Considering Interface Segregation Principle, which is one among the most “talked about” principles of Object Oriented Programming - SOLID principles, I was wondering if it were possible to have two different classes in a single Laravel controller? For example:
<?php
namespace ...;
use App\Http\Controllers\Controller;
interface VehicleInterface
{
public function ...
}
class CarController extends Controller implements VehicleInterface
{
...
}
class ElectricCar implements VehicleInterface
{
...
}
Technically you can have multiple classes in the same file.
With Laravel (or any framework), not really, if you want to use its autoloader, as classname = filename is the convention.
Also, a controller is what handles requests. You can load as many instances of your different classes inside a controller function. But defining other classes inside the controller file is not what you're supposed to do, at all.
This question has at least two problems:
I do not think that ElectricCar and CarController should share the same interface. The ElectricCar models a car, possibly with methods like accelerateTo(120mph) whereas the CarController maybe has methods like accelerateCarTo(Car5, 120mph). They are also used with a different meaning: The ElectricCar models one single car, whereas the CarController manages access to a single or multiple cars, which is also called from a abstract construct modeling an application flow.
The interface segregation principle does not speak about classes, so the question is ill-formed in the first place. The interface segregation principle says that one interface(!) specifying multiple use cases should be broken up into multiple interfaces(!) called role interfaces, each fulfilling exactly one use case. For example, an interface modeling an ATM with methods like deposit() and withdraw() should be split up into two interfaces each only fulfilling one of these functions. The goal is that a dependent entity must only use and see the parts it really requires.
In Doctrine, assuming I want to implement different types of vehicles. Maybe a car, a plane, a bicycle and so on ... all these vehicles have many different properties but also very common things like they are all made of a material.
So if i want to get this material ... should i implement a abstract class Vehicle with a property material and implement getter/setter or should i define a interface IVehicle and define the getter/setter in there to be sure there is really a method for getting and setting the material? Or should i even use both in combination?
It feels "professional" using the interface ... but it feels wrong to define getter/setters in interfaces, so my personal feeling is:
Don't use an interface, just use the abstract class.
But is the abstract class approach protected against misuse? For example on another place i definitely expect a Material type returning from the getMaterial function ... is the abstract class approach also save for not returning complete unexpected things (like the interface does)?
So if i extend this vehicle for another concrete class, a developer should not be able to return unexpected things, but should also be able to change the logic in the particular method if needed.
My 2 cents:
You can write an abstract class with methods that return null (or some other predetermined value) that would require you to implement them in every derived class.
This approach would still require you to write some boilerplate code to check against your conditions in every class.
I would personally go with an interface here as the nature of your classes seem to be quite diverse and can only be partially generalized to a single abstract class. However, I don't see anything wrong with going the abstract class way either. In this case, the devil is in the details.
I have been trying to learn OOP using PHP5 and I am having trouble wrapping my head around a couple of things.
First; I am understanding Inheritance in OOP, but what I am not understanding in PHP is why I can only use extends once to extend a parent class. I have been doing some reading online and a few times I have seen where it is not good practice to use extends more than once, that's why it's not available, is this true?
Second; Abstract vs. Interface, I read a way to Inherit one class many times is to use Interface and or Abstract. Is this correct?
Third; I am writing a very simple class using Inheritance and I am having a problem understanding how to properly contruct it. I have looked thru php.net and OS and have seen many classes and how they are written, but could anyone take the time to write a simple example in PHP using .... let's say a mammal class as the parent class and have dog, cat, and bear and the child classes.
I know I could find something online that has already been written, but this way I can converse with the person who wrote the class.
First; I am understanding Inheritance in OOP, but what I am not understanding in PHP is why I can only use extends once to extend a parent class. I have been doing some reading online and a few times I have seen where it is not good practice to use extends more than once, that's why it's not available, is this true?
In short: Its just a design decission from the PHP core developers. Multiple inheritance brings up many questions, for example what should happen, when two parent classes implements the same method? To avoid such conflicts some languages decide to not support it (e.g. Java too ;)). There is no real downside in it.
Second; Abstract vs. Interface, I read a way to Inherit one class many times is to use Interface and or Abstract. Is this correct?
You can extend every class as much as you like. There is no limitation neither for classes, abstract classes, nor interfaces. However, its possible to implement (implements keyword) more than one interface into one class.
