Hey I have being looking a lot and watching a lot of videos about inheritance.
There is just this concept of abstract classes which means that it cant be implemented.
I get that you cant use a abstract class on a gameobject but what is the force of using a abstract class?
Also can abstract method only be implemented in a abstract class, and why cant you implement the method in a abstract class but only override it?
I also read something about a abstract method taking a component<T> and I havent found out what the purpase of that was hope some of you clever minds can help a confused programmer :D
Abstract classes are used to model a shared "master class" which will never be used itself but derived classes of it will use that data/functions.
For example say your inheritance is
Animal
Bird Mammal Reptile
In your game, you will only ever use a Bird/Mammal/Reptile since those are concrete models of animals, But they all will share at least some common amount of code. Health, Hunger, and may provide some abstract functions such as Move() which will be implemented on a per subclass level.
From an engineering standpoint, this let's you do fancy things such as create an array/vector of type Animal which can contain birds, mammals, and reptiles.
So say we have a vector of type Animal with 1 of each subclass.
We can then do
foreach (Animal a in animals)
a.Move(100)
Birds/Mammals/Reptiles all move differently, but because we have that abstract function with no implementation at the base level, our code can guarantee that at some point in the inheritance tree it is implemented.
Other popular examples are an abstract class of Item and the abstract function item.Use(); Item could be a Potion, a Scroll, an apple (class Food). It doesn't matter because they all share that same base level interface.
I think reading Microsoft's article on Abstract Classes and Methods would be extremely helpful. You utilize abstract classes by providing base functionality to a class and inheriting from it.
You're also confusing some wording with implement and override: you implement an abstract method by overriding it.
Component<T> means that the class Component with the type parameter as something you define. This is defined in C# as Generics. You utilize these with base Unity3D classes like Component<GameObject> or GetComponent<GameObject>.
Related
https://dart.dev/guides/language/language-tour#implicit-interfaces
I've seen code that uses "implements", so I'm looking into "implements".
But I can't really tell the difference from extends by looking at the official docs.
Looking at the sample code in the official documentation (page above), it looks like it is just doing what it can do with extends with implements.
Then I wonder if it should use "extends".
I think I've understood about inheritance (extends) and mixins (with) so far.
The word "interface" is also mentioned in the Dart documentation, but there is a clear definition of "interface".
I can't even find where it is.
I don't have much knowledge about interfaces in other languages, so it's hard to get an image.
What exactly is the difference between inheritance and implementation?
Because of the difference, when do you use the inheritance "extends" and when do you use the implementation "implements"?
Is there somewhere in the sample that makes a clear difference?
Given class MyClass:
MyClass extends Foo is classic inheritance. MyClass is a Foo
MyClass implements Bar is declaring that the implementer conforms to the Bar interface. MyClass "looks" like a Bar
MyClass with Batz is "mixing" in the interface and implementation. MyClass "acts" like a Batz.
MyClass can implement and mixin as many interfaces as needed (some limitations apply) but can only extend from one interface.
Besides other explanations, dart does not let developers to use multiple inheritance so there can be only one extends, however there can be more than one implements.
Therefore a class can be subtype of only one class(except parents of parent class) but interfaces defines behaviour of the class which implements it and one class can have different behaviours.
A simple analogy is from animals. If we assume there is different behaviours for animals like swimming interface as ISwimmer, running interface as IRunner and flying interface as IFlyer. For example a Fish class is an Animal. Which makes it extends Animal class. Additionally a Fish has behaviour of swimming so it implements ISwimmer interface in that purpose.
If you just want to understand how they are different concept-wise then,
You extend a class. Think of it as you extended your father or your father is your base class.
You implement an interface(which is just a purely abstract class). Like Ferrari implements a car.
An interface cannot have an instance. For example - Have you seen any CAR? The answer is no, you have seen types of car i.e, Ford, Toyota which implements the CAR so the car acts as an interface which other companies or you can say, classes(Ferrari) implements.
You have to implement every feature of a car to be called a car that's why every method of an interface needs to be implemented and we say x "implements" y.
In extending you can override something and skip another , suppose your nose may look like your father but your ears do not.
Think of an interface as a skeleton or just an empty class.
It seems to me that I can make just about anything using object, trait, abstract class and in rare occasions, case class. Most of this is in the form object extends trait. So, I'm wondering, when should I, if ever, use a plain, standard class?
This is not a right place to ask this question
Looks like you are new Scala
Class is a specification for something(some entity) you want to model . It contains behavior and state
There is only one way to declare so called regular class using keyword class
Both trait and abstract class are used for inheritance.
trait is used for inheritance (generally to put common behavior in there). trait is akin to interface in Java. multiple inheritance possible with traits but not abstract class.
A class can extends one class or abstract class but can mixin any number of traits. Traits can have behavior and state.
case class is a nothing but a class but compiler produces some boilerplate code for us to make things easy and look good.
object is used when you want to declare some class but you want to have single instance of the class in the JVM (remember singleton pattern).
If an object performs stateful computations on its members i.e. its members are declared with vars;
Or, even if its member are only declared with vals but those vals store mutable data structures which can be edited in place, then it should be an ordinary (mutable) class akin to a Java mutable object.
The idiomatic way of using Case classes in Scala is as immutable types i.e. all the constructor arguments are vals. We could use vars but then we lose the advantages of case classes like equality comparisons will break over time.
Some advise from Programming in Scala by Odersky et al on deciding between using traits, abstract classes and concrete classes:
If the behavior will not be reused, then make it a concrete class. It is not reusable behavior after all.
