Delegation Through Association

Delegation is an Object Oriented Programming concept that involves using another, more specialised Class to do a big job. In other words, we can delegate a big task to various other classes to avoid large class definitions, and extend functionality. The two classes (and their subsequent Objects) are now associated, as we have defined a relationship between them that allows one to use the other in some way.

The association can be defined through Composition or Inheritance

Example §

Imagine we have a Square class, defined with a side length. Imagine we also have a Rect (Rectangle) class, defined with width and height. We know that both rectangles and squares have the same area formula: $A=h\times w$. The area of a square is just the area of a rectangle where width = height. As such, we can delegate the area formula to the rectangle

OLD:

public class Square {
	private double sideLength;
	//Other code (like constructors) goes here.
	public double getArea() 
	{
		return sideLength * sideLength;
	}
}
---
public class Rect {
	private double height, width;
 
	public Rect(double height, double width)
	{
		this.height = height;
		this.width = width;
	}
 
	public double getArea()
	{
		return height * width;
	}
}

NEW:

public class Square {
	private double sideLength;
	//Other code (like constructors) goes here.
	//Delegation via composition
	private Rect rect = new Rect(sideLength, sideLength);
	public double getArea() 
	{
		//Delegated area function to Rec
		return rect.getArea();
	}
}