How to have single overall properties for an object created with the decorator pattern?

[Scott Shipp]

Hi everyone,

Have ran into a case similar to the following. Let's use a simple example for discussion purposes. Say you want to make a "Pepperoni Pizza" and you use the decorator pattern. So you end up with some statement like:

Pizza pepperoniPizza = new Pepperoni(new Onions(new Mozzarella(new TomatoSauce(new PlainPizza()))));

Pepperoni, Onions, Mozzarella, TomatoSauce, PlainPizza all extend a "PizzaDecorator" abstract class so this works to combine their shared properties like cost, toppings into one object.

But the issue I am having is when I want a single property or properties which applies only to this specific combination, like a name property (Name="Pepperoni Pizza.")

What is the recommended way to do this?

[Ryan McGuire]

I don't know that there is one single recommended way to do this, but I can think of a couple possibilities that depend on the details of what you're after.

1. If the outermost PizzaDecorator is what determines the name, then just have each PizzaDecorator return a hardcoded name. The Pepperoni decorator would return "Pepperoni Pizza".

Pizza pizza1 = new Pepperoni(new Onions(new Mozzarella(new TomatoSauce(new PlainPizza()))));
Pizza pizza2 = new Onions(new Pepperoni(new Mozzarella(new TomatoSauce(new PlainPizza()))));
System.out.println("name1 = " + pizza1.Name() + "; name2 = " + pizza2.Name());

That would print "name1 = Pepperoni Pizza; name2 = Onion Pizza" even though the two pizzas would have the same list of ingredients.

2. Work out a name precedence scheme so that "high-priority" ingredient names like "Pepperoni" override lower ones like "Cheese", but not vice-versa.

PlainPizza would return "Plain Pizza" with a precedence of 0.

TomatoSauce would return "Sauce Pizza" with a precedence of 1.

Mozzarella would return "Cheese Pizza" with a precedence of 2.

ColbyJack would return "CJCheese Pizza" also with a precedence of 2.

Onions would return "Onion Pizza" with a precedence of 3.

Pepperoni would return "Pepperoni Pizza" with a precedence of 4.

Sausage would return "Sausage Pizza" with a precedence of 4.

Ham would return "Ham Pizza" with a precedence of 4.

class PlainPizza {
    String Name() { return "Plain Pizza"; }
    int NamePrecedence() { return 0; }
}

abstract class PizzaDecorator extends PlainPizza {
    int myNamePrecedence = 0;  // Default value
    String myName = "Decorated Pizza";  // Default value
    Pizza client = null;

    PizzaDecorator(Pizza p) { client = p; }

    String Name() {
        if (client.NamePrecedence() > myNamePrecedence)
            return client.Name();
        return nyName;   // Note: myName wins when there is a name precedence tie.
     }

    int NamePrecedence() { 
        return (Math.MAX(client.NamePrecedence(), myNamePrecedence); 
    }
}

So...

Pizza pizza1 = new Pepperoni(new Onions(new Mozzarella(new TomatoSauce(new PlainPizza()))));
Pizza pizza2 = new Onions(new Pepperoni(new Mozzarella(new TomatoSauce(new PlainPizza()))));
Pizza pizza3 = new Ham(new Pepperoni(new Mozzarella(new TomatoSauce(new PlainPizza()))));
Pizza pizza4 = new Pepperoni(new Ham(new Mozzarella(new TomatoSauce(new PlainPizza()))));
System.out.println("name1 = " + pizza1.Name() + "; name2 = " + pizza2.Name() + "; name3 = " + pizza3.Name() + "; name4 = " + pizza4.Name());

...would print...

name1 = Pepperoni Pizza; name2 = Pepperoni Pizza; name3 = Ham Pizza; name1 = Pepperoni Pizza

It would be possible to develop more complicated schemes for combining each decorator's name with those of the client. e.g. Maybe if my name precedence is equal to my client's, then I'll prepend my ingredient with an "and":
new Sausage(new Pepperoni(new Onion(new Ham(new Mozzarella(new TomatoSauce(new PlainPizza())))))).Name(); could return "Sausage and Pepperoni and Ham Pizza"

You could either work this into the PizzaDecorator class or create a separate Strategy object that looks at the whole hierarchy of PizzaDecorators and determines the correct name. e.g. If (the list of decorators contains at least one Ham and at least one Pineapple and no other meats) then name = "Hawaiian Pizza".

[Jayesh A Lalwani]

If the name is a property of the Combination, not the ingredients that decorate the pizza, then I wouldn't make name as a property of the pizza. Decorator pattern applies to behavior that can be decorated (for example cost). If a behavior cannot be decorated (for example name), you shouldn't be using a decorator pattern. Trying to put non-decorator like behavior on a decorator pattern is like putting a square peg in a round hole.

Actually, the pizza example is used to illustrate decorator pattern, but in real life, I wouldn't use a decorator pattern for pizzas, because real life pizza behavior doesn't follow decorator pattern. What if you want to model pizzas that have half the ingredients on one side, and other half on another side? What if you get a discount for using certain number of ingredients? No sirree bob.. I stick with composite pattern thankyouveddymuch

IMO, the perfect example of decorator pattern is InputStream interface. Beautifully thought out and implemented.

[Scott Shipp]

Appreciate the replies, both of you! Lots to think about. Thanks!