Is it correct to define a type within an interface? And what about an attribute?
Some programming languages only allow method signatures for an interface, but in ABAP I was considering doing something like the code below, even adding more attributes, and I'm not sure if the approach is a correct OOdesign:
INTERFACE zif_notif_note.
TYPES: BEGIN OF ty_note_id,
notif_num TYPE qmnum,
activity_key TYPE aknum,
END OF ty_note_id.
DATA: id TYPE ty_note_id READ-ONLY.
METHODS get_id
RETURNING VALUE(r_id) TYPE ty_note_id.
ENDINTERFACE.
What if I need to use this type as an input parameter for another class? Should I better define DDIC structure then?
To your first question: nothing speaks against defining a type in an interface.
To your second question...
You can do many things in ABAP OO that you cannot do in other object oriented languages like Java or C++. For example you can define a FINAL ABSTRACT CLASS which is nonsense, you can define static methods (CLASS-METHODS) in an interface which is in my opinion a heresy towards the object oriented programming.
I have never defined attributes unless they were CONSTANTS within an interface, because there is no reason for doing it. They will be automatically marked as PUBLIC which breaks in fact the encapsulation principle.
So the answer would be: It is possible but think twice before going that way because it might be breaking the encapsulation of the classes that implement the interface.
Assuming I have an interface and 3 or 4 classes that implement the interface. Now let's say I have another method but it is relevant only for one or two implementing classes.
So as I see it there are two approaches
Declare the method in the interface, implement in all the classes. In classes that the method is not relevant - throw "not supported" exception.
Declare the method only in the implementing classes where relevant but then you have to downcast the instance from interface type to the specific type in order to call the method.
So would be your design here and why?
You should apply the interface segregation principle here, part of the SOLID principles. Instead of having one large interface, split the interfaces up into a few smaller interfaces. Implement the interfaces only in the classes that need it.
I would choose a parent interface. Make the classes that satisfy the interface implement that interface. Create a second interface that extends the first one and adds the extra methods and then make the remaining classes implement that interface. You would still be coding to an interface rather than an implementation and all your design principles are satisfied.
Ask yourself this, will the object that consume the interface necessarily expect this function to be there? Is it important that no matter which instance of the interface you pass, you'll still have that method available?
If the answer to that question is yes, 1. If no, 2.
I could not find the main difference. And I am very confused when we could use inheritance and when we can use subtyping. I found some definitions but they are not very clear.
What is the difference between subtyping and inheritance in object-oriented programming?
In addition to the answers already given, here's a link to an article I think is relevant.
Excerpts:
In the object-oriented framework, inheritance is usually presented as a feature that goes hand in hand with subtyping when one organizes abstract datatypes in a hierarchy of classes. However, the two are orthogonal ideas.
Subtyping refers to compatibility of interfaces. A type B is a subtype of A if every function that can be invoked on an object of type A can also be invoked on an object of type B.
Inheritance refers to reuse of implementations. A type B inherits from another type A if some functions for B are written in terms of functions of A.
However, subtyping and inheritance need not go hand in hand. Consider the data structure deque, a double-ended queue. A deque supports insertion and deletion at both ends, so it has four functions insert-front, delete-front, insert-rear and delete-rear. If we use just insert-rear and delete-front we get a normal queue. On the other hand, if we use just insert-front and delete-front, we get a stack. In other words, we can implement queues and stacks in terms of deques, so as datatypes, Stack and Queue inherit from Deque. On the other hand, neither Stack nor Queue are subtypes of Deque since they do not support all the functions provided by Deque. In fact, in this case, Deque is a subtype of both Stack and Queue!
I think that Java, C++, C# and their ilk have contributed to the confusion, as already noted, by the fact that they consolidate both ideas into a single class hierarchy. However, I think the example given above does justice to the ideas in a rather language-agnostic way. I'm sure others can give more examples.
A relative unfortunately died and left you his bookstore.
You can now read all the books there, sell them, you can look at his accounts, his customer list, etc. This is inheritance - you have everything the relative had. Inheritance is a form of code reuse.
You can also re-open the book store yourself, taking on all of the relative's roles and responsibilities, even though you add some changes of your own - this is subtyping - you are now a bookstore owner, just like your relative used to be.
Subtyping is a key component of OOP - you have an object of one type but which fulfills the interface of another type, so it can be used anywhere the other object could have been used.
In the languages you listed in your question - C++, Java and C# - the two are (almost) always used together, and thus the only way to inherit from something is to subtype it and vice versa. But other languages don't necessarily fuse the two concepts.
Inheritance is about gaining attributes (and/or functionality) of super types. For example:
class Base {
//interface with included definitions
}
class Derived inherits Base {
//Add some additional functionality.
