I want to extend a class from File. File is abstract and I don't want to implement all the methods, so I searched for an implementation and found _File but this implementation is private.
Is there a class where it is possible to easy extend a File?
Related
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
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.
I'm trying to add a new file to my Xcode project using Xcode 6.1.1 and Xcode now has a "File type" option where you select between "Empty File, Category, Protocol, Extension"
Can someone explain the differences between these and what the default to select would be? My file is a subclass of NSObject.
Thanks
Category
Categories are used to help modularize and organize class definitions. They allow you to take a (complex) class definition and spread it over several organized classes. It is not the same as subclassing. Although categories do allow you to override methods, Objective-C has no way of determining which method definition should be used, so you should never use a category to override methods. Instead, create a subclass that overrides the method as per usual.
Categories can contain protected methods, which "allow arbitrary files to 'opt-in' to a portion of an API by simply importing the category." (Check out the articles linked below.)
Extension
Extensions provide similar functionality to categories, except that you must implement the extension's API in the main implementation file.
Extensions can also be used to create a formal private API. Ordinarily, if you wanted to create private methods, you would write them in the implementation block, but would exclude them from the interface block. However, if you have an extensive group of methods that you would like to remain private, this becomes cumbersome and difficult to read/maintain. Using extensions, you can define the private methods in both the interface and implementation blocks of the .m file. As long as you do not include it in the respective .h file, these methods will be treated as private methods.
Extensions can also be used to make previously declared properties that are read-only outside the class read-write within the class (using the "self." syntax).
Protocol
Protocols allow for abstracted horizontal relationships across various (sometimes unrelated) classes and class hierarchies. A protocol consists of an API that can be used by a variety of classes, regardless of whether or not they are related. This allows you to modify/add some class functionality through a potentially wide range of classes without having to subclass them and alter their own class hierarchies.
In order to use a protocol, a class only needs to:
1. Include the protocol's name inside angled brackets <> after the class/superclass name declaration
2. Implement the protocol's methods
Protocols can also be useful for type checking.
Empty File
An empty file is just that - an empty file. You give it a name, but it contains no class information whatsoever (no generated methods, blocks, comments, etc.).
Sources: RyPress article on Categories and Extensions and RyPress article on Protocols. Both articles have helpful examples of each tool.
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.