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I was learning JS and came across the term OOP. Then, as I was learning OOP and I found such concepts as abstraction and encapsulation. Moreover, as I was making researching on the difference between them, majority of articles says that both abstraction and encapsulation are concerned with data hiding which confused me very much. However, I made an inference that encapsulation is when we just put variables and functions that operate on them in OBJECT while abstraction is when we use access modifiers to restrict access to properties or functions in an object. Again, encapsulation is when we use capsule like object and put variables and functions in it to achieve organization and reusability. But abstraction is when we restrict access to variables and functions inside object. IS THAT TRUE?
Encapsulation means that you protect your data inside your class (or object) for the outer world and like you say you access the private or protected data via methods (functions) from the same object.
Abstraction is something totally different. Lets take an example.
Imagine that you want to use constraints on your form fields. For example you have a formfield 'email' which may not be empty and must contain an valid email.
So you want to add different kind of constraints to a large number of form fields all over your application.
All constraints must have two methods:
1) a method that tests the formfield data if it is valid
2) a method that delivers an error message if the data is not valid
Now before you write all those different constraint objects you could write one ABSTRACT parent class That includes those two methods but without any code:
(I use PHP)
abstract class Constraint
{
// Force Extending classes to define this methods
abstract protected function validate(string $data);
abstract protected function getErrorMessage();
}
Above class is abstract and cannot be used (instantiated) by its self. But you can write different constraint classes that inherit the above class:
class NotEmptyConstraint extends Constraint
{
// Mandatory overwritten methods:
protected function validate(string $data)
{
if(strlen($data) > 0 ) {
return true;
}
return false;
}
protected function getErrorMessage();
{
return 'This is a mandatory field';
}
}
Now all your constraint classes have the two mandatory methods which you can safely use without you need to know which exact constraint your dealing with.
Hope it helps..
OOP is dogma.
The author of this website wrote an article on abstraction and why it doesn't work: https://www.joelonsoftware.com/2002/11/11/the-law-of-leaky-abstractions/
In other words, cell phones and LAN phones are not based on the same hardcoded abstraction of a phone. They are free to be their own classes. Barbara Liskov (SOLID) would argue each are subtypes. I strongly discourage tying your hands by creating such arbitrary hierarchical categorization schemes. This is not how real machines are engineered. Refer to fundamental principles of mechanical design, which dictate simplicity of design, meaning less components when feasible. A separate abstract class is a waste of time and distributes the design itself into two modules.
Encapsulation is the fallacious idea that objects own the data they operate on and, therefore, must control access to it. This notion is disproven by manufacturing's model for collaborative work, which is what my architecture is based on. A screwdriver doesn't own a screw simply because it changes its position. It simply has access to it.
OOP and encapsulation led to the distributed process code model. Objects e-mail work to one another like useless office employees who don't keep e-mail trails. If you've ever gotten e-mail from people asking for help who don't give you any information, then you understand my parody. In manufacturing, steps are documented and never communicate, which creates tremendous benefits. OOP is only enough know-how to write monolithic automated processes, not processes which stop on a dime, reverse, can insert or replace steps on the fly, and try steps again. OOP is a model for distributed manufacturing, whereas manufacturing is a model for organizing everyone who creates some product in the same space.
I have seen this mentioned a few times and I am not clear on what it means. When and why would you do this?
I know what interfaces do, but the fact I am not clear on this makes me think I am missing out on using them correctly.
Is it just so if you were to do:
IInterface classRef = new ObjectWhatever()
You could use any class that implements IInterface? When would you need to do that? The only thing I can think of is if you have a method and you are unsure of what object will be passed except for it implementing IInterface. I cannot think how often you would need to do that.
Also, how could you write a method that takes in an object that implements an interface? Is that possible?
There are some wonderful answers on here to this questions that get into all sorts of great detail about interfaces and loosely coupling code, inversion of control and so on. There are some fairly heady discussions, so I'd like to take the opportunity to break things down a bit for understanding why an interface is useful.
When I first started getting exposed to interfaces, I too was confused about their relevance. I didn't understand why you needed them. If we're using a language like Java or C#, we already have inheritance and I viewed interfaces as a weaker form of inheritance and thought, "why bother?" In a sense I was right, you can think of interfaces as sort of a weak form of inheritance, but beyond that I finally understood their use as a language construct by thinking of them as a means of classifying common traits or behaviors that were exhibited by potentially many non-related classes of objects.
For example -- say you have a SIM game and have the following classes:
class HouseFly inherits Insect {
void FlyAroundYourHead(){}
void LandOnThings(){}
}
class Telemarketer inherits Person {
void CallDuringDinner(){}
void ContinueTalkingWhenYouSayNo(){}
}
Clearly, these two objects have nothing in common in terms of direct inheritance. But, you could say they are both annoying.
Let's say our game needs to have some sort of random thing that annoys the game player when they eat dinner. This could be a HouseFly or a Telemarketer or both -- but how do you allow for both with a single function? And how do you ask each different type of object to "do their annoying thing" in the same way?
The key to realize is that both a Telemarketer and HouseFly share a common loosely interpreted behavior even though they are nothing alike in terms of modeling them. So, let's make an interface that both can implement:
interface IPest {
void BeAnnoying();
}
class HouseFly inherits Insect implements IPest {
void FlyAroundYourHead(){}
void LandOnThings(){}
void BeAnnoying() {
FlyAroundYourHead();
LandOnThings();
}
}
class Telemarketer inherits Person implements IPest {
void CallDuringDinner(){}
void ContinueTalkingWhenYouSayNo(){}
void BeAnnoying() {
CallDuringDinner();
ContinueTalkingWhenYouSayNo();
}
}
We now have two classes that can each be annoying in their own way. And they do not need to derive from the same base class and share common inherent characteristics -- they simply need to satisfy the contract of IPest -- that contract is simple. You just have to BeAnnoying. In this regard, we can model the following:
class DiningRoom {
DiningRoom(Person[] diningPeople, IPest[] pests) { ... }
void ServeDinner() {
when diningPeople are eating,
foreach pest in pests
pest.BeAnnoying();
}
}
Here we have a dining room that accepts a number of diners and a number of pests -- note the use of the interface. This means that in our little world, a member of the pests array could actually be a Telemarketer object or a HouseFly object.
The ServeDinner method is called when dinner is served and our people in the dining room are supposed to eat. In our little game, that's when our pests do their work -- each pest is instructed to be annoying by way of the IPest interface. In this way, we can easily have both Telemarketers and HouseFlys be annoying in each of their own ways -- we care only that we have something in the DiningRoom object that is a pest, we don't really care what it is and they could have nothing in common with other.
This very contrived pseudo-code example (that dragged on a lot longer than I anticipated) is simply meant to illustrate the kind of thing that finally turned the light on for me in terms of when we might use an interface. I apologize in advance for the silliness of the example, but hope that it helps in your understanding. And, to be sure, the other posted answers you've received here really cover the gamut of the use of interfaces today in design patterns and development methodologies.
The specific example I used to give to students is that they should write
List myList = new ArrayList(); // programming to the List interface
instead of
ArrayList myList = new ArrayList(); // this is bad
These look exactly the same in a short program, but if you go on to use myList 100 times in your program you can start to see a difference. The first declaration ensures that you only call methods on myList that are defined by the List interface (so no ArrayList specific methods). If you've programmed to the interface this way, later on you can decide that you really need
List myList = new TreeList();
and you only have to change your code in that one spot. You already know that the rest of your code doesn't do anything that will be broken by changing the implementation because you programmed to the interface.
The benefits are even more obvious (I think) when you're talking about method parameters and return values. Take this for example:
public ArrayList doSomething(HashMap map);
That method declaration ties you to two concrete implementations (ArrayList and HashMap). As soon as that method is called from other code, any changes to those types probably mean you're going to have to change the calling code as well. It would be better to program to the interfaces.
public List doSomething(Map map);
Now it doesn't matter what kind of List you return, or what kind of Map is passed in as a parameter. Changes that you make inside the doSomething method won't force you to change the calling code.
Programming to an interface is saying, "I need this functionality and I don't care where it comes from."
Consider (in Java), the List interface versus the ArrayList and LinkedList concrete classes. If all I care about is that I have a data structure containing multiple data items that I should access via iteration, I'd pick a List (and that's 99% of the time). If I know that I need constant-time insert/delete from either end of the list, I might pick the LinkedList concrete implementation (or more likely, use the Queue interface). If I know I need random access by index, I'd pick the ArrayList concrete class.
Programming to an interface has absolutely nothing to do with abstract interfaces like we see in Java or .NET. It isn't even an OOP concept.
What it means is don't go messing around with the internals of an object or data structure. Use the Abstract Program Interface, or API, to interact with your data. In Java or C# that means using public properties and methods instead of raw field access. For C that means using functions instead of raw pointers.
EDIT: And with databases it means using views and stored procedures instead of direct table access.
Using interfaces is a key factor in making your code easily testable in addition to removing unnecessary couplings between your classes. By creating an interface that defines the operations on your class, you allow classes that want to use that functionality the ability to use it without depending on your implementing class directly. If later on you decide to change and use a different implementation, you need only change the part of the code where the implementation is instantiated. The rest of the code need not change because it depends on the interface, not the implementing class.
This is very useful in creating unit tests. In the class under test you have it depend on the interface and inject an instance of the interface into the class (or a factory that allows it to build instances of the interface as needed) via the constructor or a property settor. The class uses the provided (or created) interface in its methods. When you go to write your tests, you can mock or fake the interface and provide an interface that responds with data configured in your unit test. You can do this because your class under test deals only with the interface, not your concrete implementation. Any class implementing the interface, including your mock or fake class, will do.
