What is an interface? (POO)

[Yue Participant]

Going deeper into this object-oriented programming, I wondered what an interface is, and what its possible uses are, some simple explanation to understand.

Thank you.

[Danilo Participant]

– https://en.wikipedia.org/wiki/Interface_(object-oriented_programming)

An Interface contains declarations of methods, the parameters, and the return type.

Any class that implements the Interface is required to implement **everything**
from the Interface, exactly like it was declared.
It is like a contract/agreement and you can trust that the specific methods
are implemented in the classes.

[abakobo Participant]

The best example in monkey2 is IContainer interface.

Stack and lists for example implements IContainer and can therfore be used with eachin loops.
You can create you own container that implements icontainer and you’ll be able to use it with eachin loops.

An interface force you to respect what was planned in some way. And if you forget to implement a part of it, you’ll have a compile error.

Interfaces are usefull for generic type constrains too. (using the ‘where’ keyword)
https://www.codingame.com/playgrounds/2290/demystifying-c-generics/generics-constraints

[Danilo Participant]

Small example:

Monkey

#Import "<std>" #Import "<mojo>" Function Main() New mojo.app.AppInstance Local console := New Console Local printer := New Printer Local message := New Message HelloWorld( console ) HelloWorld( printer ) HelloWorld( message ) End #Rem Interface IOutput #End Interface IOutput Method PrintLn( text:String ) End #Rem Function HelloWorld Parameter 1: Takes any object (instance of a class) that implements IOutput #End Function HelloWorld( obj:IOutput ) obj.PrintLn( "Hello World!" ) End #Rem class Console #End Class Console Implements IOutput Method PrintLn( s:String ) Print s End End #Rem class Printer #End Class Printer Implements IOutput Method PrintLn( s:String ) Print "Printing to printer... ~n~t"+s End End #Rem class Message #End Class Message Implements IOutput Method PrintLn( s:String ) std.requesters.Notify( "Message", s ) End End

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#Import "<std>" #Import "<mojo>"   Function Main()     New mojo.app.AppInstance          Local console := New Console     Local printer := New Printer     Local message := New Message          HelloWorld( console )     HelloWorld( printer )     HelloWorld( message ) End   #Rem     Interface IOutput #End Interface IOutput     Method PrintLn( text:String ) End   #Rem     Function HelloWorld          Parameter 1: Takes any object (instance of a class)                  that implements IOutput #End Function HelloWorld( obj:IOutput )     obj.PrintLn( "Hello World!" ) End   #Rem     class Console #End Class Console Implements IOutput     Method PrintLn( s:String )         Print s     End End   #Rem     class Printer #End Class Printer Implements IOutput     Method PrintLn( s:String )         Print "Printing to printer... ~n~t"+s     End End   #Rem     class Message #End Class Message Implements IOutput     Method PrintLn( s:String )         std.requesters.Notify( "Message", s )     End End

[NoOdle Participant]

An interface allows you to add specific functionality to any class, avoiding deep/complex class hierarchies. You can store instances of any class that implement the same interface in a collection and invoke any of the interface methods.

Sorry if that makes no sense, I’m not the best at explaining things. Here’s a quick untested example:

Monkey

#Import "<std>" Using std.collections Interface IDrawable Method Draw : Void() End Interface Class Dog Implements IDrawable Method Draw : Void() Print "Draw a dog" End Method End Class Class Button Implements IDrawable Method Draw : Void() Print "Draw a button" End Method End Class Function Main() Local dog : Dog = New Dog() Local btn : Button = New Button() Local drawList : List< IDrawable > = New List< IDrawable >() drawList.AddLast( dog ) drawList.AddLast( btn ) For Local this : IDrawable = Eachin drawList this.Draw() Next End Function

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#Import "<std>"   Using std.collections   Interface IDrawable Method Draw : Void()   End Interface   Class Dog Implements IDrawable Method Draw : Void() Print "Draw a dog" End Method End Class   Class Button Implements IDrawable Method Draw : Void() Print "Draw a button" End Method End Class   Function Main() Local dog : Dog = New Dog() Local btn : Button = New Button() Local drawList : List< IDrawable > = New List< IDrawable >() drawList.AddLast( dog ) drawList.AddLast( btn ) For Local this : IDrawable = Eachin drawList this.Draw() Next End Function

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