definition

On the basis of not changing the original object, the function is attached to the object. Provides a more flexible alternative than inheritance (extend the original object function)

Types of

Structure type

Applicable scene

1. Extend the functionality of a class or add additional responsibilities to a class
2. Dynamically add functions to an object, or dynamically cancel functions.

advantage

• A powerful supplement to inheritance, which is more flexible than inheritance, and extends functions to an object without changing the original object. (Inheritance in the extension function is static and must be determined at compile time, and the decorator can be determined at runtime, and the decorator is also based on inheritance)
• Different effects can be achieved by using different decoration types and the permutation and combination of these types.
• Comply with the opening and closing principle

Disadvantage

• There will be more code, more classes, increasing the complexity of the program.
• In dynamic decoration, multi-layer decoration will be more complicated. (Using inheritance to expand functions will increase the number of classes. Using the decorator pattern will not increase the number of classes as much as inheritance, but it will increase the number of objects. When the number of objects increases to a certain level, it will undoubtedly greatly increase us. Difficulty of code debugging).

Instance

Cafe order items:

1) Coffee types: Espresso, ShortBlack (concentrated), LongBlack, Decaf (sugar-free).

2) Seasoning: Milk, Soy, Chocolate.

Code

Note: Some coffees and spices have not been realized, and they are the same as realized coffee and spices.
Key point: Calling Beverage beverage

Abstract class

public abstract class Beverage {

    public String description="Unknown Beverage";

    public String getDescription() {
        return description;
    }

    public abstract double cost();
}

Coffee

public class Espresso extends Beverage {

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

    @Override
    public double cost() {
        return 1.99;
    }
}

public class HouseBlend extends Beverage {

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

    @Override
    public double cost() {
        return 1.89;
    }
}

Abstract class-decorator (seasoning)

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

Specific decorator-seasoning

public class Mocha extends CondimentDecorator {

    public Beverage beverage;

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

    @Override
    public String getDescription() {
        return beverage.getDescription()+", Mocha";
    }

    @Override
    public double cost() {
        return 0.20+beverage.cost();
    }
}

Coffee order (main)

public class StarbuzzCoffee {
    public static void main(String args[]){
        Beverage beverage=new Espresso();
        //Single origin coffee
        System.out.println("Single Origin Coffee");
        System.out.println(beverage.getDescription()+"$"+beverage.cost());
        beverage=new Mocha(beverage);
        //Join Mocha's coffee
        System.out.println("Join Mocha's Coffee");
        System.out.println(beverage.getDescription()+"$"+beverage.cost());
    }
}

operation result