//Reuse Base without having to explicitly forward
//the functions in Base
}
Here, a Derived cannot be used where a Base is expected, but is able to act similarly to a Base, while adding behaviour or changing some aspect of Bases behaviour. Typically, Base would be a small helper class that provides both an interface and an implementation for some commonly desired functionality.
Subtype-polymorphism is about implementing an interface, and so being able to substitute different implementations of that interface at run-time:
class Interface {
//some abstract interface, no definitions included
}
class Implementation implements Interface {
//provide all the operations
//required by the interface
}
Here, an Implementation can be used wherever an Interface is required, and different implementations can be substituted at run-time. The purpose is to allow code that uses Interface to be more widely useful.
Your confusion is justified. Java, C#, and C++ all conflate these two ideas into a single class hierarchy. However, the two concepts are not identical, and there do exist languages which separate the two.
If you inherit privately in C++, you get inheritance without subtyping. That is, given:
class Derived : Base // note the missing public before Base
You cannot write:
Base * p = new Derived(); // type error
Because Derived is not a subtype of Base. You merely inherited the implementation, not the type.
Subtyping doesn't have to be implemented via inheritance. Some subtyping that is not inheritance:
Ocaml's variant
Rust's lifetime anotation
Clean's uniqueness types
Go's interface
in a simple word: subtyping and inheritance both are polymorphism, (inheritance is a dynamic polymorphism - overriding). Actually, inheritance is subclassing, it means in inheritance there is no warranty to ensure capability of the subclass with the superclass (make sure subclass do not discard superclass behavior), but subtyping(such as implementing an interface and ... ), ensure the class does not discard the expected behavior.
If interfaces have empty methods (implicit abstract method) then what is its use? Why do we say it reduces the code and provides re-usability? Give me a real life example of using an interface that shows the difference between an abstract class and an interface.
Interface is more like a contract. It doesn't provide any implementation reuse as such. Which actually makes your code de-coupled from implementation. Having a abstract class with ALL the methods abstract provides the same benefit (if we ignore the issue of multiple inheritance).
For a really good example take a look at Java Collections and how things are loosely coupled using interfaces for Collection, Map and Lists.
Because of the terminology you are using, I am going to assume you are talking about Java.
An interface is useful in lieu of an abstract class because a class can only inherit from a single class, but can implement multiple interfaces.
An interface is a contract between parts of the program. It says that one part of the program has certain expectations about classes that implement this interface. As long as those classes uphold that interface contract, the other parts of the program don't care how that contract is implemented, just that it is implemented.
It allows for polymorphism and for the reuse of code. For instance, (with respect to Java), you can take the List interface. You can write code that interfaces with a List object where you don't care about the implementation of the List. Your code then can be used with a LinkedList or an ArrayList or any other type of list that it may deal with, and it should be able to manage well enough. You can write code now that has certain expectations through this List interface contract, and 15 years down the road someone can use the latest technologies to create their own List implementation and your code will be able to use it.
Abstract class lets you describe fields and non abstract methods. It does not limit you to simply describing interface, it involves some logic. Interface on other hand does what it says and has nothing to do with logic. From client code side, you have no worries about implementation and how stuff works. It lets you exchange one interface realization with other without additional code.
On realization code side, interface lets you perform multiple "inheritance".
I like to call these types of features "implied code documentation". Using an interface can communicate a lot of information to other developers who will be working on your project, and this information can help prevent a lot of headaches.
For instance, if a class implements an interface that has 2 methods, and I'm new to the project, that may tell me that the developer who wrote those methods don't want the method signature to change.
Think about the Dog class and the Cat class that both implement the interface Sociable, where there are methods walk(int speed), sit(), layDown(), bite(int degree).
If we have a Dog class and a Cat class that implement these methods and there are dependencies on them, changing the method signature of one could have some negative effects.
Interfaces are a way to help describe a class. In this example, a Sociable Dog and a Sociable Cat have a lot in common.
As far as reusability goes, your classes become reusable because it's harder for others to come in and change the contract defined in the method signature.
Lastly, while a class can only subclass one class, it can implement multiple interfaces. Thus, the advantage of using an interface is that I can have a Dog that implements Big and Sociable, and a Cat that implements Small and Sociable.
A friend of mine goes back and forth on what "interface" means in programming.
What is the best description of an "interface"?
To me, an interface is a blueprint of a class. Is this the best definition?
An interface is one of the more overloaded and confusing terms in development.
It is actually a concept of abstraction and encapsulation. For a given "box", it declares the "inputs" and "outputs" of that box. In the world of software, that usually means the operations that can be invoked on the box (along with arguments) and in some cases the return types of these operations.
What it does not do is define what the semantics of these operations are, although it is commonplace (and very good practice) to document them in proximity to the declaration (e.g., via comments), or to pick good naming conventions. Nevertheless, there are no guarantees that these intentions would be followed.