Third; I am writing a very simple class using Inheritance and I am having a problem understanding how to properly contruct it. I have looked thru php.net and OS and have seen many classes and how they are written, but could anyone take the time to write a simple example in PHP using .... let's say a mammal class as the parent class and have dog, cat, and bear and the child classes.
abstract class Mammal {}
class Dog extends Mammal {}
class Cat extends Mammal {}
class Bear extends Mammal {}
Yes it's true PHP only officialy supports extending one class at a time, its a bug bear to all OOP because there's no good OOP reason not to be able to but there is for the language. It's also true you can circumvent it, not recommended however because it's not supported by the language for a reason (I forget now).
Not sure what you mean by versus, Abstract classes are classes that should be abstracted before using them. Think of them as 80% complete classes, there are a few functions missing for it to work which must be implemented on a per application level.
Interfaces are the kind of bones of a class, there's no functionality there but all functions are defined. They aren't to be extended or used they are to be referenced against.If a class implements and interface it commits to implement all the function stated in the interfaces (and possibly more) and PHP throws an error if that's not the case.
A very simple example
class Mammal{
function getBones(){}
}
class Dog extends Mammal{
function bark(){}
}
class Cat extends Mammal{
function meow(){}
}
$d = new Dog()
$d->bark()
//Works
$d->getBones()
//works
$d->meow()
//does not work, is a cat function
I think you are talking about Multiple Inheritance? Technically, it's a design decision of the PHP developers. There are other languages which allow it.
Multiple inheritance has some technical problems (Diamond Problem), but there is a good theoretical argument against it: A class should have one responsibility (Single Responsibility Principle, it talks of "object" because there are classless yet object-oriented languages). Two different classes usually have different responsibilities, so if a class inherits from both of them, it would have two responsibilities, violating the SRP.
See Search for "abstract class interface" – use interface to define the methods a class must implement to satisfy a contract, use abstract class if you want to partially implement something (usually an interface).
Impossible, because you did not say which aspects of the mammals should be modeled. Without knowing that, it would even be impossible to know if different mammal classes or a common base class are needed.
From a quick Google search and a the wikipedia article on Multiple Inheritance, which quotes:
Multiple inheritance refers to a feature of some object-oriented programming languages in which a class can inherit behaviors and features from more than one superclass. This contrasts with single inheritance, where a class may inherit from at most one superclass.
I understand that PHP doesn't allow multiple inheritance. What I can't find a clear answer on, however, is whether it allows more than one class to extend a superclass. Example:
class a {}
class b extends a {}
class c extends a {}
In terms of what I'm trying to do, I'm making an RPG and want a 'generic' character class to include all the methods and properties that make the template of a character. Then I want classes to include specifics for each type of character (warrior, mage, etc), such as stats modifiers and special attacks.
Is this possible?
Yes, it's perfectly possible. The entire purpose of inheritance is that multiple children can inherit common functionality from a parent.
Yes, multiple classes can extend the same class, per your code example.
This is the foundation of OOP. It's possible.
Yes, any class can extend a base class. In your example, it just isn't possible for class c to extend both a and b.
So in an adjusted example like the one above
class a {}
class b extends a {}
class c extends b {}
c could not get to properties of a ?
Interfaces allow you to create code which defines the methods of classes that implement it. You cannot however add any code to those methods.
Abstract classes allow you to do the same thing, along with adding code to the method.
Now if you can achieve the same goal with abstract classes, why do we even need the concept of interfaces?
I've been told that it has to do with OO theory from C++ to Java, which is what PHP's OO stuff is based on. Is the concept useful in Java but not in PHP? Is it just a way to keep from having placeholders littered in the abstract class? Am I missing something?
The entire point of interfaces is to give you the flexibility to have your class be forced to implement multiple interfaces, but still not allow multiple inheritance. The issues with inheriting from multiple classes are many and varied and the wikipedia page on it sums them up pretty well.
Interfaces are a compromise. Most of the problems with multiple inheritance don't apply to abstract base classes, so most modern languages these days disable multiple inheritance yet call abstract base classes interfaces and allows a class to "implement" as many of those as they want.
The concept is useful all around in object oriented programming. To me I think of an interface as a contract. So long my class and your class agree on this method signature contract we can "interface". As for abstract classes those I see as more of base classes that stub out some methods and I need to fill in the details.
Why would you need an interface, if there are already abstract classes?
To prevent multiple inheritance (can cause multiple known problems).
