I'm sorry if my question doesn't meet the standards of SO, but I really had some hard time going through the last few words within this definition of ABSTRACTION from Grady Booch
“An abstraction denotes the essential
characteristics of an object that distinguish it from all other kinds
of objects and thus provide crisply defined conceptual boundaries,
relative to the perspective of the viewer"
Please explain what does he mean by "relative to the perspective of the viewer". Any example would be really helpful.
They simply mean that from the point of view of the person trying to understand the abstraction, it should be clear what it is, what in includes and what it doesn't.
However, how is implemented might not be that clearly different from other abstractions.
For example:
A URI is a different abstraction from a Name. It's clear to a developer and a user what either are. However, implementation-wise they both might be little more than strings.
I think that what they are trying to say is that the semantics and the behaviors define abstractions correctly, not how they are going to be implemented.
Definitions in OOP world are different and not always very clear, for example, I can bring you a definition of abstraction from Tony Hoare:
"Abstraction arises from a recognition of similarities
between certain objects, situations, or processes in the real world,
and the decision to concentrate upon those similarities and to
ignore for the time being the differences."
Maybe this is clearer to you. However, I do not care too much about the words of these definitions.
What is important to understand about abstraction is that it has the function to expose to the user (or viewer) a set of behaviors (an interface) that completely describe and identify an entity (or object). Once you know these behaviors (methods) you can and should ignore the actual implementation of these methods. What the user should care is to provide input parameters and to receive the right results.
I think this is a more practical definition of abstraction.
Related
I am trying to understand the basic OOP concept called abstraction. When I say "understand", I mean not just to learn a definition, but really have a deep understanding.
On the internet, I have seen many definitions such as:
Hiding the low level implementation and providing high level specification
and
focusing on essential qualities rather than specific examples.
I understand that the iPhone button is a great example of abstraction, since I, as a user, don't have to know how the screen is displayed, all I have to know is to press the button.
What do you think of the following conclusion, when it comes to abstraction:
Abstraction takes many specific instances of objects and extracts their common information and functions by providing a single, generalised concept.
So based on this, a class is actually an abstraction of many instances, right?
I disagree with both of your examples. An iPhone button is not an abstraction of the screen, it is an interface to use the phone. A class is also not an abstraction of its instances.
An abstraction can be thought of treating a specific concept as a form of a more general concept.
To repeat an overused example: all vehicles can move. Cars rotate wheels, airplanes use jets, trains run on tracks.
Given a collection of vehicles, instead of being burdened with knowing the specifics of each vehicles' inner workings, and having to:
car.RotateWheel();
airplane.StartJet();
train.MoveOnTrack();
we could treat these objects as the more abstract vehicle, and tell them to
vehicle.Move();
In this case vehicle is an abstraction. It does not represent any specific object, but represents the common functionality of cars, airplanes and trains and allows us to interact with these specific objects without knowing anything about them except that they are a type of vehicle.
In the context of OOP, vehicle would most likely be a base class of the more specific types of vehicles.
IMHO there are actually 2 underlying concepts that needs to be understood here.
Abstraction: The idea of dealing only with "What" of something rather than "How" of something. For example: When you call an object method you only care about what the method does and not how it does what it does. There are layers of abstraction i.e the upper layer is only interested in what the below layer does and not how it does it. Another example: When you are writing assembly instruction you only care what a particular instruction does and not how the underlying circuit in the CPU execute the instruction.
Generalization: The idea of comparing a bunch of things (objects, functions, basically anything) and figure out the commonality between them and then extracting that commonality. A class with a bunch of properties is the generalization of the instances of the classes as all the instances have the same properties but different values for those properties.
The goal of object-oriented programming is to take the real-world thinking into software development as much as possible. That is, abstraction means what any dictionary may define.
For example, one of possible definitions of abstraction in Oxford Dictionary:
The quality of dealing with ideas rather than events.
WordReference.com's definition is even more eloquent:
the act of considering something as a general quality or characteristic, apart from concrete realities, specific objects, or actual instances.
In fact, WordReference.com's one is one of possible definitions of abstraction and you should be surprised because it's not a programming explanation of abstraction.
Perhaps you want a more programming alike definition of abstraction, and I'll try to provide a good summary:
Abstraction is the process of turning concrete realities into object representations which could be used as archetypes. Usually, in most OOP languages, archetypes are represented by types which in turn could be defined by classes, structures and interfaces. Types may abstract data or behaviors.
One good example of abstraction would be that a chair made of oak wood is still a chair. That's the way our mind works. You learn that certain forms are the most basic definition of many things. Your brain doesn't see all details of a given chair, but it sees that it fulfills the requirements to consider something a chair. Object-oriented programming and abstraction just mirrors this.
