I want to understand the difference between extends, implements and with. When do I use each keyword?
Extends:
Use extends to create a subclass, and super to refer to the superclass.
Extends is the typical OOP class inheritance. If class a extends class b all properties, variables, functions implemented in class b are also available in class a. Additionally you can override functions etc.
You use extend if you want to create a more specific version of a class. For example the class car could extend the class vehicle. In Dart a class can only extend one class.
Implements:
Every class implicitly defines an interface containing all the instance members of the class and of any interfaces it implements. If you want to create a class A that supports class B’s API without inheriting B’s implementation, class A should implement the B interface.
Implements can be used if you want to create your own implementation of another class or interface. When class a implements class b. All functions defined in class b must be implemented.
When you're implementing another class, you do not inherit code from the class. You only inherit the type. In Dart you can use the implements keyword with multiple classes or interfaces.
With (Mixins):
Mixins are a way of reusing a class’s code in multiple class hierarchies.
With is used to include Mixins. A mixin is a different type of structure, which can only be used with the keyword with.
They are used in Flutter to include common code snippets. A common used Mixin is the SingleTickerProviderStateMixin.
extend can only be used with a single class at the time, BUT... you can easily extend a class which extends another class which extends another class which...! ;)
In fact, most Flutter widgets are already built like that.
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I couldn't understand the difference between these two keywords.
By using extends we can get features from parent class. I think implements does that too.
First I thought the difference is overriding methods but with extends I can do that.(I might be wrong)
Is the difference of these two keywords about overriding methods or what? Thank you
extends means we get the implementation of a given class and we can then override members if we want our own implementation for certain variables or methods. You can also add new variables and methods.
implements means you get nothing from the class you implement from. But you promise that your class will be compatible with the interface of the class you are implementing. So no, you are not getting any implementation from the super class and you need to implement everything or declare your class abstract.
I was learning dart but it was still an error when I entered into the mixin, I don't know the fault where it is always an error when:
Abstract classes cannot be instantiated
I want to implement a mixin for Cat, Elang and Hiu with a subclass of Mamalia, Burung, Ikan
This is the Github code:
The error are rather clear. You cannot make an instance of an abstract class since an abstract class per definition is a class which can define methods and fields which are yet to be implemented and the class is therefore not complete.
The purpose of abstract classes is to let other classes extend from them and implement the missing methods.
I don't know why all your classes are marked as abstract but you can just remove the abstract keyword from the classes: Mamalia, Ikan and Burung and it should work since all of these classes are not needed to be abstract.
Abstract Class -
Use the abstract modifier to define an abstract class—a class that can’t be instantiated. Abstract classes are useful for defining interfaces, often with some implementation.
Don't instantiate Mamalia, Burung or Ikan classes. Instead instantiate Cat, Elang and Hiu.
Mamlia mamal = Cat(); // Will allow accessing methods defined only in Mamal class
Cat cat = Cat() // Will allow accessing all the methods defined in Cat class
From:
https://medium.com/dartlang/announcing-dart-2-1-improved-performance-usability-9f55fca6f31a
Under Mixins:
mixin SomeClass<T extends SomeOtherClass>
on State<T>
implements ThirdClass
What is "on"?
This mixin can only be applied to classes that extend or implement State<T> which is effectively the state of a stateful widget.
Figuratively speaking on is the extends for mixins.
mixin A on B
is like
class A extends B
To choose which one to use is like to choose between composition or inheritance. More on that composition vs inheritance thing.
What is the difference?
class ExtraPowers{}
class ClassPowers extends ExtraPowers{}
mixin MixinPowers on ExtraPowers{}
Let's say our main character is class "C" and we want to give it some extra capabilities and powers. We can do that in two ways:
// e.g. inheritance
class C extends ClassPowers{}
or
// e.g. composition
class C with ExtraPowers, MixinPowers{}
So if we choose to extend we may need to comply to some requirements. And we can do that by passing arguments to super.
If we choose to gain powers by using with can't use super to pass requirements. The requirements are satisfied by having the same extra powers as our mixin (nothing is left hidden e.g composition) and that on keyword tells us what extra powers our mixin has. So in order to get powers from MixinPowers we first must get ExtraPowers.
class C with ExtraPowers, MixinPowers{}
In conclusion on just like extends gives access to members of another object but on is for mixins extends for classes. The difference is when you use mixin you must implement the objects after the on keyword.
More info
i want to do something like the following so bad
abstract class A{}
abstract class B extends A{}
abstract class C extends A{}
abstract class D extends B with C{} //C cannot be used as a mixin because it extends a class other than object
is there any solution other than copying the content of C in D?
the real names of my classes, to give you an idea of what i am trying to do
//A Observable
//B DynamicObservable
//C ObservableWithValidationErrors
//D DynamicObservableWithValidationErrors
There are some restrictions on the class you can use as mixin (See Mixins in Dart - Syntax and semantics).
However, in this proposal, a mixin may only be extracted from a class that obeys the following restrictions:
The class has no declared constructors.
The class’ superclass is Object.
The class contains no super calls.
Those restrictions may be removed in the future.
The semantics are deliberately restricted in several ways, so as to reduce disruption to our existing implementations, while allowing future evolution toward a full-fledged mixin implementation. This restricted version already provides considerable value.
In some circumstances it might be possible to restructure your class to use multiple mixins instead:
abstract class Observable{}
abstract class Dynamic{}
abstract class ValidationErrors{}
abstract class DynamicObservable extends Observable with Dynamic{}
abstract class ObservableWithValidationErrors extends Observable with ValidationErrors{}
abstract class DynamicObservableWithValidationErrors extends Observable with Dynamic, ValidationErrors{}
Of course, if Dynamic or ValidationErrors cannot be isolated this way, and rely on inheriting from Observable, this will not be possible.
i've been poking around RIM's PlayBook SDK and noticed they extend Object on a lot of their classes.
are there benefits to extending Object on a custom class that would normally be unextended?
package qnx.notificationManager
{
public class Notification extends Object
{
...
If you do not specify that your class extends another class, the compiler automatically makes your class extend Object. So the answer is that you don't gain any benefit; they must have just been trying to be explicit, or maybe the code was generated through decompilation.