One of such problems:
The "diamond problem" (sometimes referred to as the "deadly diamond of
death") is an ambiguity that arises when two classes B and C inherit
from A and class D inherits from both B and C. If there is a method
in A that B and C have overridden, and D does not override it, then
which version of the method does D inherit: that of B, or that of C?
Source: https://en.wikipedia.org/wiki/Multiple_inheritance#The_diamond_problem
Why/When to use an interface?
An example... All cars in the world have the same interface (methods)... AccelerationPedalIsOnTheRight(), BrakePedalISOnTheLeft(). Imagine that each car brand would have these "methods" different from another brand. BMW would have The brakes on the right side, and Honda would have brakes on the left side of the wheel. People would have to learn how these "methods" work every time they would buy a different brand of car. That's why it's a good idea to have the same interface in multiple "places."
What does an interface do for you (why would someone even use one)?
An interface prevents you from making "mistakes" (it assures you that all classes which implement a specific interface, will all have the methods which are in the interface).
// Methods inside this interface must be implemented in all classes which implement this interface.
interface IPersonService
{
public function Create($personObject);
}
class MySqlPerson implements IPersonService
{
public function Create($personObject)
{
// Create a new person in MySql database.
}
}
class MongoPerson implements IPersonService
{
public function Create($personObject)
{
// Mongo database creates a new person differently then MySQL does. But the code outside of this method doesn't care how a person will be added to the database, all it has to know is that the method Create() has 1 parameter (the person object).
}
}
This way, the Create() method will always be used the same way. It doesn't matter if we are using the MySqlPerson class or the MongoPerson class. The way how we are using a method stays the same (the interface stays the same).
For example, it will be used like this (everywhere in our code):
new MySqlPerson()->Create($personObject);
new MongoPerson()->Create($personObject);
This way, something like this can't happen:
new MySqlPerson()->Create($personObject)
new MongoPerson()->Create($personsName, $personsAge);
It's much easier to remember one interface and use the same one everywhere, than multiple different ones.
This way, the inside of the Create() method can be different for different classes, without affecting the "outside" code, which calls this method. All the outside code has to know is that the method Create() has 1 parameter ($personObject), because that's how the outside code will use/call the method. The outside code doesn't care what's happening inside the method; it only has to know how to use/call it.
You can do this without an interface as well, but if you use an interface, it's "safer" (because it prevents you to make mistakes). The interface assures you that the method Create() will have the same signature (same types and a same number of parameters) in all classes that implement the interface. This way you can be sure that ANY class which implements the IPersonService interface, will have the method Create() (in this example) and will need only 1 parameter ($personObject) to get called/used.
A class that implements an interface must implement all methods, which the interface does/has.
I hope that I didn't repeat myself too much.
The difference between using an interface and an abstract class has more to do with code organization for me, than enforcement by the language itself. I use them a lot when preparing code for other developers to work with so that they stay within the intended design patterns. Interfaces are a kind of "design by contract" whereby your code is agreeing to respond to a prescribed set of API calls that may be coming from code you do not have aceess to.
While inheritance from abstract class is a "is a" relation, that isn't always what you want, and implementing an interface is more of a "acts like a" relation. This difference can be quite significant in certain contexts.
For example, let us say you have an abstract class Account from which many other classes extend (types of accounts and so forth). It has a particular set of methods that are only applicable to that type group. However, some of these account subclasses implement Versionable, or Listable, or Editable so that they can be thrown into controllers that expect to use those APIs. The controller does not care what type of object it is
By contrast, I can also create an object that does not extend from Account, say a User abstract class, and still implement Listable and Editable, but not Versionable, which doesn't make sense here.
In this way, I am saying that FooUser subclass is NOT an account, but DOES act like an Editable object. Likewise BarAccount extends from Account, but is not a User subclass, but implements Editable, Listable and also Versionable.
Adding all of these APIs for Editable, Listable and Versionable into the abstract classes itself would not only be cluttered and ugly, but would either duplicate the common interfaces in Account and User, or force my User object to implement Versionable, probably just to throw an exception.
Interfaces are essentially a blueprint for what you can create. They define what methods a class must have, but you can create extra methods outside of those limitations.
I'm not sure what you mean by not being able to add code to methods - because you can. Are you applying the interface to an abstract class or the class that extends it?
