hire a babysitter. The former option can give you the complete monitoring data, while the latter cannot. If you fix sensors or devices, which can be reached from anywhere, you have the flexibility to monitor as well as control those devices to the best of their ability; this can make your home or baby 99.99% secure. 

Your home security system, which already enables you to remotely control your locks and thermostats, can cool down your home and open your windows, based on your preferences.

British Gas's Hive Active Heating enables consumers to control their home heating from their smartphone, laptop or tablet. It even has the ability to turn off when no one is home by detecting whether your smartphone is in the house or not.

IoT can help in reducing accidents and save valuable human life.

Let’s look at one example. In 2007, a bridge collapsed in Minnesota, killing many people, because of steel plates that were inadequate to handle the bridge’s load. 

When we rebuild bridges, we can use smart cement: cement equipped with sensors to monitor stresses, cracks, and war-pages. This is cement that alerts us to fix problems before they cause a catastrophe. And these technologies aren’t limited to the bridge’s structure.

If there’s ice on the bridge, the same sensors in the concrete will detect it and communicate the information via the wireless internet to your car. Once your car knows there’s a hazard ahead, it will instruct the driver to slow down, and if the driver doesn’t, then the car will slow down for him.

And thus bridges become smart bridges, and cars smart cars. And soon, we have smart cities, and….

Airplane manufacturers are building air-frames with networked sensors that send continuous data on product wear and tear to their computers, allowing for proactive maintenance and reducing unplanned downtime. 

Now consider some more examples of Real World IoT:

Some insurance companies, for example, are offering to install location sensors in customers’ cars. That allows these companies to base the price of policies on how a car is driven as well as where it travels. Pricing can be customized to the actual risks of operating a vehicle rather than based on proxies such as a driver’s age, gender, or place of residence.

Sensors in even the domestic animals. In the world of IoT, even the domestic animals will be connected and monitored with the help of embedded sensors. This allows farmers to monitor their animal's health and track their movements, ensuring a healthier, more plentiful supply of milk and meat for people to consume. 

Some more...

What if your office equipment knew when it was running low on supplies and automatically re-ordered more? 

What if your refrigerators can warn you when you’re out of milk. 

What if smart dustbins can signal when they need to be emptied.

What if smart tea maker that knows just when you’re in need of a cup of tea.

Truly speaking, you can think of infinite examples of IoT in the real world. It cannot be summarized in an article. There is no limit. Just keep thinking...

Other articles on IoT:

Internet of Things (IoT) - Next Stage of Information Revolution

Internet of Things (IoT) - Next Stage of Information Revolution

Internet of Things (IoT) - Next Stage of Information Revolution

IoT

September 12, 2017 Add Comment

The term "Internet of Things" was first coined by Kevin Ashton, cofounder and executive director of the Auto-ID Center at MIT in 1999.

The "Internet of Things (IoT)" is the next stage of the Information Revolution. 

What is IoT (Internet of Things)?

IoT refers to the connection of devices (other than computers, smartphones and tablets) to the Internet via embedded sensors. It allows devices to talk to us and talk to each other. So, IoT can also be defined as a network of internet-connected devices able to collect and exchange data using embedded sensors. 

A thing, in the "Internet of Things", can be a person with a heart monitor implant, a farm animal with a bio-chip transponder, an automobile that has built-in sensors to alert the driver when tire pressure is low or any other natural or man-made object (almost anything else you can think of) that can be assigned an IP address and provided with the ability to transfer data over a network with the help of embedded sensors. 

Earlier the data was created by people on the internet, but now the data will be created by the things (living or non-living) without any human intervention.

IoT Devices

Any stand-alone internet-connected device that can be monitored and/or controlled from anywhere. It should have embedded sensors and on/off switch. It should have the ability to represent itself digitally means it can be assigned an IP address and have the ability to collect and transfer data over a network without manual assistance or intervention. 

Due to the limited address space of IPv4 (which allows for 4.3 billion unique addresses), IoT devices will have to use the next generation of the Internet protocol (IPv6) to scale to the extremely large address space required.

Examples of IoT Devices: Smartwatch, TV,  Refrigerators, Washing Machines, Kitchen Appliances, Thermostats, Cars, Switches, Lights, Blood Pressure and Heart Rate Monitors, Smart grids, Virtual Power Plants, Intelligent Transportation and anything you can think of.

Basically, if your fridge or TV has an Internet connection, then it becomes an IoT device.

If your coffee maker connects to an app on your smartphone that allows you to begin brewing with a tap on your screen, that coffee maker becomes part of the Internet of Things.

