Class delegation

You might have already heard about the delegation pattern or at least used it without even knowing it had a name. It allows a type to forward one or more of its methods call to a different type. Therefore, you need two types to achieve this: the delegate and the delegator.

This could easily sound like a proxy pattern, but it isn't. A proxy pattern is meant to provide a placeholder for an instance to get full control while accessing it. Let's say you are writing a UI framework and you start where your abstraction is UIElement. Each of the components define a getHeight and getWidth.

Class delegation via association

Below you see the UML translated into Kotlin. We defined the UIElement interface with both Panel and Rectangle classes inheriting from:

    interface UIElement { 
      fun getHeight(): Int 
      fun getWidth(): Int 
    } 
    class Rectangle(val x1: Int, val x2: Int, val y1: Int, val y2: Int) : UIElement { 
      override fun getHeight...