EDIT: Below is a link to an article where Erich Gamma discusses his quote, "Program to an interface, not an implementation."
http://www.artima.com/lejava/articles/designprinciples.html
You should look into Inversion of Control:
Martin Fowler: Inversion of Control Containers and the Dependency Injection pattern
Wikipedia: Inversion of Control
In such a scenario, you wouldn't write this:
IInterface classRef = new ObjectWhatever();
You would write something like this:
IInterface classRef = container.Resolve<IInterface>();
This would go into a rule-based setup in the container object, and construct the actual object for you, which could be ObjectWhatever. The important thing is that you could replace this rule with something that used another type of object altogether, and your code would still work.
If we leave IoC off the table, you can write code that knows that it can talk to an object that does something specific, but not which type of object or how it does it.
This would come in handy when passing parameters.
As for your parenthesized question "Also, how could you write a method that takes in an object that implements an Interface? Is that possible?", in C# you would simply use the interface type for the parameter type, like this:
public void DoSomethingToAnObject(IInterface whatever) { ... }
This plugs right into the "talk to an object that does something specific." The method defined above knows what to expect from the object, that it implements everything in IInterface, but it doesn't care which type of object it is, only that it adheres to the contract, which is what an interface is.
For instance, you're probably familiar with calculators and have probably used quite a few in your days, but most of the time they're all different. You, on the other hand, knows how a standard calculator should work, so you're able to use them all, even if you can't use the specific features that each calculator has that none of the other has.
This is the beauty of interfaces. You can write a piece of code, that knows that it will get objects passed to it that it can expect certain behavior from. It doesn't care one hoot what kind of object it is, only that it supports the behavior needed.
Let me give you a concrete example.
We have a custom-built translation system for windows forms. This system loops through controls on a form and translate text in each. The system knows how to handle basic controls, like the-type-of-control-that-has-a-Text-property, and similar basic stuff, but for anything basic, it falls short.
Now, since controls inherit from pre-defined classes that we have no control over, we could do one of three things:
Build support for our translation system to detect specifically which type of control it is working with, and translate the correct bits (maintenance nightmare)
Build support into base classes (impossible, since all the controls inherit from different pre-defined classes)
Add interface support
So we did nr. 3. All our controls implement ILocalizable, which is an interface that gives us one method, the ability to translate "itself" into a container of translation text/rules. As such, the form doesn't need to know which kind of control it has found, only that it implements the specific interface, and knows that there is a method where it can call to localize the control.
Code to the Interface Not the Implementation has NOTHING to do with Java, nor its Interface construct.
This concept was brought to prominence in the Patterns / Gang of Four books but was most probably around well before that. The concept certainly existed well before Java ever existed.
The Java Interface construct was created to aid in this idea (among other things), and people have become too focused on the construct as the centre of the meaning rather than the original intent. However, it is the reason we have public and private methods and attributes in Java, C++, C#, etc.
It means just interact with an object or system's public interface. Don't worry or even anticipate how it does what it does internally. Don't worry about how it is implemented. In object-oriented code, it is why we have public vs. private methods/attributes. We are intended to use the public methods because the private methods are there only for use internally, within the class. They make up the implementation of the class and can be changed as required without changing the public interface. Assume that regarding functionality, a method on a class will perform the same operation with the same expected result every time you call it with the same parameters. It allows the author to change how the class works, its implementation, without breaking how people interact with it.
And you can program to the interface, not the implementation without ever using an Interface construct. You can program to the interface not the implementation in C++, which does not have an Interface construct. You can integrate two massive enterprise systems much more robustly as long as they interact through public interfaces (contracts) rather than calling methods on objects internal to the systems. The interfaces are expected to always react the same expected way given the same input parameters; if implemented to the interface and not the implementation. The concept works in many places.
Shake the thought that Java Interfaces have anything what-so-ever to do with the concept of 'Program to the Interface, Not the Implementation'. They can help apply the concept, but they are not the concept.
It sounds like you understand how interfaces work but are unsure of when to use them and what advantages they offer. Here are a few examples of when an interface would make sense:
// if I want to add search capabilities to my application and support multiple search
// engines such as Google, Yahoo, Live, etc.
interface ISearchProvider
{
string Search(string keywords);
}
then I could create GoogleSearchProvider, YahooSearchProvider, LiveSearchProvider, etc.
// if I want to support multiple downloads using different protocols
// HTTP, HTTPS, FTP, FTPS, etc.
interface IUrlDownload
{
void Download(string url)
}
// how about an image loader for different kinds of images JPG, GIF, PNG, etc.
interface IImageLoader
{
Bitmap LoadImage(string filename)
}
then create JpegImageLoader, GifImageLoader, PngImageLoader, etc.
Most add-ins and plugin systems work off interfaces.
Another popular use is for the Repository pattern. Say I want to load a list of zip codes from different sources
interface IZipCodeRepository
{
IList<ZipCode> GetZipCodes(string state);
}
then I could create an XMLZipCodeRepository, SQLZipCodeRepository, CSVZipCodeRepository, etc. For my web applications, I often create XML repositories early on so I can get something up and running before the SQL Database is ready. Once the database is ready I write an SQLRepository to replace the XML version. The rest of my code remains unchanged since it runs solely off of interfaces.
Methods can accept interfaces such as:
PrintZipCodes(IZipCodeRepository zipCodeRepository, string state)
{
foreach (ZipCode zipCode in zipCodeRepository.GetZipCodes(state))
{
Console.WriteLine(zipCode.ToString());
}
}
It makes your code a lot more extensible and easier to maintain when you have sets of similar classes. I am a junior programmer, so I am no expert, but I just finished a project that required something similar.
I work on client side software that talks to a server running a medical device. We are developing a new version of this device that has some new components that the customer must configure at times. There are two types of new components, and they are different, but they are also very similar. Basically, I had to create two config forms, two lists classes, two of everything.
I decided that it would be best to create an abstract base class for each control type that would hold almost all of the real logic, and then derived types to take care of the differences between the two components. However, the base classes would not have been able to perform operations on these components if I had to worry about types all of the time (well, they could have, but there would have been an "if" statement or switch in every method).
I defined a simple interface for these components and all of the base classes talk to this interface. Now when I change something, it pretty much 'just works' everywhere and I have no code duplication.
A lot of explanation out there, but to make it even more simpler. Take for instance a List. One can implement a list with as:
An internal array
A linked list
Other implementations
By building to an interface, say a List. You only code as to definition of List or what List means in reality.
You could use any type of implementation internally say an array implementation. But suppose you wish to change the implementation for some reason say a bug or performance. Then you just have to change the declaration List<String> ls = new ArrayList<String>() to List<String> ls = new LinkedList<String>().
Nowhere else in code, will you have to change anything else; Because everything else was built on the definition of List.
If you program in Java, JDBC is a good example. JDBC defines a set of interfaces but says nothing about the implementation. Your applications can be written against this set of interfaces. In theory, you pick some JDBC driver and your application would just work. If you discover there's a faster or "better" or cheaper JDBC driver or for whatever reason, you can again in theory re-configure your property file, and without having to make any change in your application, your application would still work.
I am a late comer to this question, but I want to mention here that the line "Program to an interface, not an implementation" had some good discussion in the GoF (Gang of Four) Design Patterns book.
It stated, on p. 18:
Program to an interface, not an implementation
Don't declare variables to be instances of particular concrete classes. Instead, commit only to an interface defined by an abstract class. You will find this to be a common theme of the design patterns in this book.
and above that, it began with:
There are two benefits to manipulating objects solely in terms of the interface defined by abstract classes:
Clients remain unaware of the specific types of objects they use, as long as the objects adhere to the interface that clients expect.
Clients remain unaware of the classes that implement these objects. Clients only know about the abstract class(es) defining the interface.
So in other words, don't write it your classes so that it has a quack() method for ducks, and then a bark() method for dogs, because they are too specific for a particular implementation of a class (or subclass). Instead, write the method using names that are general enough to be used in the base class, such as giveSound() or move(), so that they can be used for ducks, dogs, or even cars, and then the client of your classes can just say .giveSound() rather than thinking about whether to use quack() or bark() or even determine the type before issuing the correct message to be sent to the object.
Programming to Interfaces is awesome, it promotes loose coupling. As #lassevk mentioned, Inversion of Control is a great use of this.
In addition, look into SOLID principals. here is a video series
It goes through a hard coded (strongly coupled example) then looks at interfaces, finally progressing to a IoC/DI tool (NInject)
To add to the existing posts, sometimes coding to interfaces helps on large projects when developers work on separate components simultaneously. All you need is to define interfaces upfront and write code to them while other developers write code to the interface you are implementing.
It can be advantageous to program to interfaces, even when we are not depending on abstractions.
Programming to interfaces forces us to use a contextually appropriate subset of an object. That helps because it:
prevents us from doing contextually inappropriate things, and
lets us safely change the implementation in the future.
For example, consider a Person class that implements the Friend and the Employee interface.
class Person implements AbstractEmployee, AbstractFriend {
}
In the context of the person's birthday, we program to the Friend interface, to prevent treating the person like an Employee.
function party() {
const friend: Friend = new Person("Kathryn");
friend.HaveFun();
}
In the context of the person's work, we program to the Employee interface, to prevent blurring workplace boundaries.
function workplace() {
const employee: Employee = new Person("Kathryn");
employee.DoWork();
}
Great. We have behaved appropriately in different contexts, and our software is working well.
Far into the future, if our business changes to work with dogs, we can change the software fairly easily. First, we create a Dog class that implements both Friend and Employee. Then, we safely change new Person() to new Dog(). Even if both functions have thousands of lines of code, that simple edit will work because we know the following are true:
Function party uses only the Friend subset of Person.
Function workplace uses only the Employee subset of Person.
Class Dog implements both the Friend and Employee interfaces.
On the other hand, if either party or workplace were to have programmed against Person, there would be a risk of both having Person-specific code. Changing from Person to Dog would require us to comb through the code to extirpate any Person-specific code that Dog does not support.
The moral: programming to interfaces helps our code to behave appropriately and to be ready for change. It also prepares our code to depend on abstractions, which brings even more advantages.
