Abstract class and Interfaces

[ajairajan]

Hi ,

I am new to the field of IT .I actually want to know according to what a designer selects it is good to use Abstract class or Interfaces in apartcular solution.

It would be more helpful if you describe it with some code examples?

Thanks in advance,
Ajai.

[Motoma]

Hi ,

I am new to the field of IT .I actually want to know according to what a designer selects it is good to use Abstract class or Interfaces in apartcular solution.

It would be more helpful if you describe it with some code examples?

Thanks in advance,
Ajai.

The way I usually decide when to use inheritance is the is/has test. I say, is does my new class have this other class or is it of this other class.

An example:
I have classes Monster, and Hammer, and I want to create a class of type Troll.
I say to myself "Self, is a Troll a Monster, or does it have a Monster?" To which I answer "It is a Monster." I then say "Now Motoma, is a Troll a Hammer, or does it have a Hammer?" I then write my Troll class to be a subclass of Monster, and have a Hammer property.

[JosAH]

The distinction of an abstract class and an interface differs per programming
language; e.g. C++ doesn't even have a keyword for an interface. Using C++
you simply build a class with all pure virtual functions.

Java does make the distinction and so it has an 'interface' keyword and its
associated semantics.

There's a little mantra in the OO world that says: "code to the interface, not the
implementation". If you follow that little mantra at least one part of your design
will probably be fine. The next pitfall is the abuse of inheritance of both type
(interface) and implementation (classes).

Before you want to exend a type/class ask yourself the question if the inheritance
can stand the Liskov Substitution Principle (LSP) and even then think again
and consider composition over extension.

kind regards,

Jos

[Ganon11]

In the specific example of having to choose between an interface and an abstract class, it can get kind of tricky. I usually rely on the definitions of the two for what I'm doing.

An interface cannot define any of the methods it includes. Any class using the interface defines these methods - the interface itself has no idea of how its methods are implemented. Consider the Bird interface. Now, every Bird can fly, but the interface shouldn't know exactly how each bird flies. A hummingbird flies differently than a Robin, which flies differently than a Sparrow, and a Penguin doesn't even fly! Bird best fits as an interface.

In an abstract class, you can provide definitions for methods that all subclasses will share - you can choose as little as 1 method to be abstract. Consider the abstract class Employee. Now, every Employee has a first name, a last name, and an ID number. These are common to all Employees, so you can write the code to get/set these elements in the Employee class. But every Employee also gets a salary - however, an Hourly worker gets paid differently than a Contracted Worker. In this case, the getSalary() method would be abstract in the Employee class.

To summarize, use an interface when you don't know exactly how any implementing class will work, and use an abstract class when any subclasses will have some behavior in common.

[EnglishToUML]

I have seen the abuse of interfaces, as noted above to achieve multiple inheritance. This is irritating. An interface is to specify behavior. Then we dont need to use it to define class models, or data models.

Interfaces were introduced in CORBA/DCOM/COM era to facilitate distributed components specification only in terms of behavior. At that level, you are mainly focused in a component's behavior. But that does not mean that, every class you create will have to have an interface. In other words, you dont create interfaces while defining a good data model.

If you allow a component's organization in terms of entity classes, their associations, and relations, and a control class to manage these entities, collections, and relations, then specify an interface for the control class, and not for others.

No doubt you see, interfaces in libraries. Such interfaces could be for collections, etc. That is fine for those defining libraries. But designers have to think of using interfaces only for control classes. Otherwise, you get into doubling of classes. Also its a burden for the one who maintains code.

[praveen2gupta]

Hi ,

I am new to the field of IT .I actually want to know according to what a designer selects it is good to use Abstract class or Interfaces in apartcular solution.

It would be more helpful if you describe it with some code examples?

Thanks in advance,
Ajai.

In Java Both are ahving different purpose. It seems that there is some similarity that method are abstract in the both.
Interface is used to implement the multiple inhertance. Since java does not supports multiple inheritance so this is an alternate for it.

abstract is used when classes are at initial stage and we wants to bind the developers with certain conditions.