As per IoT, Anything that can be connected, will be connected. Connect everything in this world. The Ultimate Goal of IOT is to Automate Human Life. In this way, IoT creates a relationship among people-people, people-things, and things-things.

Applications of IoT

Wearables (like Smartwatches to track health and exercise progress, sleep patterns, send text messages and calls).

Smart Home (hundreds of products in the market that users can control even with their voices).

Smart Cities (solves traffic congestion issues, smart parking, reduces noise, crime, and pollution).

Connected Cars (to assist drivers and reduce accidents).

Internet of Things Devices & Examples

Amazon Echo - Smart Home: The Amazon Echo works through its voice assistant, Alexa, which users can talk to in order to perform a variety of functions. Users can tell Alexa to play music, provide a weather report, get sports scores, order an Uber, and more.

Fitbit One - Wearables: The Fitbit One tracks your steps, floors climbed, calories burned, and sleep quality. The device also wirelessly syncs with computers and smartphones in order to transmit your fitness data in understandable charts to monitor your progress.

Barcelona - Smart Cities: The Spanish city is one of the foremost smart cities in the world after it implemented several IoT initiatives that have helped enhance smart parking and the environment.

AT&T - Connected Car: AT&T added 1.3 million cars to its network in the second quarter of 2016, bringing the total number of cars it connects to 9.5 million. Drivers don't have to subscribe or pay a monthly fee for data in order for AT&T to count them as subscribers.

Other articles on IoT:

Real World Examples of IoT (Internet of Things) - How will IoT change our lives?

Oracle Forms Debugging: Using the “DEBUG_MESSAGES=YES” runtime option - Handling "Please Acknowledge" message

Oracle Forms Debugging: Using the “DEBUG_MESSAGES=YES” runtime option - Handling "Please Acknowledge" message

Oracle Forms

Maret 14, 2016 Add Comment

Oracle Forms Debugging: Using the “DEBUG_MESSAGES=YES” runtime option - Handling "Please Acknowledge" message

“DEBUG_MESSAGES=YES” is a “quick and dirty” technique for pinpointing code problems. This causes a message to automatically display to let you know each trigger as it executes. Once you encounter the runtime error, you will know the last trigger that fired and should be able to trace through the code from that point. For anyone that was around in the SQL*Forms 3.0 days, this was the default behavior when running a form in debug mode.

To use this option from the Form Builder, check the “debug_messages” option on the runtime tab after choosing Tools -> preferences from the menu. From the command line, simply add the “debug_messages=yes” option. 

If you are using an icon in Windows to start your form, add the option to the shortcut command. It would look something like this for Windows platforms:

c:\orant\bin\ifrun60.exe module=myform userid=scott/tiger debug_messages=yes

On Unix:

f60runm module=myform userid=scott/tiger debug_messages=yes

The major disadvantage of using this technique is that you have to acknowledge every message as each trigger executes, which can be very annoying. You will keep getting "Please acknowledge message" prompt each time whenever a new trigger is encountered. Although it shows the triggers that fire, it does not show program units that execute, and there is no way to display the values of variables. It can also cause the same focus problems and program flow interruption that you see with the “MESSAGE” built-in. However, this method is useful if you have no idea which trigger is causing the problem; once you have narrowed down the scope, other troubleshooting techniques would be more appropriate.

Oracle Forms Exception Handling: NO_DATA_FOUND, TOO_MANY_ROWS and OTHERS

Oracle Forms Exception Handling: NO_DATA_FOUND, TOO_MANY_ROWS and OTHERS

Oracle Oracle Forms PLSQL

Februari 29, 2016 Add Comment

Oracle Forms Exception Handling: NO_DATA_FOUND, TOO_MANY_ROWS and OTHERS

EXCEPTION block in PLSQL Oracle Forms is used to track the exceptions. Following is the PLSQL code snippet which uses NO_DATA_FOUND, TOO_MANY_ROWS and OTHERS exceptions. If the SQL SELECT query does not return any data, NO_DATA_FOUND exception is fired. If the SQL SELECT query returns more than one row where it was expected to return only one row, TOO_MANY_ROWS exception can be used to track this kind of exception. If you are not sure what kind of exception can the code throw, use OTHERS exception.