If I'm writing a new class Swimmer to add the functionality swim() and need to use an object of class say Dog, and this Dog class implements interface Animal which declares swim().
At the top of the hierarchy (Animal), it's very abstract while at the bottom (Dog) it's very concrete. The way I think about "programming to interfaces" is that, as I write Swimmer class, I want to write my code against the interface that's as far up that hierarchy which in this case is an Animal object. An interface is free from implementation details and thus makes your code loosely-coupled.
The implementation details can be changed with time, however, it would not affect the remaining code since all you are interacting with is with the interface and not the implementation. You don't care what the implementation is like... all you know is that there will be a class that would implement the interface.
It is also good for Unit Testing, you can inject your own classes (that meet the requirements of the interface) into a class that depends on it
Short story: A postman is asked to go home after home and receive the covers contains (letters, documents, cheques, gift cards, application, love letter) with the address written on it to deliver.
Suppose there is no cover and ask the postman to go home after home and receive all the things and deliver to other people, the postman can get confused.
So better wrap it with cover (in our story it is the interface) then he will do his job fine.
Now the postman's job is to receive and deliver the covers only (he wouldn't bothered what is inside in the cover).
Create a type of interface not actual type, but implement it with actual type.
To create to interface means your components get Fit into the rest of code easily
I give you an example.
you have the AirPlane interface as below.
interface Airplane{
parkPlane();
servicePlane();
}
Suppose you have methods in your Controller class of Planes like
parkPlane(Airplane plane)
and
servicePlane(Airplane plane)
implemented in your program. It will not BREAK your code.
I mean, it need not to change as long as it accepts arguments as AirPlane.
Because it will accept any Airplane despite actual type, flyer, highflyr, fighter, etc.
Also, in a collection:
List<Airplane> plane; // Will take all your planes.
The following example will clear your understanding.
You have a fighter plane that implements it, so
public class Fighter implements Airplane {
public void parkPlane(){
// Specific implementations for fighter plane to park
}
public void servicePlane(){
// Specific implementatoins for fighter plane to service.
}
}
The same thing for HighFlyer and other clasess:
public class HighFlyer implements Airplane {
public void parkPlane(){
// Specific implementations for HighFlyer plane to park
}
public void servicePlane(){
// specific implementatoins for HighFlyer plane to service.
}
}
Now think your controller classes using AirPlane several times,
Suppose your Controller class is ControlPlane like below,
public Class ControlPlane{
AirPlane plane;
// so much method with AirPlane reference are used here...
}
Here magic comes as you may make your new AirPlane type instances as many as you want and you are not changing the code of ControlPlane class.
You can add an instance...
JumboJetPlane // implementing AirPlane interface.
AirBus // implementing AirPlane interface.
You may remove instances of previously created types too.
So, just to get this right, the advantage of a interface is that I can separate the calling of a method from any particular class. Instead creating a instance of the interface, where the implementation is given from whichever class I choose that implements that interface. Thus allowing me to have many classes, which have similar but slightly different functionality and in some cases (the cases related to the intention of the interface) not care which object it is.
For example, I could have a movement interface. A method which makes something 'move' and any object (Person, Car, Cat) that implements the movement interface could be passed in and told to move. Without the method every knowing the type of class it is.
Imagine you have a product called 'Zebra' that can be extended by plugins. It finds the plugins by searching for DLLs in some directory. It loads all those DLLs and uses reflection to find any classes that implement IZebraPlugin, and then calls the methods of that interface to communicate with the plugins.
This makes it completely independent of any specific plugin class - it doesn't care what the classes are. It only cares that they fulfill the interface specification.
Interfaces are a way of defining points of extensibility like this. Code that talks to an interface is more loosely coupled - in fact it is not coupled at all to any other specific code. It can inter-operate with plugins written years later by people who have never met the original developer.
You could instead use a base class with virtual functions - all plugins would be derived from the base class. But this is much more limiting because a class can only have one base class, whereas it can implement any number of interfaces.
C++ explanation.
Think of an interface as your classes public methods.
You then could create a template that 'depends' on these public methods in order to carry out it's own function (it makes function calls defined in the classes public interface). Lets say this template is a container, like a Vector class, and the interface it depends on is a search algorithm.
Any algorithm class that defines the functions/interface Vector makes calls to will satisfy the 'contract' (as someone explained in the original reply). The algorithms don't even need to be of the same base class; the only requirement is that the functions/methods that the Vector depends on (interface) is defined in your algorithm.
The point of all of this is that you could supply any different search algorithm/class just as long as it supplied the interface that Vector depends on (bubble search, sequential search, quick search).
You might also want to design other containers (lists, queues) that would harness the same search algorithm as Vector by having them fulfill the interface/contract that your search algorithms depends on.
This saves time (OOP principle 'code reuse') as you are able to write an algorithm once instead of again and again and again specific to every new object you create without over-complicating the issue with an overgrown inheritance tree.
As for 'missing out' on how things operate; big-time (at least in C++), as this is how most of the Standard TEMPLATE Library's framework operates.
Of course when using inheritance and abstract classes the methodology of programming to an interface changes; but the principle is the same, your public functions/methods are your classes interface.
This is a huge topic and one of the the cornerstone principles of Design Patterns.
In Java these concrete classes all implement the CharSequence interface:
CharBuffer, String, StringBuffer, StringBuilder
These concrete classes do not have a common parent class other than Object, so there is nothing that relates them, other than the fact they each have something to do with arrays of characters, representing such, or manipulating such. For instance, the characters of String cannot be changed once a String object is instantiated, whereas the characters of StringBuffer or StringBuilder can be edited.
Yet each one of these classes is capable of suitably implementing the CharSequence interface methods:
char charAt(int index)
int length()
CharSequence subSequence(int start, int end)
String toString()
In some cases, Java class library classes that used to accept String have been revised to now accept the CharSequence interface. So if you have an instance of StringBuilder, instead of extracting a String object (which means instantiating a new object instance), it can instead just pass the StringBuilder itself as it implements the CharSequence interface.
The Appendable interface that some classes implement has much the same kind of benefit for any situation where characters can be appended to an instance of the underlying concrete class object instance. All of these concrete classes implement the Appendable interface:
BufferedWriter, CharArrayWriter, CharBuffer, FileWriter, FilterWriter, LogStream, OutputStreamWriter, PipedWriter, PrintStream, PrintWriter, StringBuffer, StringBuilder, StringWriter, Writer
Previous answers focus on programming to an abstraction for the sake of extensibility and loose coupling. While these are very important points,
readability is equally important. Readability allows others (and your future self) to understand the code with minimal effort. This is why readability leverages abstractions.
An abstraction is, by definition, simpler than its implementation. An abstraction omits detail in order to convey the essence or purpose of a thing, but nothing more.
Because abstractions are simpler, I can fit a lot more of them in my head at one time, compared to implementations.
As a programmer (in any language) I walk around with a general idea of a List in my head at all times. In particular, a List allows random access, duplicate elements, and maintains order. When I see a declaration like this: List myList = new ArrayList() I think, cool, this is a List that's being used in the (basic) way that I understand; and I don't have to think any more about it.
On the other hand, I do not carry around the specific implementation details of ArrayList in my head. So when I see, ArrayList myList = new ArrayList(). I think, uh-oh, this ArrayList must be used in a way that isn't covered by the List interface. Now I have to track down all the usages of this ArrayList to understand why, because otherwise I won't be able to fully understand this code. It gets even more confusing when I discover that 100% of the usages of this ArrayList do conform to the List interface. Then I'm left wondering... was there some code relying on ArrayList implementation details that got deleted? Was the programmer who instantiated it just incompetent? Is this application locked into that specific implementation in some way at runtime? A way that I don't understand?
I'm now confused and uncertain about this application, and all we're talking about is a simple List. What if this was a complex business object ignoring its interface? Then my knowledge of the business domain is insufficient to understand the purpose of the code.
So even when I need a List strictly within a private method (nothing that would break other applications if it changed, and I could easily find/replace every usage in my IDE) it still benefits readability to program to an abstraction. Because abstractions are simpler than implementation details. You could say that programming to abstractions is one way of adhering to the KISS principle.
An interface is like a contract, where you want your implementation class to implement methods written in the contract (interface). Since Java does not provide multiple inheritance, "programming to interface" is a good way to achieve multiple inheritance.
If you have a class A that is already extending some other class B, but you want that class A to also follow certain guidelines or implement a certain contract, then you can do so by the "programming to interface" strategy.
Q: - ... "Could you use any class that implements an interface?"
A: - Yes.
Q: - ... "When would you need to do that?"
A: - Each time you need a class(es) that implements interface(s).
Note: We couldn't instantiate an interface not implemented by a class - True.
Why?
Because the interface has only method prototypes, not definitions (just functions names, not their logic)
AnIntf anInst = new Aclass();
// we could do this only if Aclass implements AnIntf.
// anInst will have Aclass reference.
Note: Now we could understand what happened if Bclass and Cclass implemented same Dintf.
Dintf bInst = new Bclass();
// now we could call all Dintf functions implemented (defined) in Bclass.
Dintf cInst = new Cclass();
// now we could call all Dintf functions implemented (defined) in Cclass.
What we have: Same interface prototypes (functions names in interface), and call different implementations.
Bibliography:
Prototypes - wikipedia
program to an interface is a term from the GOF book. i would not directly say it has to do with java interface but rather real interfaces. to achieve clean layer separation, you need to create some separation between systems for example: Let's say you had a concrete database you want to use, you would never "program to the database" , instead you would "program to the storage interface". Likewise you would never "program to a Web Service" but rather you would program to a "client interface". this is so you can easily swap things out.
i find these rules help me:
1. we use a java interface when we have multiple types of an object. if i just have single object, i dont see the point. if there are at least two concrete implementations of some idea, then i would use a java interface.
2. if as i stated above, you want to bring decoupling from an external system (storage system) to your own system (local DB) then also use a interface.
notice how there are two ways to consider when to use them.