If it might be reused in multiple, unrelated classes, make it a trait.
Only traits can be mixed into different parts of the class hierarchy.
If you want to inherit from it in Java code, use an abstract class.
Since traits with code do not have a close Java analog, it tends to be
awkward to inherit from a trait in a Java class. Inheriting from a
Scala class, meanwhile, is exactly like inheriting from a Java class.
As one exception, a Scala trait with only abstract members translates
directly to a Java interface, so you should feel free to define such
traits even if you expect Java code to inherit from it. See Chapter 29
for more information on working with Java and Scala together.
If you plan to distribute it in compiled form, and you expect outside
groups to write classes inheriting from it, you might lean towards
using an abstract class. The issue is that when a trait gains or loses
a member, any classes that inherit from it must be recompiled, even if
they have not changed. If outside clients will only call into the
behavior, instead of inheriting from it, then using a trait is fine.
If efficiency is very important, lean towards using a class. Most Java
runtimes make a virtual method invocation of a class member a faster
operation than an interface method invocation. Traits get compiled to
interfaces and therefore may pay a slight performance overhead.
However, you should make this choice only if you know that the trait
in question constitutes a performance bottleneck and have evidence
that using a class instead actually solves the problem.
If you still do not know, after considering the above, then start by
making it as a trait. You can always
change it later, and in general using a trait keeps more options open.
The following code using UIGestureRecognizer:
UIGestureRecognizer *gestureRecog = [[UIGestureRecognizer alloc]
initWithTarget:self
action:#selector(handletap:)];
[self.view addGestureRecognizer:gestureRecog];
can actually compile and run. I thought abstract class cannot be instantiated?
Abstract classes are not a language feature in Objective-C (unlike Java, for example), so it isn't something the compiler could enforce.
When a class is marked as abstract in the documentation, it is just a hint how it is intended to be used, but neither the runtime, nor the compiler will actually prevent you from instantiating it directly.
The section on abstract classes in the Objective-C Programming Language Guide actually states that NSView is an example of an abstract class that you may sometimes use without subclassing, so the concept as such is not as strict as in other languages/frameworks.
Thanks for omz's answer. This is the related excerpt from Apple's documentation:
Abstract Classes
Some classes are designed only or primarily so that other classes can
inherit from them. These abstract classes group methods and instance
variables that can be used by a number of subclasses into a common
definition. The abstract class is typically incomplete by itself, but
contains useful code that reduces the implementation burden of its
subclasses. (Because abstract classes must have subclasses to be
useful, they’re sometimes also called abstract superclasses.)
Unlike some other languages, Objective-C does not have syntax to mark
classes as abstract, nor does it prevent you from creating an instance
of an abstract class.
The NSObject class is the canonical example of an abstract class in
Cocoa. You never use instances of the NSObject class in an
application—it wouldn’t be good for anything; it would be a generic
object with the ability to do nothing in particular.
The NSView class, on the other hand, provides an example of an
abstract class, instances of which you might occasionally use
directly.
Abstract classes often contain code that helps define the structure of
an application. When you create subclasses of these classes, instances
of your new classes fit effortlessly into the application structure
and work automatically with other objects.
With reference to UML diagrams, what is an interface? and can someone explain in more simpler words. I cant understand anything from googling it.
An interface is like a template design for a class that contains no data or implemetnation; only definitions for methods, properties etc. These are abstract and cannot be instantiated but can be inherited from at which point all specified methods etc must be implemented by the concrete class inheriting the interface.
An interface is a design item describing a behaviour.
Classes implementing the interface will/must behave according to its definition.
Interfaces are used to promote loose coupling and the base of many IoC patterns (Inversion of Control)
A Interface is just a description of a class nothing concrete.
You use it to create a new class "with the same description" without knowing the concrete implementation.
In one word: it's a contract. Every class that implements this contract (interface) will have to implement the methods defined on it.
Whenever I create an abstract class I tend to create an interface to go along with it and have other code refer to the interface and not the abstract class. Usually when I don't create an interface to start with I regret it (such as having to override all implimented methods to stub the class for unit testing or later down the line new classes don't need any of the implimentation and override everything also finding themselves unable to extend any other class).
At first I tried to distinguish when to use an interface and when to use an abstract class by considering is-a vs able-to but I still would end up suffering later down the line for not making an interface to start with.
So the question is when is it a good idea to only have an abstract class and no interface at all?
When you wish to "give" some base class functionality to derived classes but when this functionality is not sufficient to instantiate a usable class, then go for abstract classes.
When you wish that some classes completely implement a set of methods (a public contract), then it is a convenient to define such contract with interfaces and enforce them onto classes by making them inherit this interface.
In short:
With abstract classes you give some common base functionality to derived classes. No further actions are necessary unless abstract class has some stubs (which have to be implemented down there).
With interfaces you require derived classes to implement a set of functions and you do not pass along any implementation.
So the question is when is it a good idea to only have an abstract class and no interface at all?
When you do not wish to enforce any public contract (a set of methods/properties defined by an interface).
Also when you do not plan to use certain coding techniques like casting object to an interface type (run-time polymorphism) or limit allowed input (some method argument will only accept object of types which implement certain interfaces).
Well, the main case it is useful to have only an abstract class without any interface is to mark a certain type. It is useful to be able to check if an object "is-a" something. These interface "mark" an objet to be of a certain type. Depending on the language you use, different design patterns apply ...
These sort of abstract classes exist in java. You can also use them in C++ with RTTI.
my2c