A method in the interface applied to the abstract class will need to be implemented in that abstract class. However apply that interface to the extending class and the method only needs implementing in the extending class. I could be wrong here - I don't use interfaces as often as I could/should.
I've always thought of interfaces as a pattern for external developers or an extra ruleset to ensure things are correct.
You will use interfaces in PHP:
To hide implementation - establish an access protocol to a class of objects an change the underlying implementation without refactoring in all the places you've used that objects
To check type - as in making sure that a parameter has a specific type $object instanceof MyInterface
To enforce parameter checking at runtime
To implement multiple behaviours into a single class (build complex types)
class Car implements EngineInterface, BodyInterface, SteeringInterface {
so that a Car object ca now start(), stop() (EngineInterface) or goRight(),goLeft() (Steering interface)
and other things I cannot think of right now
Number 4 it's probably the most obvious use case that you cannot address with abstract classes.
From Thinking in Java:
An interface says, “This is what all classes that implement this particular interface will look like.” Thus, any code that uses a particular interface knows what methods can be called for that interface, and that’s all. So the interface is used to establish a “protocol” between classes.
Interfaces exist not as a base on which classes can extend but as a map of required functions.
The following is an example of using an interface where an abstract class does not fit:
Lets say I have a calendar application that allows users to import calendar data from external sources. I would write classes to handle importing each type of data source (ical, rss, atom, json) Each of those classes would implement a common interface that would ensure they all have the common public methods that my application needs to get the data.
<?php
interface ImportableFeed
{
public function getEvents();
}
Then when a user adds a new feed I can identify the type of feed it is and use the class developed for that type to import the data. Each class written to import data for a specific feed would have completely different code, there may otherwise be very few similarities between the classes outside of the fact that they are required to implement the interface that allows my application to consume them. If I were to use an abstract class, I could very easily ignore the fact that I have not overridden the getEvents() method which would then break my application in this instance whereas using an interface would not let my app run if ANY of the methods defined in the interface do not exist in the class that implemented it. My app doesn't have to care what class it uses to get data from a feed, only that the methods it needs to get that data are present.
To take this a step further, the interface proves to be extremely useful when I come back to my calendar app with the intent of adding another feed type. Using the ImportableFeed interface means I can continue adding more classes that import different feed types by simply adding new classes that implement this interface. This allows me to add tons of functionality without having to add unnecessarily bulk to my core application since my core application only relies on there being the public methods available that the interface requires so as long as my new feed import classes implement the ImportableFeed interface then I know I can just drop it in place and keep moving.
This is just a very simple start. I can then create another interface that all my calendar classes can be required to implement that offers more functionality specific to the feed type the class handles. Another good example would be a method to verify the feed type, etc.
This goes beyond the question but since I used the example above:
Interfaces come with their own set of issues if used in this manner. I find myself needing to ensure the output that is returned from the methods implemented to match the interface and to achieve this I use an IDE that reads PHPDoc blocks and add the return type as a type hint in a PHPDoc block of the interface which will then translate to the concrete class that implements it. My classes that consume the data output from the classes that implement this interface will then at the very least know it's expecting an array returned in this example:
<?php
interface ImportableFeed
{
/**
* #return array
*/
public function getEvents();
}
There isn't much room in which to compare abstract classes and interfaces. Interfaces are simply maps that when implemented require the class to have a set of public interfaces.
Interfaces aren't just for making sure developers implement certain methods. The idea is that because these classes are guaranteed to have certain methods, you can use these methods even if you don't know the class's actual type. Example:
interface Readable {
String read();
}
List<Readable> readables; // dunno what these actually are, but we know they have read();
for(Readable reader : readables)
System.out.println(reader.read());
In many cases, it doesn't make sense to provide a base class, abstract or not, because the implementations vary wildly and don't share anything in common besides a few methods.
Dynamically typed languages have the notion of "duck-typing" where you don't need interfaces; you are free to assume that the object has the method that you're calling on it. This works around the problem in statically typed languages where your object has some method (in my example, read()), but doesn't implement the interface.
In my opinion, interfaces should be preferred over non-functional abstract classes. I wouldn't be surprised if there would be even a performance hit there, as there is only one object instantiated, instead of parsing two, combining them (although, I can't be sure, I'm not familiar with the inner workings of OOP PHP).
It is true that interfaces are less useful/meaningful than compared to, say, Java. On the other hand, PHP6 will introduce even more type hinting, including type hinting for return values. This should add some value to PHP interfaces.
tl;dr: interfaces defines a list of methods that need to be followed (think API), while an abstract class gives some basic/common functionality, which the subclasses refine to specific needs.