Here is an analogy: Take a look at your television when it is off. Its interface are the buttons it has, the various plugs, and the screen. Its semantics and behavior are that it takes inputs (e.g., cable programming) and has outputs (display on the screen, sound, etc.). However, when you look at a TV that is not plugged in, you are projecting your expected semantics into an interface. For all you know, the TV could just explode when you plug it in. However, based on its "interface" you can assume that it won't make any coffee since it doesn't have a water intake.
In object oriented programming, an interface generally defines the set of methods (or messages) that an instance of a class that has that interface could respond to.
What adds to the confusion is that in some languages, like Java, there is an actual interface with its language specific semantics. In Java, for example, it is a set of method declarations, with no implementation, but an interface also corresponds to a type and obeys various typing rules.
In other languages, like C++, you do not have interfaces. A class itself defines methods, but you could think of the interface of the class as the declarations of the non-private methods. Because of how C++ compiles, you get header files where you could have the "interface" of the class without actual implementation. You could also mimic Java interfaces with abstract classes with pure virtual functions, etc.
An interface is most certainly not a blueprint for a class. A blueprint, by one definition is a "detailed plan of action". An interface promises nothing about an action! The source of the confusion is that in most languages, if you have an interface type that defines a set of methods, the class that implements it "repeats" the same methods (but provides definition), so the interface looks like a skeleton or an outline of the class.
Consider the following situation:
You are in the middle of a large, empty room, when a zombie suddenly attacks you.
You have no weapon.
Luckily, a fellow living human is standing in the doorway of the room.
"Quick!" you shout at him. "Throw me something I can hit the zombie with!"
Now consider:
You didn't specify (nor do you care) exactly what your friend will choose to toss;
...But it doesn't matter, as long as:
It's something that can be tossed (He can't toss you the sofa)
It's something that you can grab hold of (Let's hope he didn't toss a shuriken)
It's something you can use to bash the zombie's brains out (That rules out pillows and such)
It doesn't matter whether you get a baseball bat or a hammer -
as long as it implements your three conditions, you're good.
To sum it up:
When you write an interface, you're basically saying: "I need something that..."
Interface is a contract you should comply to or given to, depending if you are implementer or a user.
I don't think "blueprint" is a good word to use. A blueprint tells you how to build something. An interface specifically avoids telling you how to build something.
An interface defines how you can interact with a class, i.e. what methods it supports.
In Programming, an interface defines what the behavior a an object will have, but it will not actually specify the behavior. It is a contract, that will guarantee, that a certain class can do something.
Consider this piece of C# code here:
using System;
public interface IGenerate
{
int Generate();
}
// Dependencies
public class KnownNumber : IGenerate
{
public int Generate()
{
return 5;
}
}
public class SecretNumber : IGenerate
{
public int Generate()
{
return new Random().Next(0, 10);
}
}
// What you care about
class Game
{
public Game(IGenerate generator)
{
Console.WriteLine(generator.Generate())
}
}
new Game(new SecretNumber());
new Game(new KnownNumber());
The Game class requires a secret number. For the sake of testing it, you would like to inject what will be used as a secret number (this principle is called Inversion of Control).
The game class wants to be "open minded" about what will actually create the random number, therefore it will ask in its constructor for "anything, that has a Generate method".
First, the interface specifies, what operations an object will provide. It just contains what it looks like, but no actual implementation is given. This is just the signature of the method. Conventionally, in C# interfaces are prefixed with an I.
The classes now implement the IGenerate Interface. This means that the compiler will make sure, that they both have a method, that returns an int and is called Generate.
The game now is being called two different object, each of which implementant the correct interface. Other classes would produce an error upon building the code.
Here I noticed the blueprint analogy you used:
A class is commonly seen as a blueprint for an object. An Interface specifies something that a class will need to do, so one could argue that it indeed is just a blueprint for a class, but since a class does not necessarily need an interface, I would argue that this metaphor is breaking. Think of an interface as a contract. The class that "signs it" will be legally required (enforced by the compiler police), to comply to the terms and conditions in the contract. This means that it will have to do, what is specified in the interface.
This is all due to the statically typed nature of some OO languages, as it is the case with Java or C#. In Python on the other hand, another mechanism is used:
import random
# Dependencies
class KnownNumber(object):
def generate(self):
return 5
class SecretNumber(object):
def generate(self):
return random.randint(0,10)
# What you care about
class SecretGame(object):
def __init__(self, number_generator):
number = number_generator.generate()
print number
Here, none of the classes implement an interface. Python does not care about that, because the SecretGame class will just try to call whatever object is passed in. If the object HAS a generate() method, everything is fine. If it doesn't: KAPUTT!
