Decorator#

The intent of the decorator pattern is to give objects new responsibilities without overusing inheritance and therefore creating a bunch of classes. Instead Components get decorated to further enhance objects.

Structure#

classDiagram Component <|-- Decorator Component <|-- ConcreteComponent Decorator <|-- ConcreteDecoratorA Decorator <|-- ConcreteDecoratorB Component <-- Decorator class Component{ +operationA() +operationB() } class Decorator{ Component wrappedObj +operationA() +operationB() } class ConcreteComponent{ +operationA() +operationB() } class ConcreteDecoratorA{ Object newState // can extend state +operationA() +operationB() } class ConcreteDecoratorB{ +operationA() +operationB() +newBehavior() // can add new }

With this structure you can do things in the decorator before after calling wrappedObj.operationA().

Example#

public abstract class Beverage {
    String description = "Unknown Beverage";

    public String getDescription() {
        return description;
    }

    public abstract double cost();
}
public abstract class CondimentDecorator extends Beverage {
    public abstract String getDescription();
}
public class Espresso extends Beverage {

    public Espresso() {
        description = "Espresso";
    }

    public double cost() {
        return 1.99;
    }
}
public class Milk extends CondimentDecorator {
    Beverage beverage;

    public Milk(Beverage beverage) {
        this.beverage = beverage;
    }

    public String getDescription() {
        return beverage.getDescription() + ", Milk";
    }

    public double cost() {
        return .10 + beverage.cost();
    }
}