I can't remember if PHP is different in this respect, but in Java, you can implement multiple Interfaces, but you can't inherit multiple abstract classes. I'd assume PHP works the same way.
In PHP you can apply multiple interfaces by seperating them with a comma (I think, I don't find that a clean soloution).
As for multiple abstract classes you could have multiple abstracts extending each other (again, I'm not totally sure about that but I think I've seen that somewhere before). The only thing you can't extend is a final class.
Interfaces will not give your code any performance boosts or anything like that, but they can go a long way toward making it maintainable. It is true that an abstract class (or even a non-abstract class) can be used to establish an interface to your code, but proper interfaces (the ones you define with the keyword and that only contain method signatures) are just plain easier to sort through and read.
That being said, I tend to use discretion when deciding whether or not to use an interface over a class. Sometimes I want default method implementations, or variables that will be common to all subclasses.
Of course, the point about multiple-interface implementation is a sound one, too. If you have a class that implements multiple interfaces, you can use an object of that class as different types in the same application.
The fact that your question is about PHP, though, makes things a bit more interesting. Typing to interfaces is still not incredibly necessary in PHP, where you can pretty much feed anything to any method, regardless of its type. You can statically type method parameters, but some of that is broken (String, I believe, causes some hiccups). Couple this with the fact that you can't type most other references, and there isn't much value in trying to force static typing in PHP (at this point). And because of that, the value of interfaces in PHP, at this point is far less than it is in more strongly-typed languages. They have the benefit of readability, but little else. Multiple-implementation isn't even beneficial, because you still have to declare the methods and give them bodies within the implementor.
Interfaces are like your genes.
Abstract classes are like your actual parents.
Their purposes are hereditary, but in the case of abstract classes vs interfaces, what is inherited is more specific.
I don't know about other languages, what is the concept of interface there. But for PHP, I will try my best to explain it. Just be patient, and Please comment if this helped.
An interface works as a "contracts", specifying what a set of subclasses does, but not how they do it.
The Rule
An Interface can't be instantiate.
You can't implement any method in an interface,i.e. it only contains .signature of the method but not details(body).
Interfaces can contain methods and/or constants, but no attributes. Interface constants have the same restrictions as class constants. Interface methods are implicitly abstract.
Interfaces must not declare constructors or destructors, since these are implementation details on the class
level.
All the methods in an interface must have public visibility.
Now let's take an example.
Suppose we have two toys: one is a Dog, and other one is a Cat.
As we know a dog barks, and cat mews.These two have same speak method, but with different functionality or implementation.
Suppose we are giving the user a remote control that has a speak button.
When the user presses speak button, the toy have to speak it doesn't matter if it's Dog or a Cat.
This a good case to use an interface, not an abstract class because the implementations are different.
Why? Remember
If you need to support the child classes by adding some non-abstract method, you should use abstract classes. Otherwise, interfaces would be your choice.
Below are the points for PHP Interface
It is used to define required no of methods in class [if you want to load html then id and name is required so in this case interface include setID and setName].
Interface strictly force class to include all the methods define in it.
You can only define method in interface with public accessibility.
You can also extend interface like class. You can extend interface in php using extends keyword.
Extend multiple interface.
You can not implement 2 interfaces if both share function with same name. It will throw error.
Example code :
interface test{
public function A($i);
public function B($j = 20);
}
class xyz implements test{
public function A($a){
echo "CLASS A Value is ".$a;
}
public function B($b){
echo "CLASS B Value is ".$b;
}
}
$x = new xyz();
echo $x->A(11);
echo "<br/>";
echo $x->B(10);
We saw that abstract classes and interfaces are similar in that they provide abstract methods that must be implemented in the child classes. However, they still have the following differences:
1.Interfaces can include abstract methods and constants, but cannot contain concrete methods and variables.
2.All the methods in the interface must be in the public visibility
scope.
3.A class can implement more than one interface, while it can inherit
from only one abstract class.
interface abstract class
the code - abstract methods - abstract methods
- constants - constants
- concrete methods
- concrete variables
access modifiers
- public - public
- protected
- private
etc.
number of parents The same class can implement
more than 1 interface The child class can
inherit only from 1 abstract class
Hope this will helps to anyone to understand!