Coding to an interface is a philosophy, rather than specific language constructs or design patterns - it instructs you what is the correct order of steps to follow in order to create better software systems (e.g. more resilient, more testable, more scalable, more extendible, and other nice traits).
What it actually means is:
===
Before jumping to implementations and coding (the HOW) - think of the WHAT:
What black boxes should make up your system,
What is each box' responsibility,
What are the ways each "client" (that is, one of those other boxes, 3rd party "boxes", or even humans) should communicate with it (the API of each box).
After you figure the above, go ahead and implement those boxes (the HOW).
Thinking first of what a box' is and what its API, leads the developer to distil the box' responsibility, and to mark for himself and future developers the difference between what is its exposed details ("API") and it's hidden details ("implementation details"), which is a very important differentiation to have.
One immediate and easily noticeable gain is the team can then change and improve implementations without affecting the general architecture. It also makes the system MUCH more testable (it goes well with the TDD approach).
===
Beyond the traits I've mentioned above, you also save A LOT OF TIME going this direction.
Micro Services and DDD, when done right, are great examples of "Coding to an interface", however the concept wins in every pattern from monoliths to "serverless", from BE to FE, from OOP to functional, etc....
I strongly recommend this approach for Software Engineering (and I basically believe it makes total sense in other fields as well).
Program to an interface allows to change implementation of contract defined by interface seamlessly. It allows loose coupling between contract and specific implementations.
IInterface classRef = new ObjectWhatever()
You could use any class that implements IInterface? When would you need to do that?
Have a look at this SE question for good example.
Why should the interface for a Java class be preferred?
does using an Interface hit performance?
if so how much?
Yes. It will have slight performance overhead in sub-seconds. But if your application has requirement to change the implementation of interface dynamically, don't worry about performance impact.
how can you avoid it without having to maintain two bits of code?
Don't try to avoid multiple implementations of interface if your application need them. In absence of tight coupling of interface with one specific implementation, you may have to deploy the patch to change one implementation to other implementation.
One good use case: Implementation of Strategy pattern:
Real World Example of the Strategy Pattern
"Program to interface" means don't provide hard code right the way, meaning your code should be extended without breaking the previous functionality. Just extensions, not editing the previous code.
Also I see a lot of good and explanatory answers here, so I want to give my point of view here, including some extra information what I noticed when using this method.
Unit testing
For the last two years, I have written a hobby project and I did not write unit tests for it. After writing about 50K lines I found out it would be really necessary to write unit tests.
I did not use interfaces (or very sparingly) ... and when I made my first unit test, I found out it was complicated. Why?
Because I had to make a lot of class instances, used for input as class variables and/or parameters. So the tests look more like integration tests (having to make a complete 'framework' of classes since all was tied together).
Fear of interfaces
So I decided to use interfaces. My fear was that I had to implement all functionality everywhere (in all used classes) multiple times. In some way this is true, however, by using inheritance it can be reduced a lot.
Combination of interfaces and inheritance
I found out the combination is very good to be used. I give a very simple example.
public interface IPricable
{
int Price { get; }
}
public interface ICar : IPricable
public abstract class Article
{
public int Price { get { return ... } }
}
public class Car : Article, ICar
{
// Price does not need to be defined here
}
This way copying code is not necessary, while still having the benefit of using a car as interface (ICar).
I am developing a class library which will include the object Car.
The dilemma is, Car itself will be a class with fields such as Registration Number, and other general information on the car.
But a car has an engine, chassis, etc. These objects need to be modelled too. Should they be classes embedded within Car? If not, what is the usage scenario of an embedded class?
I've learnt that composition is "part of", so you can model seperate classes and use the engine type, for example, at the field level of the car to achieve this. However, "aggregation", which is a "has a" relationship with the type being passed in the ctor, also applies (a car "has an" engine).
Which way do I go?
EDIT: I am currently on homework hence the lack of a reply from me. The class library is for a web app based around cars. I am a professional developer (I develop in .NET for a living but as a junior) so this is not a homework question.
Thanks
It really depends on your application.
For example, you could implement the wheels as separate classes, containing information about what tyre is on it, how worn it is, etc. but if your app doesn't even care about the wheels then the entire class is a waste of code.
I can see three use cases for composition:
The owning class has gotten overly complicated and should be broken down.
The owning class has multiple copies of a set of properties that could be mapped into a class. This allows you to bind all those properties together.
The contained object may need to be inspected or considered separately from the object that owns it (eg. you might want to move the Engine object to another car) or may be replaced as a single unit.
In summary: Use composition as a tool for encapsulating complexity or eliminating repetition. If it doesn't serve one of those purposes it probably isn't worth making a new class for.
A class should have as few responsibilities as possible and encapsulate and delegate other functionality to other classes. Lots of a small, simple classes that do one thing is a sign of a readable, stable codebase.
Yes, a car will "have" an engine, but I'd suggest using an interface for this and similar "has a" relationships. Again, depending on the professor, you might get bonus points for having a factory create different cars (appropriate, no?):
public class Car
{
private Engine engine;
public Car(Engine engine)
{
this.engine = engine;
}
public void accelerate()
{
this.engine.goFaster();
}
public void decelerate()
{
this.engine.goSlower();
}
}
public interface Engine
{
public void goFaster();
public void goSlower();
}
public class ReallyFastEngine implements Engine
{
public void goFaster()
{
// some code that goes really fast
}
public void goSlower()
{
// some code that goes slower
}
}
public class NotAsFastEngine implements Engine
{
public void goFaster()
{
// some code that goes not as fast
}
public void goSlower()
{
// some code that goes slower
}
}
public class CarFactory()
{
public static Car createFastCar()
{
return new Car(new ReallyFastEngine());
}
public static Car createNotAsFastCar()
{
return new Car(new NotAsFastEngine());
}
}
Seeing as it is homework, and depending on the inclinations of your tutor/professor/teacher, you are probably better to go down the route of writing a separate classes for the engine, wheels and so on. Even though it may be completely over-engineered, and your application may not care about them, it is possible that your homework will be marked by standards such as:
"Did they identify an engine class"
"Does it have sensible methods like Start()"
"Mark them down for lumping everything in one big class that is actually simpler, because they clearly don't understand composition"
Or whatever, and not the kinds of standards that the more pragmatic people in this thread apply to their own designs.
Only break down the model of the car into pieces that will be exposed as separate entities outside the scope of the car. Another way to think about it is do you really understand how your car gets started when you turn the key? As far as the typical driver is concerned, everything under the hood is one big (and noisy) black box. The auto-engineers know the common parts that need maintenance by the car owner and have explicitly designed them for a different level of user interaction, things like the oil dipstick or coolant reservoir refill cap.
Can you model each piece of the car? Sure. Is it helpful to model the individual spark plugs? Probably not.
Do you need cars with different attributes like color or size? Do you need cars with different capabilities like passenger or towing capacity? The one place that is different is if you need cars with different behaviors. This is where you really need to think about modeling a Driver object which has attributes, from simple ones like reaction-time to complex ones like aggressiveness.
Modeling vehicles as examples of object orientation or inheritance is problematic because the examples don't really explain the true distinctions between essential attributes that define a class. It's not new to StackOverflow but this question isn't a duplicate either, see this SO thread. I had this same discussion with a friend of mine and posted a log of it on my blog. Read up on the different aircraft types the FAA recognizes and how the regulations for each type are subdivided. There are lots of different types of aircraft, the biggest separation is between powered and unpowered.
Check out the definitions used by the FAA:
Aircraft means a device that is used
or intended to be used for flight in
the air.
Airplane means an engine-driven
fixed-wing aircraft heavier than air,
that is supported in flight by the
dynamic reaction of the air against
its wings.
Airship means an engine-driven
lighter-than-air aircraft that can be
steered.
There is also lighter-than-air and heavier-than-air. A hot-air balloon is unpowered and lighter-than-air. A blimp is powered and lighter-than-air. A glider is unpowered and heavier-than-air. A Boeing 757 is powered and heavier-than air but adds another category of 'fixed-wing' which is unlike a helicopter which is also powered and heavier-than-air but is 'rotary-wing'.
Here is the first four in the form of a table:
| Powered | Unpowered
---------------------------------------------------
Lighter-than-air | Blimp | Hot-air balloon
Heavier-than-air | 737 | Glider
You get the picture.
You can't just say you'll model the engine separately from the car because a car without an engine might be a whole different animal. A car without an engine is nothing like a trailer, which also doesn't have an engine but never will either. In these cases neither 'is-a' nor 'has-a' fits in the concrete way we build objects. You don't declare a blimp as being a aircraft that 'is-a' lighter-than-air, so is a hot-air balloon. The fact that they are both lighter-than-air doesn't make them related in any way except the physics they exploit. The distinction is important because the rules and regulations that apply are different. From the other angle, we don't describe a blimp as a hot-air balloon that 'has-a' engine. The aircraft aren't physically related, the relationship is how they should be handled.
If you don't need to define your objects to that level of detail, you may not need to model them to that level of detail either.
Car will be an top hierarchy object. Including simple fields like Number, ID or description.
And will have complicated fields like Engine, which is an object by itself.
So the Car will look something like:
class Car{
String ID;
Engine engine;
}
That a has-a relation.
One criteria you can have to decide whether the classes for Engine, Chasis etc.
needs to be present as an inner class (embedded class) is whether instance of
these classes can be used elsewhere in your application. In such cases the
decision is simple and it is to make these classes exist separately
(not as inner classes).
Even if these classes are not used elsewhere in your application then other
criteria can be testability. With these classes embedded inside and with your
design is it possible to have unit tests that can appropriately test your
code providing a good coverage.
For example say, if you have made an instance variable which references an
Engine object and this variable is being initialized in the Constructor of Car.And
your Engine class has some methods which needs to be tested. Then how can
you add unit tests to check the code in Engine class ? Probably you would
have some methods in Car class which expose the behavior or Engine class allowing
you to write unit tests. Then the question is if there is a need to expose
the behavior of Engine class wouldn't it be better that the Engine class
stands on it own?