I've read the other posts discussing abstraction and encapsulation, but I'm not confident I understand them; or maybe I understand them but feel unsatisfied with the clarity of their content. Here are my understandings of abstraction and encapsulation. In what regards are they accurate/inaccurate/complete/incomplete?
"Abstractions are data types created by programmers to extend a language when primitive data types are insufficient. Like primitive data types, abstractions have specifications which list the inputs they require and the outputs they return, but the specifications do not overwhelm programmers with the methods, functions, and variables used to operate on the inputs. A class is an example of an abstraction. An API is another example of an abstraction."
"Encapsulation is the state of having abstract data types — i.e. classes — isolated from each other so their methods, functions, and variables do not conflict with each other, and so programmers can easily reuse an existing class in other programs without being concerned that doing so would interfere with the rest of the program (presuming the programmer correctly provides the required inputs and correctly handles the data that get returns)."
I prefer Robert C. Martin's definition in APPP:
Abstraction is the elimination of the irrelevant and the amplification of the essential.
I'd say your understanding is correct ... so much, so, that I hesitate to comment more specifically.
However, if I were to comment, I might say that "Data types can be used to implement abstractions ...", rather than "Abstractions are data types ...", since abstractions can exist outside of software (it hurt me to say that :-).
But that's just nitpicking. I think you understand. I hope I do, after 36 years of coding ... mostly in languages that support reasonable levels of abstraction (PL/1, Pascal, C, C++, Java).
There are a lot of nice intelligent people in industry, though, who have no concept of abstraction in software, and consider it pretentiously high brow.
Personally, I think that good clear misnomer-free abstraction is a key technical ingredient of solid software engineering.
I've never come across that definition of encapsulation before. That definition sounds more like what namespaces are for. I've always read about encapsulation being purely about the ability to restrict access to certain components of your code, such as access modifiers in OOP languages. However, there seems to be a two definitions of encapsulation on wikipedia, which is news to me:
Encapsulation is the packing of data and functions into a single
component. The features of encapsulation are supported using classes
in most object-oriented programming languages, although other
alternatives also exist. It allows selective hiding of properties and
methods in an object by building an impenetrable wall to protect the
code from accidental corruption.
In programming languages, encapsulation is used to refer to one of two
related but distinct notions, and sometimes to the combination
thereof:
A language mechanism for restricting access to some of the object's
components.
A language construct that facilitates the bundling
of data with the methods (or other functions) operating on that
data.
Some programming language researchers and academics use
the first meaning alone or in combination with the second as a
distinguishing feature of object-oriented programming, while other
programming languages which provide lexical closures view
encapsulation as a feature of the language orthogonal to object
orientation.
The second definition is motivated by the fact that in many OOP
languages hiding of components is not automatic or can be overridden;
thus, information hiding is defined as a separate notion by those who
prefer the second definition.
source
So, I guess I've always defined encapsulation in terms of point #1, but it looks like some people define it as the ability to bundle methods and data together, and term #1 "information hiding".
I'm wondering whether an object asking a question to another object that indirectly holds it is "bad" design. For example...
Requirements:
Character (an object) moves on a grid. When it tries to move to another spot, it needs to know whether that spot is already occupied by something that blocks it, or if that part of the grid is completely inaccessible. (Note that the character itself needs to know).
In the application, a state holds a tilemanager and a charactermanager. The tilemanager knows what tiles are accessible and which aren't. The charactermanager knows the characters' tile locations.
Would it be reasonable for the character to call a function from the state, say AuthorizeMovement, which determines whether movement is possible via its TileManager and CharacterManager, and returns true if so, false if not?
Is this violating any important principles, leading to trouble down the road?
Obviously this is generalized and stripped down to what is necessary to understand the problem.
I'd suggest that it is likely a bad design, yes. The "red flag," so to speak, is the circular reference. You said:
... an object asking a question to another object that indirectly holds it
So, the "holding" object has a reference to the "held" object, and also in order to "ask a question" the "held" object will need a reference to the "holding" object.
That's makes a circular object dependency graph and is often a code smell.
It would seem some other class should have the responsibility of knowing about both the character and the TileManager and/or CharacterManager.
I don't see a problem. Good OO Design comes with many principles. But at its core you have the big 4: Encapsulation, Inheritance, Polymorphism, and Abstraction. In addition, you want high cohesion and low coupling. Meaning your objects/classes can fit anywhere and aren't tied to a particular implementation or class.
With that said, it sounds like you have used the above to encapsulated movement and characters abstract them into separate classes. So your Character class isn't modifying the board directly, which would be bad if it were.
As you gain a deeper understanding of your problem, you can always refactor you code to improve your design to take advantage of other principles.