DECLARE
  DEPARTMENT_NAME VARCHAR(60);
BEGIN
  SELECT DEPTNAME INTO DEPARTMENT_NAME FROM DEPT WHERE DEPTNO = 20;
EXCEPTION
  WHEN NO_DATA_FOUND THEN
    MESSAGE('No data found');
  WHEN TOO_MANY_ROWS THEN
    MESSAGE('More than one row found');
  WHEN OTHERS THEN
    NULL; -- don't do anything and just return from the procedure
END;

Difference between WHEN-VALIDATE-ITEM and KEY-NEXT-ITEM triggers

Difference between WHEN-VALIDATE-ITEM and KEY-NEXT-ITEM triggers

Oracle Oracle Forms Oracle Reports PLSQL

Februari 29, 2016 Add Comment

Difference between WHEN-VALIDATE-ITEM and KEY-NEXT-ITEM triggers

WHEN-VALIDATE-ITEM and KEY-NEXT-ITEM triggers are very close to each other and create a lot of confusion. Following are three differences between them to clear the picture a little bit.

1. Whenever the user changes the value in the item and tries to move out of that item using ENTER or TAB or MOUSE, WHEN-VALIDATE-ITEM trigger is fired. But, in case of KEY-NEXT-ITEM trigger, if user moves out using MOUSE, it will not fire. So, the validation written on this trigger will not fire. Better use, WHEN-VALIDATE-ITEM trigger in this case as it also works with MOUSE.

2. KEY-NEXT-ITEM trigger fires before the WHEN-VALIDATE-ITEM trigger.

3. KEY-NEXT-ITEM trigger will fire every time you move to the next field from that field but WHEN-VALIDATE-ITEM will fire only when you have acutally made any changes to that item. If you have made no changes in the item, it will not fire when you move out this item.

Personally, I prefer to use WHEN-VALIDATE-ITEM trigger in many situations.

Oracle Forms Tutorials: WHEN-VALIDATE-ITEM trigger

Oracle Forms Tutorials: WHEN-VALIDATE-ITEM trigger

Oracle Oracle Forms Oracle Reports PLSQL

Februari 28, 2016 Add Comment

Oracle Forms Tutorials: WHEN-VALIDATE-ITEM trigger

Consider that you have an oracle form on which there is a datablock which uses EMP table. EMP table has a column called SALARY. Now there is a contraint on the SALARY column that it should be greater than or equal to $1000. Your requirement is that whenever any user fills salary in the Oracle Forms ITEM (say ITEM_SALARY) and tabs out from that item, a validation should fire and if the value filled is not valid, it should give you an error message and does not let the cursor go to the other item. In these situations WHEN-VALIDATE-ITEM trigger is used. Following is the PLSQL code you should write on the WHEN-VALIDATE-ITEM trigger of the ITEM_SALARY item.

IF :ITEM_SALARY < 1000 THEN

    MESSAGE('ERROR: Salary must be at least $1000 or more.');

    RAISE FORM_TRIGGER_FAILURE; -- To keep the cursor in the item

END IF;

You should also go through this video on YOUTUBE by Edward Honour.

In this video, he tries to pick the department name when the user enters the department number. If department number does not exist in the database, he shows the error message and does not let the cursor to go to the other item using FORM_TRIGGER_FAILURE trigger. He has used NO_DATA_FOUND exception for showing the error message. Following is the code used in this video:

BEGIN

SELECT DEPT_NAME INTO :BLOCKNAME.ITEMNAME FROM DEPT WHERE        DEPT_NO = :BLOCKNAME.ITEMNAME2;

EXCEPTION

WHEN NO_DATA_FOUND THEN

MESSAGE('Invalid Department Number');

RAISE FORM_TRIGGER_FAILURE;

END

5 Things you should never discuss your manager

5 Things you should never discuss your manager

Management

Februari 28, 2016 Add Comment

5 Things you should never discuss your manager

An employee should be fair and transparent with his manager, but few revelations can spoil your bonding with your manager. Try never ever talking about these things to your manager:

1. Your doubt on manager' decisions - At times, you may want to raise questions on his judgement. Do yourself a favour - give it a pass. You may not know what business pressures he/she is living through.

2. Snooping on manager's life - Every employee wants to check what his or her manager is doing in social life. Don't blow your cover by admitting to doing it unashamed.

3. Office gossip - Never share office hearsay with your manager without checking its truthfulness. Sharing something random that you heard in the cafeteria and reacting to it may get him or her thinking that you are not mature or trustworthy.

4. Your personal secrets - Your life and its constant struggles are for you to handle. Pouring it all out in front of your manager will put you in a vulnerable position.

5. Your expectations from the manager - The manager is mandated to put forward his/her expectations from you. But it does not mean you to go and tell the manager how you evaluate his/her skill sets!

Difference between Free and FreeAndNil in Delphi: What to use when?

Difference between Free and FreeAndNil in Delphi: What to use when?

Delphi

September 05, 2014 Add Comment

Difference between Free and FreeAndNil in Delphi: What to use when?