Alternatively there might not be a need to explicitly test the methods in
Engine class and unit testing the methods in Car covers the Engine class code
as well. Then it reflects tight integration of Engine class with the Car class
and would mean it can remain as an inner class.
It depends on what it is you're trying to do. Trying to design a 'Car' class (or any other class for that matter) without an idea of the use cases is an exercise in futility.
You will design the classes and their relationships and interactions very differently depending on the use cases you're trying to enable.
What is the precise difference between encapsulation and abstraction?
Most answers here focus on OOP but encapsulation begins much earlier:
Every function is an encapsulation; in pseudocode:
point x = { 1, 4 }
point y = { 23, 42 }
numeric d = distance(x, y)
Here, distance encapsulates the calculation of the (Euclidean) distance between two points in a plane: it hides implementation details. This is encapsulation, pure and simple.
Abstraction is the process of generalisation: taking a concrete implementation and making it applicable to different, albeit somewhat related, types of data. The classical example of abstraction is C’s qsort function to sort data:
The thing about qsort is that it doesn't care about the data it sorts — in fact, it doesn’t know what data it sorts. Rather, its input type is a typeless pointer (void*) which is just C’s way of saying “I don't care about the type of data” (this is also called type erasure). The important point is that the implementation of qsort always stays the same, regardless of data type. The only thing that has to change is the compare function, which differs from data type to data type. qsort therefore expects the user to provide said compare function as a function argument.
Encapsulation and abstraction go hand in hand so much so that you could make the point that they are truly inseparable. For practical purposes, this is probably true; that said, here’s an encapsulation that’s not much of an abstraction:
class point {
numeric x
numeric y
}
We encapsulate the point’s coordinate, but we don’t materially abstract them away, beyond grouping them logically.
And here’s an example of abstraction that’s not encapsulation:
T pi<T> = 3.1415926535
This is a generic variable pi with a given value (π), and the declaration doesn’t care about the exact type of the variable. Admittedly, I’d be hard-pressed to find something like this in real code: abstraction virtually always uses encapsulation. However, the above does actually exist in C++(14), via variable templates (= generic templates for variables); with a slightly more complex syntax, e.g.:
template <typename T> constexpr T pi = T{3.1415926535};
Many answers and their examples are misleading.
Encapsulation is the packing of "data" and "functions operating on that data" into a single component and restricting the access to some of the object's components.
Encapsulation means that the internal representation of an object is generally hidden from view outside of the object's definition.
Abstraction is a mechanism which represent the essential features without including implementation details.
Encapsulation:-- Information hiding.
Abstraction:-- Implementation hiding.
Example (in C++):
class foo{
private:
int a, b;
public:
foo(int x=0, int y=0): a(x), b(y) {}
int add(){
return a+b;
}
}
Internal representation of any object of foo class is hidden outside of this class. --> Encapsulation.
Any accessible member (data/function) of an object of foo is restricted and can only be accessed by that object only.
foo foo_obj(3, 4);
int sum = foo_obj.add();
Implementation of method add is hidden. --> Abstraction.
Encapsulation is hiding the implementation details which may or may not be for generic or specialized behavior(s).
Abstraction is providing a generalization (say, over a set of behaviors).
Here's a good read: Abstraction, Encapsulation, and Information Hiding by Edward V. Berard of the Object Agency.
encapsulation puts some things in a box and gives you a peephole; this keeps you from mucking with the gears.
abstraction flat-out ignores the details that don't matter, like whether the things have gears, ratchets, flywheels, or nuclear cores; they just "go"
examples of encapsulation:
underpants
toolbox
wallet
handbag
capsule
frozen carbonite
a box, with or without a button on it
a burrito (technically, the tortilla around the burrito)
examples of abstraction:
"groups of things" is an abstraction (which we call aggregation)
"things that contains other things" is an abstraction (which we call composition)
"container" is another kind of "things that contain other things" abstraction; note that all of the encapsulation examples are kinds of containers, but not all containers exhibit/provide encapsulation. A basket, for example, is a container that does not encapsulate its contents.
Encapsulation means-hiding data like using getter and setter etc.
Abstraction means- hiding implementation using abstract class and interfaces etc.
Abstraction is generalized term. i.e. Encapsulation is subset of Abstraction.
Abstraction
Encapsulation
It solves an issue at the design level.
Encapsulation solves an issue at implementation level.
hides the unnecessary detail but shows the essential information.
It hides the code and data into a single entity or unit so that the data can be protected from the outside world.
Focuses on the external lookout.
Focuses on internal working.
Lets focus on what an object does instead of how it does it.
Lets focus on how an object does something.
Example: Outer look of mobile, like it has a display screen and buttons.
Example: Inner details of mobile, how button and display screen connect with each other using circuits.
Example: The solution architect is the person who creates the high-level abstract technical design of the entire solution, and this design is then handed over to the the development team for implementation.
Here, solution architect acts as a abstract and development team acts as a Encapsulation.
Example: Encapsulation(networking) of user data
image courtesy
Abstraction (or modularity) – Types enable programmers to think at a higher level than the bit or byte, not bothering with low-level implementation. For example, programmers can begin to think of a string as a set of character values instead of as a mere array of bytes. Higher still, types enable programmers to think about and express interfaces between two of any-sized subsystems. This enables more levels of localization so that the definitions required for interoperability of the subsystems remain consistent when those two subsystems communicate.
Source
Java example
A lot of good answers are provided above but I am going to present my(Java) viewpoint here.
Data Encapsulation simply means wrapping and controlling access of logically grouped data in a class. It is generally associated with another keyword - Data Hiding. This is achieved in Java using access modifiers.
A simple example would be defining a private variable and giving access to it using getter and setter methods or making a method private as it's only use is withing the class. There is no need for user to know about these methods and variables.
Note : It should not be misunderstood that encapsulation is all about data hiding only. When we say encapsulation, emphasis should be on grouping or packaging or bundling related data and behavior together.
Data Abstraction on the other hand is concept of generalizing so that the underneath complex logic is not exposed to the user. In Java this is achieved by using interfaces and abstract classes.
Example -
Lets say we have an interface Animal and it has a function makeSound(). There are two concrete classes Dog and Cat that implement this interface. These concrete classes have separate implementations of makeSound() function. Now lets say we have a animal(We get this from some external module). All user knows is that the object that it is receiving is some Animal and it is the users responsibility to print the animal sound. One brute force way is to check the object received to identify it's type, then typecast it to that Animal type and then call makeSound() on it. But a neater way is to abstracts thing out. Use Animal as a polymorphic reference and call makeSound() on it. At runtime depending on what the real Object type is proper function will be invoked.
More details here.
Complex logic is in the circuit board which is encapsulated in a touchpad and a nice interface(buttons) is provided to abstract it out to the user.
PS: Above links are to my personal blog.
These are somewhat fuzzy concepts that are not unique to Computer Science and programming. I would like to offer up some additional thoughts that may help others understand these important concepts.
Short Answer
Encapsulation - Hiding and/or restricting access to certain parts of a system, while exposing the necessary interfaces.
Abstraction - Considering something with certain characteristics removed, apart from concrete realities, specific objects, or actual instances, thereby reducing complexity.
The main similarity is that these techniques aim to improve comprehension and utility.
The main difference is that abstraction is a means of representing things more simply (often to make the representation more widely applicable), whereas encapsulation is a method of changing the way other things interact with something.
Long Answer
Encapsulation
Here's an example of encapsulation that hopefully makes things more clear:
Here we have an Arduino Uno, and an Arduino Uno within an enclosure. An enclosure is a great representation of what encapsulation is all about.
Encapsulation aims to protect certain components from outside influences and knowledge as well as expose components which other things should interface with. In programming terms, this involves information hiding though access modifiers, which change the extent to which certain variables and/or properties can be read and written.
But beyond that, encapsulation also aims to provide those external interfaces much more effectively. With our Arduino example, this could include the nice buttons and screen which makes the user's interaction with the device much simpler. They provide the user with simple ways to affect the device's behavior and gain useful information about its operation which would otherwise be much more difficult.
In programming, this involves the grouping of various components into a separable construct, such as a function, class, or object. It also includes providing the means of interacting with those constructs, as well as methods for gaining useful information about them.
Encapsulation helps programmers in many many additional ways, not least of which is improved code maintainability and testability.
Abstraction
Although many other answers here defined abstraction as generalization, I personally think that definition is misguided. I would say that generalization is actually a specific type of abstraction, not the other way around. In other words, all generalizations are abstractions, but all abstractions are not necessarily generalizations.
Here's how I like to think of abstraction:
Would you say the image there is a tree? Chances are you would. But is it really a tree? Well, of course not! It's a bunch of pixels made to look like something we might call a tree. We could say that it represents an abstraction of a real tree. Notice that several visual details of the tree are omitted. Also, it does not grow, consume water, or produce oxygen. How could it? it's just a bunch of colors on a screen, represented by bytes in your computer memory.
And here is the essence of abstraction. It's a way of simplifying things so they are easier to understand. Every idea going through your head is an abstraction of reality. Your mental image of a tree is no more an actual tree than this jpeg is.
In programming, we might use this to our advantage by creating a Tree class with methods for simulated growing, water consuming, and oxygen production. Our creation would be something that represents our experience of actual trees, and only includes those elements that we really care about for our particular simulation. We use abstraction as a way of representing our experience of something with bytes and mathematics.
Abstract Classes
Abstraction in programming also allows us to consider commonalities between several "concrete" object types (types that actually exist) and define those commonalities within a unique entity. For example, our Tree class may inherit from an abstract class Plant, which has several properties and methods which are applicable to all of our plant-like classes, but removes those that are specific to each type of plant. This can significantly reduce duplication of code, and improves maintainability.