It's not against any OOP principle. The details of the call are totally abstracted and you depend on the State object anyway. How on earth else would you possibly implement this function?
Abstraction and principles are useful tools. But you shouldn't depend on them to qualify your code as good. Not every abstraction or every principle is good for every scenario or every possible implementation. They're guidelines, not rules. If you can't quickly see an alternative implementation, use this one and then come back to it.
In the SRP, a 'responsibility' is usually described as 'a reason to change', so that each class (or object?) should have only one reason someone should have to go in there and change it.
But if you take this to the extreme fine-grain you could say that an object adding two numbers together is a responsibility and a possible reason to change. Therefore the object should contain no other logic, because it would produce another reason for change.
I'm curious if there is anyone out there that has any strategies for 'scoping', the single-responsibility principle that's slightly less objective?
it comes down to the context of what you are modeling. I've done some extensive writing and presenting on the SOLID principles and I specifically address your question in my discussions of Single Responsibility.
The following first appeared in the Jan/Feb 2010 issue of Code Magazine, and is available online at "S.O.L.I.D. Software Development, One Step at a Time"
The Single Responsibility Principle
says that a class should have one, and
only one, reason to change.
This may seem counter-intuitive at
first. Wouldn’t it be easier to say
that a class should only have one
reason to exist? Actually, no-one
reason to exist could very easily be
taken to an extreme that would cause
more harm than good. If you take it to
that extreme and build classes that
have one reason to exist, you may end
up with only one method per class.
This would cause a large sprawl of
classes for even the most simple of
processes, causing the system to be
difficult to understand and difficult
to change.
The reason that a class should have
one reason to change, instead of one
reason to exist, is the business
context in which you are building the
system. Even if two concepts are
logically different, the business
context in which they are needed may
necessitate them becoming one and the
same. The key point of deciding when a
class should change is not based on a
purely logical separation of concepts,
but rather the business’s perception
of the concept. When the business
perception and context has changed,
then you have a reason to change the
class. To understand what
responsibilities a single class should
have, you need to first understand
what concept should be encapsulated by
that class and where you expect the
implementation details of that concept
to change.
Consider an engine in a car, for
example. Do you care about the inner
working of the engine? Do you care
that you have a specific size of
piston, camshaft, fuel injector, etc?
Or, do you only care that the engine
operates as expected when you get in
the car? The answer, of course,
depends entirely on the context in
which you need to use the engine.
If you are a mechanic working in an
auto shop, you probably care about the
inner workings of the engine. You need
to know the specific model, the
various part sizes, and other
specifications of the engine. If you
don’t have this information available,
you likely cannot service the engine
appropriately. However, if you are an
average everyday person that only
needs transportation from point A to
point B, you will likely not need that
level of information. The notion of
the individual pistons, spark plugs,
pulleys, belts, etc., is almost
meaningless to you. You only care that
the car you are driving has an engine
and that it performs correctly.
The engine example drives straight to
the heart of the Single Responsibility
Principle. The contexts of driving the
car vs. servicing the engine provide
two different notions of what should
and should not be a single concept-a
reason for change. In the context of
servicing the engine, every individual
part needs to be separate. You need to
code them as single classes and ensure
they are all up to their individual
specifications. In the context of
driving a car, though, the engine is a
single concept that does not need to
be broken down any further. You would
likely have a single class called
Engine, in this case. In either case,
the context has determined what the
appropriate separation of
responsibilities is.
I tend to think in term of "velocity of change" of the business requirements rather than "reason to change" .
The question is indeed how likely stuffs will change together, not whether they could change or not.
The difference is subtle, but helps me. Let's consider the example on wikipedia about the reporting engine:
if the likelihood that the content and the template of the report change at the same time is high, it can be one component because they are apparently related. (It can also be two)
but if the likelihood that the content change without the template is important, then it must be two components, because they are not related. (Would be dangerous to have one)
But I know that's a personal interpretation of the SRP.
Also, a second technique that I like is: "Describe your class in one sentence". It usually helps me to identify if there is a clear responsibility or not.
I don't see performing a task like adding two numbers together as a responsibility. Responsibilities come in different shapes and sizes but they certainly should be seen as something larger than performing a single function.
To understand this better, it is probably helpful to clearly differentiate between what a class is responsible for and what a method does. A method should "do only one thing" (e.g. add two numbers, though for most purposes '+' is a method that does that already) while a class should present a single clear "responsibility" to it's consumers. It's responsibility is at a much higher level than a method.
A class like Repository has a clear and singular responsibility. It has multiple methods like Save and Load, but a clear responsibility to provide persistence support for Person entities. A class may also co-ordinate and/or abstract the responsibilities of dependent classes, again presenting this as a single responsibility to other consuming classes.