This mistake will not be seen at compile time, but at runtime, so possibly when your program is already deployed and running. C# would notify you way before you came close to that.
The reason this mechanism is used, naively stated, because in OO languages naturally functions aren't first class citizens. As you can see, KnownNumber and SecretNumber contain JUST the functions to generate a number. One does not really need the classes at all. In Python, therefore, one could just throw them away and pick the functions on their own:
# OO Approach
SecretGame(SecretNumber())
SecretGame(KnownNumber())
# Functional Approach
# Dependencies
class SecretGame(object):
def __init__(self, generate):
number = generate()
print number
SecretGame(lambda: random.randint(0,10))
SecretGame(lambda: 5)
A lambda is just a function, that was declared "in line, as you go".
A delegate is just the same in C#:
class Game
{
public Game(Func<int> generate)
{
Console.WriteLine(generate())
}
}
new Game(() => 5);
new Game(() => new Random().Next(0, 10));
Side note: The latter examples were not possible like this up to Java 7. There, Interfaces were your only way of specifying this behavior. However, Java 8 introduced lambda expressions so the C# example can be converted to Java very easily (Func<int> becomes java.util.function.IntSupplier and => becomes ->).
To me an interface is a blueprint of a class, is this the best definition?
No. A blueprint typically includes the internals. But a interface is purely about what is visible on the outside of a class ... or more accurately, a family of classes that implement the interface.
The interface consists of the signatures of methods and values of constants, and also a (typically informal) "behavioral contract" between classes that implement the interface and others that use it.
Technically, I would describe an interface as a set of ways (methods, properties, accessors... the vocabulary depends on the language you are using) to interact with an object. If an object supports/implements an interface, then you can use all of the ways specified in the interface to interact with this object.
Semantically, an interface could also contain conventions about what you may or may not do (e.g., the order in which you may call the methods) and about what, in return, you may assume about the state of the object given how you interacted so far.
Personally I see an interface like a template. If a interface contains the definition for the methods foo() and bar(), then you know every class which uses this interface has the methods foo() and bar().
Let us consider a Man(User or an Object) wants some work to be done. He will contact a middle man(Interface) who will be having a contract with the companies(real world objects created using implemented classes). Few types of works will be defined by him which companies will implement and give him results.
Each and every company will implement the work in its own way but the result will be same. Like this User will get its work done using an single interface.
I think Interface will act as visible part of the systems with few commands which will be defined internally by the implementing inner sub systems.
An interface separates out operations on a class from the implementation within. Thus, some implementations may provide for many interfaces.
People would usually describe it as a "contract" for what must be available in the methods of the class.
It is absolutely not a blueprint, since that would also determine implementation. A full class definition could be said to be a blueprint.
An interface defines what a class that inherits from it must implement. In this way, multiple classes can inherit from an interface, and because of that inherticance, you can
be sure that all members of the interface are implemented in the derived class (even if its just to throw an exception)
Abstract away the class itself from the caller (cast an instance of a class to the interface, and interact with it without needing to know what the actual derived class IS)
for more info, see this http://msdn.microsoft.com/en-us/library/ms173156.aspx
In my opinion, interface has a broader meaning than the one commonly associated with it in Java. I would define "interface" as a set of available operations with some common functionality, that allow controlling/monitoring a module.
In this definition I try to cover both programatic interfaces, where the client is some module, and human interfaces (GUI for example).
As others already said, an interface always has some contract behind it, in terms of inputs and outputs. The interface does not promise anything about the "how" of the operations; it only guarantees some properties of the outcome, given the current state, the selected operation and its parameters.
As above, synonyms of "contract" and "protocol" are appropriate.
The interface comprises the methods and properties you can expect to be exposed by a class.
So if a class Cheetos Bag implements the Chip Bag interface, you should expect a Cheetos Bag to behave exactly like any other Chip Bag. (That is, expose the .attemptToOpenWithoutSpillingEverywhere() method, etc.)
A boundary across which two systems communicate.
Interfaces are how some OO languages achieve ad hoc polymorphism. Ad hoc polymorphism is simply functions with the same names operating on different types.
Conventional Definition - An interface is a contract that specifies the methods which needs to be implemented by the class implementing it.
The Definition of Interface has changed over time. Do you think Interface just have method declarations only ? What about static final variables and what about default definitions after Java 5.
Interfaces were introduced to Java because of the Diamond problem with multiple Inheritance and that's what they actually intend to do.
Interfaces are the constructs that were created to get away with the multiple inheritance problem and can have abstract methods , default definitions and static final variables.
http://www.quora.com/Why-does-Java-allow-static-final-variables-in-interfaces-when-they-are-only-intended-to-be-contracts
In short, The basic problem an interface is trying to solve is to separate how we use something from how it is implemented. But you should consider interface is not a contract. Read more here.