The practical difference of an abstract class and plain class is that conceptually there's no "real" instances of the abstract class. It wouldn't make sense to construct a Plant object because that's not specific enough. Every "real" Plant is also a more specific type of Plant.
Also, if we want our program to be more realistic, we might want to consider the fact that our Tree class might be too abstract itself. In reality, every Tree is a more specific type of Tree, so we could create classes for those types such as Birch, Maple, etc. which inherit from our, perhaps now abstract, Tree class.
JVM
Another good example of abstraction is the Java Virtual Machine (JVM), which provides a virtual or abstract computer for Java code to run on. It essentially takes away all of the platform specific components of a system, and provides an abstract interface of "computer" without regard to any system in particular.
The Difference
Encapsulation differs from abstraction in that it doesn't have anything to do with how 'real' or 'accurate' something is. It doesn't remove components of something to make it simpler or more widely applicable. Rather it may hide certain components to achieve a similar purpose.
Abstraction lets you focus on what the object does instead of how it does it
Encapsulation means hiding the internal details or mechanics of how an object does something.
Like when you drive a car, you know what the gas pedal does but you may not know the process behind it because it is encapsulated.
Let me give an example in C#. Suppose you have an integer:
int Number = 5;
string aStrNumber = Number.ToString();
you can use a method like Number.ToString() which returns you characters representation of the number 5, and stores that in a string object. The method tells you what it does instead of how it does it.
Encapsulation: Is hiding unwanted/un-expected/propriety implementation details from the actual users of object.
e.g.
List<string> list = new List<string>();
list.Sort(); /* Here, which sorting algorithm is used and hows its
implemented is not useful to the user who wants to perform sort, that's
why its hidden from the user of list. */
Abstraction: Is a way of providing generalization and hence a common way to work with objects of vast diversity. e.g.
class Aeroplane : IFlyable, IFuelable, IMachine
{ // Aeroplane's Design says:
// Aeroplane is a flying object
// Aeroplane can be fueled
// Aeroplane is a Machine
}
// But the code related to Pilot, or Driver of Aeroplane is not bothered
// about Machine or Fuel. Hence,
// pilot code:
IFlyable flyingObj = new Aeroplane();
flyingObj.Fly();
// fighter Pilot related code
IFlyable flyingObj2 = new FighterAeroplane();
flyingObj2.Fly();
// UFO related code
IFlyable ufoObj = new UFO();
ufoObj.Fly();
// **All the 3 Above codes are genaralized using IFlyable,
// Interface Abstraction**
// Fly related code knows how to fly, irrespective of the type of
// flying object they are.
// Similarly, Fuel related code:
// Fueling an Aeroplane
IFuelable fuelableObj = new Aeroplane();
fuelableObj.FillFuel();
// Fueling a Car
IFuelable fuelableObj2 = new Car(); // class Car : IFuelable { }
fuelableObj2.FillFuel();
// ** Fueling code does not need know what kind of vehicle it is, so far
// as it can Fill Fuel**
Difference Between Abstraction and Encapsulation.
Abstraction: The idea of presenting something in a simplified / different way, which is either easier to understand and use or more pertinent to the situation.
Consider a class that sends an email... it uses abstraction to show itself to you as some kind of messenger boy, so you can call emailSender.send(mail, recipient). What it actually does - chooses POP3 / SMTP, calling servers, MIME translation, etc, is abstracted away. You only see your messenger boy.
Encapsulation: The idea of securing and hiding data and methods that are private to an object. It deals more with making something independent and foolproof.
Take me, for instance. I encapsulate my heart rate from the rest of the world. Because I don't want anyone else changing that variable, and I don't need anyone else to set it in order for me to function. Its vitally important to me, but you don't need to know what it is, and you probably don't care anyway.
Look around you'll find that almost everything you touch is an example of both abstraction and encapsulation. Your phone, for instance presents to you the abstraction of being able to take what you say and say it to someone else - covering up GSM, processor architecture, radio frequencies, and a million other things you don't understand or care to. It also encapsulates certain data from you, like serial numbers, ID numbers, frequencies, etc.
It all makes the world a nicer place to live in :D
Abstraction: Only necessary information is shown. Let's focus on the example of switching on a computer. The user does not have to know what goes on while the system is still loading (that information is hidden from the user).
Let's take another example, that of the ATM. The customer does not need to know how the machine reads the PIN and processes the transaction, all he needs to do is enter the PIN, take the cash and leave.
Encapsulation: Deals with hiding the sensitive data of a clas hence privatising part of it. It is a way of keeping some information private to its clients by allowing no access to it from outside.
Another example:
Suppose I created an immutable Rectangle class like this:
class Rectangle {
public:
Rectangle(int width, int height) : width_(width), height_(height) {}
int width() const { return width_; }
int height() const { return height_; }
private:
int width_;
int height_;
}
Now it's obvious that I've encapsulated width and height (access is somehow restricted), but I've not abstracted anything (okay, maybe I've ignored where the rectangle is located in the coordinates space, but this is a flaw of the example).
Good abstraction usually implies good encapsulation.
An example of good abstraction is a generic database connection class. Its public interface is database-agnostic, and is very simple, yet allows me to do what I want with the connection. And you see? There's also encapsulation there, because the class must have all the low-level handles and calls inside.
A mechanism that prevents the data of a particular objects safe from intentional or accidental misuse by external functions is called "data Encapsulation"
The act of representing essential features without including the background details or explanations is known as abstraction
Abstraction and Encapsulation by using a single generalized example
------------------------------------------------------------------------------------------------------------------------------------
We all use calculator for calculation of complex problems !
Abstraction : Abstraction means to show What part of functionality.
Encapsulation : Encapsulation means to hide the How part of the functionality.
Lets take a very simple example
/// <summary>
/// We have an Employee class having two properties EmployeeName and EmployeeCode
/// </summary>
public class Employee
{
public string EmplpyeeName { get; set; }
public string EmployeeCode { get; set; }
// Add new employee to DB is the main functionality, so are making it public so that we can expose it to external environment
// This is ABSTRACTION
public void AddEmployee(Employee obj)
{
// "Creation of DB connection" and "To check if employee exists" are internal details which we have hide from external environment
// You can see that these methods are private, external environment just need "What" part only
CreateDBConnection();
CheckIfEmployeeExists();
}
// ENCAPLUSATION using private keyword
private bool CheckIfEmployeeExists()
{
// Here we can validate if the employee already exists
return true;
}
// ENCAPLUSATION using private keyword
private void CreateDBConnection()
{
// Create DB connection code
}
}
Program class of Console Application
class Program
{
static void Main(string[] args)
{
Employee obj = new Employee();
obj.EmplpyeeName = "001";
obj.EmployeeCode = "Raj";
// We have exposed only what part of the functionality
obj.AddEmployee(obj);
}
}
Let's take the example of a stack. It could be implemented using an array or a linked list. But the operations it supports are push and pop.
Now abstraction is exposing only the interfaces push and pop. The underlying representation is hidden (is it an array or a linked list?) and a well-defined interface is provided. Now how do you ensure that no accidental access is made to the abstracted data? That is where encapsulation comes in. For example, classes in C++ use the access specifiers which ensure that accidental access and modification is prevented. And also, by making the above-mentioned interfaces as public, it ensures that the only way to manipulate the stack is through the well-defined interface. In the process, it has coupled the data and the code that can manipulate it (let's not get the friend functions involved here). That is, the code and data are bonded together or tied or encapsulated.
Encapsulation is wrapping up complexity in one capsule that is class & hence Encapsulation…
While abstraction is the characteristics of an object which differentiates from other object...
Abstraction can be achieved by making class abstract having one or more methods abstract. Which is nothing but the characteristic which should be implemented by the class extending it.
e.g. when you inventing/designing a car you define a characteristics like car should have 4 doors, break, steering wheel etc… so anyone uses this design should include this characteristics. Implementation is not the head each of abstraction. It will just define characteristics which should be included.
Encapsulation is achieved keeping data and the behaviour in one capsule that is class & by making use of access modifiers like public, private, protected along with inheritance, aggregation or composition. So you only show only required things, that too, only to the extent you want to show. i.e. public, protected, friendly & private ka funda……
e.g. GM decides to use the abstracted design of car above. But they have various products having the same characteristics & doing almost same functionality. So they write a class which extends the above abstract class. It says how gear box should work, how break should work, how steering wheel should work. Then all the products just use this common functionality. They need not know how the gear box works or break works or steering wheal works. Indivisual product can surely have more features like a/c or auto lock etc…..
Both are powerful; but using abstraction require more skills than encapsulation and bigger applications/products can not survive with out abstraction.
I will try to demonstrate Encapsulation in a simple way.. Lets see..
The wrapping up of data and functions into a single unit (called
class) is known as encapsulation. Encapsulation containing and hiding
information about an object, such as internal data structures and
code.
Encapsulation is -
Hiding Complexity,
Binding Data and Function together,
Making Complicated Method's Private,
Making Instance Variable's Private,
Hiding Unnecessary Data and Functions from End User.
Encapsulation implements Abstraction.
And Abstraction is -
Showing Whats Necessary,
Data needs to abstract from End User,
Lets see an example-
The below Image shows a GUI of "Customer Details to be ADD-ed into a Database".
By looking at the Image we can say that we need a Customer Class.
Step - 1: What does my Customer Class needs?
i.e.
2 variables to store Customer Code and Customer Name.
1 Function to Add the Customer Code and Customer Name into Database.
namespace CustomerContent
{
public class Customer
{
public string CustomerCode = "";
public string CustomerName = "";
public void ADD()
{
//my DB code will go here
}
Now only ADD method wont work here alone.
Step -2: How will the validation work, ADD Function act?