The bottom line is if the application of SRP is leading to single-method classes who's whole purpose seems to be just to wrap the functionality of that method in a class then SRP is not being applied correctly.
A simple rule of thumb I use is that: the level or grainularity of responsibility should match the level or grainularity of the "entity" in question. Obviously the purpose of a method will always be more precise than that of a class, or service, or component.
A good strategiy for evaluating the level of responsibility can be to use an appropriate metaphor. If you can relate what you are doing to something that exists in the real world it can help give you another view of the problem you're trying to solve - including being able to identify appropriate levels of abstraction and responsibility.
#Derick bailey: nice explanation
Some additions: It is totally acceptable that application of SRP is contextual base.
The question still remains: are there any objective ways to define if a given class violates SRP ?
Some design contexts are quite obvious ( like the car example by Derick ) but otherwise contexts in which a class's behaviour has to defined remains fuzzy many-a-times.
For such cases, it might well be helpful if the fuzzy class behaviour is analysed by splitting it's responsibilities into different classes and then measuring the impact of new behavioural and structural relations that has emanated because of the split.
As soon the split is done, the reasons to keep the splitted responsibilities or to back-merge them into single responsibility becomes obvious at once.
I have applied this approach and which has lead good results for me.
But my search to look for 'objective ways of defining a class responsibility' still continues.
I respectful don't agree when Chris Nicola's above says that "a class should presents a single clear "responsibility" to it's consumers
I think SRP is about having a good design inside the class, not class' customers.
To me it's not very clear what a responsability is, and the prove is the number of questions that this concept arises.
"single reason to change"
or
"if the description contains the word
"and" then it needs to be split"
leads to the question: where is the limit? At the end, any class with 2 public methods has 2 reasons to change, isn't it?
For me, the true SRP leads to the Facade pattern, where you have a class that simply delegades the calls to other classes
For example:
class Modem
send()
receive()
Refactors to ==>
class ModemSender
class ModelReceiver
+
class Modem
send() -> ModemSender.send()
receive() -> ModemReceiver.receive()
Opinions are wellcome
I 'm concern about what techniques should I use to choose the right object in OOP
Is there any must-read book about OOP in terms of how to choose objects?
Best,
Just write something that gets the job done, even if it's ugly, then refactor continuously:
eliminate duplicate code (don't repeat yourself)
increase cohesion
reduce coupling
But:
don't over-engineer; keep it simple
don't write stuff you ain't gonna need
It's not a precise recipe, just some general guidelines. Keep practicing.
P.S.
Code objects are not related to tangible real-life objects; they are just constructs that hold related information together.
Don't believe what the Java books/schools teach about objects; they're lying.
You probably mean "the right class", rather than "the right object". :-)
There are a few techniques, such as text analysis (a.k.a. underlining the nouns) and Class Responsibility Collaborator (CRC).
With "underlining the nouns", you basically start with a written, natural language (i.e. plain English) description of the problem you want to solve and underline the nouns. That gives you a list of candidate classes. You will need to perform several passes to refine it into a list of classes to implement.
For CRC, check out the Wikipedia.
I suggest The OPEN Toolbox of Techniques for full reference.
Hope it helps.
I am assuming that there is understanding of what is sctruct, type, class, set, state, alphabet, scalar and vector and relationship.
Object is a noun, method is a verb. Object members can represent identity, state or scalar value per field. Relationships between objects usually are represented with references, where references are members of objects. In cases, when relationships are complex, multidirectional, have arity greater than 2, represent some sort of grouping or containment, then relationships can be expressed as objects.
For other, broader technical reasons objects are most likely the only way to represent any form of information in OOP languages.
I am adding a second answer due to demian's comment:
Sometimes the class is so obvious
because it's tangible, but other times
the concept of object it's to abstract
like a db connector.
That is true. My preferred approach is to perform a behavioural analysis of the system (using use cases, for example), and then derive system operations. Once you have a stable list of system operations (such as PrintDocument, SaveDocument, SpellCheck, MergeMail, etc. for a word processor) you need to assign each of them to a class. If you have developed a list of candidate classes with some of the techniques that I mentioned earlier, you will be able to allocate some of the operations. But some will remain unallocated. These will signal the need of more abstract or unintuitive classes, which you will need to make up, using your good judgment.
The whole method is documented in a white paper at www.openmetis.com.
You should check out Domain-Driven Design, by Eric Evans. It provides very useful concepts in thinking about the objects in your model, what their function are in the domain, and how they could be organized to work together. It's not a cookbook, and probably not a beginner book - but then, I read it at different stages of my career, and every time I found something valuable in it...
(source: domaindrivendesign.org)