Free and FreeAndNil are the main concepts of memory management in Delphi. FreeAndNil is a function declared in the SysUtils unit and was introduced in Delphi 5. Below is the implementation of FreeAndNil:

procedure FreeAndNil(var Obj);

var

  Temp: TObject;

begin

  Temp := TObject(Obj);

  Pointer(Obj) := nil;

  Temp.Free;

end;

Difference between Free and FreeAndNil: 

FreeAndNil first sets the object reference to nil and then frees the object. 

When to use FreeAndNil instead of Free?

If you want a rational answer, the question to ask is, why would you want to set an object reference to nil once you’re done with it?  And there’s only one good reason to ever do that: If you want to reuse the variable later.  There are some times when you’ll have an object that may or may not be initialized. If it’s not, then the variable will be nil, and if you find it’s nil, then you create it before using it.  This is a pretty common pattern.  It’s called “lazy creation”.  But every once in a while, you want to use lazy creation on an object, then destroy it and create it again. This is when FreeAndNil comes in handy.

Use FreeAndNil for Defensive Programming

You can compare FreeAndNil with seat (safety) belts in cars: if you application runs normally - then FreeAndNil won’t be useful. But if your code mess up with something - then FreeAndNil (as seat belts too) will protect you from consequences. By clearing the reference, FreeAndNil will help you to catch your wrong access immediately. Without it (i.e. by using Free only), your code may continue to run (even without raising an exception) and will give the wrong results or damage global state. It is quite dangerous situation and may lead to corrupt your data too. By not setting it to nil (in case of Free)- you're lying to the compiler. You're telling that there is allocated memory at this location, and the truth is that it isn't. And lying to machine just isn't good. FreeAndNil is very strict in these situation will immediately give you Access Violation error which I think is good thing and you can fix the problem on the spot because you know the problem!! Compare this with case, when your code silently produce wrong results! By always using FreeAndNil you’ll make your code bullet-proof for modifications.

Example of FreeAndNil

var

  myList : TList;

begin

  try

    myList := TList.Create;

    ....

    ....

  finally

    FreeAndNil(myList);

  end;

end;

Difference between Override and Reintroduce in Delphi

Difference between Override and Reintroduce in Delphi

Delphi

Agustus 31, 2014 Add Comment

Difference between Override and Reintroduce in Delphi

When you decide to declare a method as virtual, you are giving permission to derived classes to extend and override the method with their own implementation.

Use the reintroduce keyword to introduce a new implementation of a parent method (this hides the parent method). You can hide a method without using reintroduce but you will get a compiler warning. Using reintroduce will suppress the warning.

You tell the compiler that you know that you hide the ancestor function and replace it with this new function and do so deliberately.

Difference between Override and Reintroduce

1. The reintroduce and override modifiers have different meanings. The reintroduce modifier creates a new member with the same name, signature, and visibility and hides the original member. The override modifier extends the implementation for an inherited member and allows you to implement inheritance-based polymorphism.

2. Override is used in conjuction with Virtual/Dynamic methods of parent class but for Reintroduce, parent method should not be necassarily Virtual/Dynamic. 

The only reason to override an ancestor method is that you can call inherited from within the descendent method. If you don't intend to call inherited from within a descendent method, the descendent method should be declared with Reintroduce rather than Override.

Avoid Reintroduce

Sometimes there are clear reasons to introduce a new method with the same name, signature, and visibility of a parent method. In those clear cases, introducing a new member is a powerful feature. However, if you do not have a clear reason, then avoid using Reintroduce.

Difference between Virtual, Dynamic and Abstract methods in Delphi

Difference between Virtual, Dynamic and Abstract methods in Delphi

Delphi

Agustus 30, 2014 Add Comment

Difference between Virtual, Dynamic and Abstract methods in Delphi

Virtual and Dynamic methods are the strong concepts of Polymorphism in Delphi. Dynamic is semantically equivalent to Virtual. Both Virtual and Dynamic directives allows a class method to be override (replaced) by a same named method in a derived class. 

You would mark a function or procedure as Virtual or Dynamic when you allow a programmer who creates a class based on your class to replace its functionality. 

Virtual and Dynamic may be followed by the Abstract directive. This modifies the effect of the Virtual and Dynamic directives. It means that the current class must not code the method - it is there only as a placeholder to remind and ensure that derived classes implement it.

Difference between Virtual and Dynamic methods in Delphi

Although Virtual and Dynamic methods appear same but there is some differnce in their internal implementation. Virtual methods are implemented with a Virtual Method Table (VMT). There is one VMT for each class. The VMT contains one entry for each virtual method in the class. And that entry is the address of the method.