We will need Database Connection code and Validation Code (Extra Methods).
public bool Validate()
{
//Granular Customer Code and Name
return true;
}
public bool CreateDBObject()
{
//DB Connection Code
return true;
}
class Program
{
static void main(String[] args)
{
CustomerComponent.Customer obj = new CustomerComponent.Customer;
obj.CustomerCode = "s001";
obj.CustomerName = "Mac";
obj.Validate();
obj.CreateDBObject();
obj.ADD();
}
}
Now there is no need of showing the Extra Methods(Validate(); CreateDBObject() [Complicated and Extra method] ) to the End User.End user only needs to see and know about Customer Code, Customer Name and ADD button which will ADD the record.. End User doesn't care about HOW it will ADD the Data to Database?.
Step -3: Private the extra and complicated methods which doesn't involves End User's Interaction.
So making those Complicated and Extra method as Private instead Public(i.e Hiding those methods) and deleting the obj.Validate(); obj.CreateDBObject(); from main in class Program we achieve Encapsulation.
In other words Simplifying Interface to End User is Encapsulation.
So now the code looks like as below -
namespace CustomerContent
{
public class Customer
{
public string CustomerCode = "";
public string CustomerName = "";
public void ADD()
{
//my DB code will go here
}
private bool Validate()
{
//Granular Customer Code and Name
return true;
}
private bool CreateDBObject()
{
//DB Connection Code
return true;
}
class Program
{
static void main(String[] args)
{
CustomerComponent.Customer obj = new CustomerComponent.Customer;
obj.CustomerCode = "s001";
obj.CustomerName = "Mac";
obj.ADD();
}
}
Summary :
Step -1: What does my Customer Class needs? is Abstraction.
Step -3: Step -3: Private the extra and complicated methods which doesn't involves End User's Interaction is Encapsulation.
P.S. - The code above is hard and fast.
Abstraction--- Hiding Implementation--at Design---Using Interface/Abstract calsses
Encapsulation--Hiding Data --At Development---Using access modifiers(public/private)
From this
Difference between Encapsulation and Abstraction in OOPS
Abstraction and Encapsulation are two important Object Oriented Programming (OOPS) concepts. Encapsulation and Abstraction both are interrelated terms.
Real Life Difference Between Encapsulation and Abstraction
Encapsulate means to hide. Encapsulation is also called data hiding.You can think Encapsulation like a capsule (medicine tablet) which hides medicine inside it. Encapsulation is wrapping, just hiding properties and methods. Encapsulation is used for hide the code and data in a single unit to protect the data from the outside the world. Class is the best example of encapsulation.
Abstraction refers to showing only the necessary details to the intended user. As the name suggests, abstraction is the "abstract form of anything". We use abstraction in programming languages to make abstract class. Abstract class represents abstract view of methods and properties of class.
Implementation Difference Between Encapsulation and Abstraction
Abstraction is implemented using interface and abstract class while Encapsulation is implemented using private and protected access modifier.
OOPS makes use of encapsulation to enforce the integrity of a type (i.e. to make sure data is used in an appropriate manner) by preventing programmers from accessing data in a non-intended manner. Through encapsulation, only a predetermined group of functions can access the data. The collective term for datatypes and operations (methods) bundled together with access restrictions (public/private, etc.) is a class.
The below paragraph helped me to understand how they differ from each other:
Data encapsulation is a mechanism of bundling the data, and the
functions that use them and data abstraction is a mechanism of
exposing only the interfaces and hiding the implementation details
from the user.
You can read more here.
Information hiding is not strictly required for abstraction or encapsulation. Information might be ignored, but does not have to be hidden.
Encapsulation is the ability to treat something as a single thing, even though it may be composed of many complex parts or ideas. For example, I can say that I'm sitting in a "chair" rather than referring to the many various parts of that chair each with a specific design and function, all fitting together precisely for the purpose of comfortably holding my butt a few feet away from the floor.
Abstraction is enabled by encapsulation. Because we encapsulate objects, we can think about them as things which relate to each other in some way rather than getting bogged down in the subtle details of internal object structure. Abstraction is the ability to consider the bigger picture, removed from concern over little details. The root of the word is abstract as in the summary that appears at the top of a scholarly paper, not abstract as in a class which can only be instantiated as a derived subclass.
I can honestly say that when I plop my butt down in my chair, I never think about how the structure of that chair will catch and hold my weight. It's a decent enough chair that I don't have to worry about those details. So I can turn my attention toward my computer. And again, I don't think about the component parts of my computer. I'm just looking at a part of a webpage that represents a text area that I can type in, and I'm communicating in words, barely even thinking about how my fingers always find the right letters so quickly on the keyboard, and how the connection is ultimately made between tapping these keys and posting to this forum. This is the great power of abstraction. Because the lower levels of the system can be trusted to work with consistency and precision, we have attention to spare for greater work.
The more I read, more I got confused. So, simply here is what I understood:
Encapsulation:
We generally see a watch from outside and it's components are encapsulated inside it's body. We have some kind of control for different operations. This way of hiding details and exposing control (e.g. setting time) is encapsulation.
Abstraction:
So far we were talking about a watch. But we didn't specify what kind of watch. It could be digital or analog, for hand or wall. There are many possibilities. What we do know is, it is a watch and it tells time and that is the only thing we are interested in, the time. This way of hiding details and exposing generic feature or use case is abstraction.
class Aeroplane : IFlyable, IFuelable, IMachine
{ // Aeroplane's Design says:
// Aeroplane is a flying object
// Aeroplane can be fueled
// Aeroplane is a Machine
}
// But the code related to Pilot, or Driver of Aeroplane is not bothered
// about Machine or Fuel. Hence,
// pilot code:
IFlyable flyingObj = new Aeroplane();
flyingObj.Fly();
// fighter Pilot related code
IFlyable flyingObj2 = new FighterAeroplane();
flyingObj2.Fly();
// UFO related code
IFlyable ufoObj = new UFO();
ufoObj.Fly();
// **All the 3 Above codes are genaralized using IFlyable,
// Interface Abstraction**
// Fly related code knows how to fly, irrespective of the type of
// flying object they are.
// Similarly, Fuel related code:
// Fueling an Aeroplane
IFuelable fuelableObj = new Aeroplane();
fuelableObj.FillFuel();
// Fueling a Car
IFuelable fuelableObj2 = new Car(); // class Car : IFuelable { }
fuelableObj2.FillFuel();
// ** Fueling code does not need know what kind of vehicle it is, so far
// as it can Fill Fuel**
abstraction is hiding non useful data from users
and encapsulation is bind together data into a capsule (a class).
I think encapsulation is way that we achieve abstraction.
The process of Abstraction and Encapsulation both generate interfaces.
An interface generated via encapsulation hides implementation details.
An interface generated via abstraction becomes applicable to more data types, compared to before abstraction.
Abstraction is a contract for the implementation we are going to do. Implementation may get changed over period of time. The various implementations themselves may or may not be hidden but are Masked behind the Abstraction.
Suppose we define all the APIs of a class in an interface then ask the users of our code to depened upon the defined APIs of the interface. We are free to improve or modify the implementation only we must follow the set contract. The users are not coupled with our implementation.
We EXPOSE all the NECESSARY Rules (methods) in abstraction, the implementation of the rules are left for the implementor entities, also the implemention is not part of the abstraction. Its just the signature and declaration what makes the abstraction.
Encapsulation is simply HIDING the internal details by reducing the acess of the states and behaviors. An encapsulated class may or may not have well defined Abstraction.
java.util.List is an abstraction for java.util.ArrayList. The internal states of java.util.ArrayList being marked with non public access modifiers is encapsulation.
Edit
Suppose a class Container.nava implements IContainer , IContainer may declare methods like addElement, removeElements, contains, etc. Here IContainer represents the abstraction for its implementing class. Abstraction is declaring the APIs of the class or a module or a system to the outer world. These APIs become the contract.
That system may be or may not be developed yet. The users of the system now can depend on the declared APIs and are sure any system implementing such a contract will always adhere to the APIs declared, they will always provide tge implementation for those APIs. Once we are writing some concrete entity then deciding to hide our internal states is encapsulation
I Think Encapsulation is a way to implement abstraction. Have a look at the following link.
Abstraction and Encapsulation
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My relative is studying programming and has a hard time understanding classes. He has trouble understanding for example that you need to instantiate it, that methods cannot access variables in other methods and if you change a variable in one instance of a class it doesn't change for other instances.
I've tried to use analogies like a class definition is like a blueprint of a house. And instances are houses made from that blueprint.
How do you explain classes and OO in general?
Seriously use Animals, it works great. And that's what nailed the concept for me years ago. Just found this C# code. It seems good
// Assembly: Common Classes
// Namespace: CommonClasses
public interface IAnimal
{
string Name
{
get;
}
string Talk();
}
// Assembly: Animals
// Namespace: Animals
public class AnimalBase
{
private string _name;
AnimalBase(string name)
{
_name = name;
}
public string Name
{
get
{
return _name;
}
}
}
// Assembly: Animals
// Namespace: Animals
public class Cat : AnimalBase, IAnimal
{
public Cat(String name) :
base(name)
{
}
public string Talk() {
return "Meowww!";
}
}
// Assembly: Animals
// Namespace: Animals
public class Dog : AnimalBase, IAnimal
{
public Dog(string name) :
base(name)
{
}
public string Talk() {
return "Arf! Arf!";
}
}
// Assembly: Program
// Namespace: Program
// References and Uses Assemblies: Common Classes, Animals
public class TestAnimals
{
// prints the following:
//
// Missy: Meowww!
// Mr. Bojangles: Meowww!
// Lassie: Arf! Arf!
//
public static void Main(String[] args)
{
List<IAnimal> animals = new List<IAnimal>();
animals.Add(new Cat("Missy"));
animals.Add(new Cat("Mr. Bojangles"));
animals.Add(new Dog("Lassie"));
foreach(IAnimal animal in animals)
{
Console.WriteLine(animal.Name + ": " + animal.Talk());
}
}
}
And once he's got this nailed, you challenge him to define Bird (fly), and then Penguin (fly!?)
The best way I got it through to my wife (a chartered accountant) is as follows.