This allows for very efficient calling. You simply get the address of the VMT which is located at a fixed offset from Self. Then you look up the method pointer by index and call the method.

What this does mean is that if you have a class with a lot of virtual methods, and you derive a sub-class, you will make a brand new VMT with all the virtual methods. And if you have not overridden many of them, then you'll find that the VMTs have a lot of overlap.

This used to matter in the days of 16 bit. The VMTs could take up a lot of space in the executable image (that's what is meant by code size) and you could run out of space for the VMTs. So dynamic methods were introduced. The analogue to the VMT is the dynamic method table, DMT. This is implemented differently to avoid the repetition when methods are not overridden. The downside is that calling dynamic methods is more expensive.

In modern times, since 32 bit, and especially with the very fat executables that Delphi produces, these size issues don't matter. And so all sound advice is to use virtual methods exclusively.

Abstract methods must be virtual or dynamic

Any abstract method should be virtual or dynamic. For example:

function Age: Integer; Abstract;

will not work and throw following error:

E2167 Abstract methods must be virtual or dynamic

Conclusion: Virtual methods are optimized for speed and dynamic methods are optimized for size. In most of the situations, speed is more crucial than size, so I would prefer to use Virtual methods.

Difference between Owner and Parent in Delphi

Difference between Owner and Parent in Delphi

Delphi

Agustus 29, 2014 Add Comment

Difference between Owner and Parent in Delphi

Parent is the Window control where the control is displayed. Owner is the component responsible for making sure it's destroyed. Parent refers to the component that another component is contained in, such as TForm, TGroupBox or a TPanel. If one control (parent) contains others, the contained controls are child controls of the parent.

Example of Owner and Parent

Whenever we try add a button on a panel in a form, we try to make an invisible connection between the components. When a component is dropped on a form at design time, the owner is the form and the parent is the container on which it is dropped.

As we add the button on the panel, the panel automatically is set as the parent of the button while the form becomes the owner.  

Another example, drop a group box on a form. Its owner and parent are both the form. Drop a check box on the group box. Its owner is the form and its parent is the group box.

Parent is required to make a control visible

For a component to be visible, it must have a parent to display itself within. Controls that appear on the screen need to know where to paint themselves, hence the parent property. It specifies the visual container for the control. This property must be specified before a control can be displayed. When you want to create a TButton at run-time, it is important that we remember to assign a parent - the control that contains the button. 

The Parent component on the other end determines how the child (i.e. contained component is to be displayed). All the properties like left, top etc are relative to the parent component.

Owner is Readonly while Parent can be changed

Owner property is a read only property and cannot be modified, however parent for a control can be assigned and chagned easily at runtime. The Owner is being set while invoking the create constructor and becomes a read-only property so can't be changed later on.

Allocating and Freeing the memory

The Owner component is responsible for freeing the allocated memory of the owned components. Form is the owner of all the components residing inside it. So whenever the form is destroyed, all the components memory is freed automatically. This is the purpose of ownership.

HasParent Property

Not all components have the Parent. Many forms do not have a Parent. For example, forms that appear directly on the Windows desktop have Parent set to nil. A component's HasParent method returns a boolean value indicating whether or not the component has been assigned a parent.

Parent Property

We use the Parent property to get or set the parent of a control. If we try to create a new control, then we need to set the parent property of the control to make it visible. For example, 

Button1.Parent := Panel1;

Above code has set Panel1 as the parent of Button1.

Difference between Components and Controls in Delphi

Difference between Components and Controls in Delphi

Delphi

Agustus 29, 2014 Add Comment

Difference between Components and Controls in Delphi

Controls are the visible elements on the Delphi Forms to which you can focus and interact. All the controls are components and vice versa is not true. So we can say that Controls are the subset of Components in Delphi.

TComponent is the common ancestor of all component objects in Delphi. TControl is the abstract base class for all visual components. Everything that goes into the component palette is a Component. Everything that goes into the component and is something that the user of the program can control by using mouse or keyboard is a Control.

For example: TTimer component is a component that you can place on a form, but it is not visible to the end-user. So TTimer is only the component but not a control in Delphi. Other examples like TTimer are TXMLDocument, TDatabase, TQuery, TTable, TDataSet, TDataSource etc. These all components play a vital role but never appear in front of the end user.

A TEdit or TButton is visible and can be controlled by the user (you can add events to TEdit and TButton and with those events a user can actually focus/select/click on the TEdit and TButton components). So both TEdit and TButton are the components as well as controls in Delphi. 

A control takes up more resources than a component because a control needs extra info about how to draw itself.