In 'regular' programming you have data (things that are manipulated) and code (things that manipulate) and they're separate. Sometimes you get mixed up because a certain piece of code tries to manipulate the wrong thing.
In my wife's case, I said a invoice arrived (which involves no physical money changing hands) and accidentally updated a bank balance, something she immediately saw as potential fraud (she used to do forensic accounting, everything is potential fraud to her, including most of my share trades :-).
You could just as easily say that a piece of code meant to wash a floor with a huge mop decided to do it with your toothbrush.
With OO programming, the manipulators and manipulatees are inextricably entwined. You don't apply the floor washing process to the floor, instead you command the floor to wash itself. It knows how to do this because the code is part of the object, not something external to it.
In the accounting case above, I think we ended up having the chart of accounts as the object and we told it to apply a invoice to itself. Since it understood the process, it knew which accounts were allowed to be updated (creditors liability account and an expense account if I remember correctly).
Anyway, that's irrelevant and I'm just meandering now. What I'm saying is to express it in terms your target audience will understand. I suppose that's the secret of most teaching.
Like all old farts, I'd like to answer this with a story from my own life.
I started programming basic on a VIC-20. Not knowing anything else, I though this was how all computers were programmed. I thought it was a bit hard to keep track of which variable names I had used and which were still free, (scope problem). I also thought it was hard to divide my program into repeatable chunks using gosub-return and setting and reading the variables that these would use, (lack of methods).
Then I got into Turbo C over MS-DOS. Now I could create my own methods and functions! I was no longer stuck with the old finite set of commands in basic. I felt like I was creating a new language for every program I wrote. C gave me more expressive power.
C++ was the first object oriented language I heard about. The big moment for me was when I understood that I could create my own data types, and even overload the operators. Again, it felt like I could create my own language containing both new functions and data types, complete with operators.
That's how I would sell OO to a new programmer. Explain that it gives expressive power because they can define their own data types. I always thought encapsulation was a better selling point than inheritance.
I assume the target knows how to use graphical user interfaces. I found the best way is to describe OOP with stuff that they are really used for. Say
Class
A Window is a class. It has methods like
Show a window
Enable a window
Set the window's title
A Window has attributes. That is data associated with it. It is encapsulated into the class, together with the functions that operate on them
A Window has dimensions. Width and height.
A Window has possibly a parent window, and possibly children.
A Window has a title
Object
There are many windows. Each particular window is an object of the class Window. A Parent window containing 10 windows makes 11 Window objects.
Deriveration
A Button is a Window. It has dimensions has a parent window and has a title, the label of a button. It's a special kind of a window. When you ask for a window object, someone can give you a Button. A Button can add functions and data that are specific for a button:
A Button has a state. It can be in a pressed state, and unpressed state.
A Button can be the default button in a Window.
While you are explaining OO with animals, do not forget to illustrate the "is-a" relationship with Stinger missiles-armed kangaroos ;-)
The kangaroos scattered, as predicted, and the Americans nodded appreciatively . . . and then did a double-take as the kangaroos reappeared from behind a hill and launched a barrage of stinger missiles at the hapless helicopter. (Apparently the programmers had forgotten the remove "that" part of the infantry coding).
The lesson? Objects are defined with certain attributes, and any new object defined in terms of the old one inherits all the attributes. The embarrassed programmers had learned to be careful when reusing object-oriented code, and the Yanks left with the utmost respect for the Australian wildlife.
Read the Java tutorials for some good ideas and real world examples.
How about "each molding is built using a mold", or "each model is built using a template", and so "each object is built using a class" ?
Note that it works for class-oriented OOP (which is what you want), but not for prototype-oriented OOP.
As for explaining OOP to a programmer, I'd add examples illustrating:
Separating state from behavior
Most of the time, an instance describe a state, and a class describe a behavior.
Delegation
An instance delegates its behavior to its class, and the class in turn can delegate its behavior to its superclasses (or mixins or traits)
Polymorphism
If class A inherits from class B, an instance of A can be used anywhere an instance of class B can be used.
Messages & methods
A message (or generic function, or virtual function) is like a question. Most of the time, several classes can answer to this question.
A corresponding method is a possible answer to the question, that resides in a class.
When sending a message to an instance, the instance looks up for a corresponding method in its class. If found, it calls it (with the instance bound to 'self' or 'this'. Otherwise, it looks for a corresponding method in its mixins, traits, or superclasses, and calls it.
If they're old enough to have ever filled out a tax form, show them a 1040EZ and explain that an instance of a class is like a filled-out form: each blank is a member variable of the object, and the form also includes instructions for what to do with the member variables, and those instructions are the member functions of the object. A class itself is like a master copy of the form, from which you can print off an endless number of blank forms to fill out.
One thing that I would counsel to AVOID in trying to communicate the concepts of OO to new programmers is using only examples where objects (in the OO sense) represent real-world physical objects. This will actually make students more confused when they encounter objects used to represent non-physical objects (such as a color scheme, or most of the behavioral patterns in "Design Patterns") or objects used just as a useful way to store related functions and related data in the same place (think Java's java.lang.Math for an example.)
Believe it or not, sports!
I've had success in teaching and mentoring by talking about the way that e.g. a play for a football team is described in terms of how the various positions (Center, Quarterback, Runningback, etc.) interact to accomplish a particular goal. In one version, the positions correspond to classes, and specific persons (Tony Romo, Johnny Unitas, etc.) are instances of the class -- individuals who exhibit the same behaviors as defined by the positions.
The second version of this metaphor is to explain that the positions may be interfaces (in the Java sense) rather than classes. An interface really represents a role fulfilled by any object that implements the methods of the interface. And it's perfectly reasonable for an object (via its class, in Java) to implement multiple interfaces, just as it is possible for a talented individual to play more than one position on a sports team.
Finally, the play is like a pattern, in that it describes how a set of roles interact to accomplish some specific goal.
An object is a black box, which you can't see through. Public methods are buttons on them. Protected methods are buttons hidden on the bottom, private methods are dip switches inside.
Let's see a washer as an object. We don't know how it works. We don't care if it's powered by natural gas, diesel, electricity, or plutonium. However, the mechanism and internal structure will vary greatly depending on the energy source like a combustion engine is needed for some. We don't care as long as if we push a "Wash" button, it washes our clothes.
Let's turn the washer not Object-oriented. Expose all the buttons by arranging them on the top. Customers can now turbo-charge the engine by tweaking some dip switches. Make the chassis transparent. Now, you can see your energy-saving washing machine is actually hybrid-powered. There are some monkeys in it. You free them into the wild, and the machine eats up your utility bill like a gas-guzzler.
Object-oriented programming is one technique of raising the level of abstraction by means of which the programmer communicates with the computer: from the level of flipping individual bits on and off, from the level of punching holes in paper cards, from the level of extraordinarily complex sequences of basic instruction codes, from the level of less complicated definitions of reusable templates for blocks of data and reusable blocks of code (structs and procedures), to the level of transcribing the concepts in the programmer's mind into code, so that what goes on inside the computer comes to resemble, for the programmer, what goes on outside the computer in the world of physical objects, intangible assets, and cause-and-effect.
the best book i've ever on object-oriented programming is Betrand's "Object-Oriented Software Construction" - if you really want to get the basics, there is no way around it.
I explain that procedural program is built around the "verbs" of the system, the things you want the system to do, whereas object-oriented programming is build about the "nouns," the things in the system, and what they are capable of, and that for many people this allows for a more straightforward mapping from the problem domain to software.
For the example, I use cars -- "Honda Accord" is a class, whereas the vehicle sitting in the parking lot is an object, an instance of a Honda Accord. A Honda Accord is a sedan, which is a car, which is an automobile, which is a motorized vehicle, which is a mode of transportation, etc. I cannot do anything with a car until I have a physical car to work with -- it doesn't help me that the idea of a Honda Accord exists.
It also helps for discussing interfaces and polymorphism -- the gas pedal means accelerate, regardless what the car does behind the scenes to make that happen. There are "private" parts of the car that I as user do not have access to -- I cannot directly apply an individual brake.
Since the issue is to explain to a new programmer and not to a mother or a wife, I would go right straight to the point. OO is about three main concepts:
Inheritance: a dog is an animal, parent-child, is-a relationship test, etc.
Encapsulation: public-private (protected), information hiding, internal underlying details are not important to the users of the class, protect users from future changes in the implementation.
Polymorphism: run-time binding, late binding, method that gets invoked depends on the type of the object and not the reference or pointer to the object.
Also, depending on how much the new programmer has been doing a procedural language, I would need to help him/her unlearn that the functions or procedures are no longer central.
Games are good.
There are gameobjects, from this walls, enemies and players inherit.
The gameobjects should be renderable have collision-logic etc. The enemies have ai-logic while the player is keyboard controlled.
Some UI-elements are also good, there are buttons, inputboxes etc that all inherit from some baseobject that has code for managing mouse-events etc.
I don't like the animal-example because i've never seen a "real" program that has ever had to use of animals in that way. It will only make people use inheritance all over the place and you will end up with cubes inheriting from rectangles that inherit from lines (why does so many books insist on using this as example? ).
OOP is a higher level of abstraction, a programmer can't really come to grasp it unless he has a good understanding of the normal (read: procedural) way of programming, and he must be able to write some programs that do something useful.
For me it took a series of several lectures by one of my university profs, where he discussed many theoretical aspects of programming, he tried to convince us that programming is about manipulating data, and that this data is a representation of the "state(s)" of the program, and some other abstract stuff that I forgot now! But the point is, it's hard to understand OOP without some theoretical abstract discussion first, and this discussion wouldn't make any sense to a person who hadn't had experience writing some real code.
After the theoretical discussion, you give an example of a moderately complex program, written in procedural style, and slowly convert it, step by step, into object oriented style. After the concrete example, you should go back to the theoretical discussion and just summarize the main points, directly relate the theoretical constructs to the concrete example, e.g you can talk about how the name, age, and salary of an employee represent his state.