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1. 装饰者模式Overview

• Attach additional responsibilities to an object dynamically. Decorator provides a flexible alternative to subclassing for extending functionality.
  • inheriting behavior at runtime through composition and delegation
  • open for extension but close for modification
  • decorator has the same super type as the objects they decorate
  • we could use one or more decorators to wrap an object
  • the decorator adds its own behavior before and/or after delegating to the object it decorates to do the rest of the job

2. 使用场景

• when we want to give some other responsibility at run time
• kind of like continue to wrap the intrinsic object with some new responsibility, still an is-a relationship

3. 使用目的

• Extends the class at run time instead of compile time
• To give objects some new responsibilities without making any code changes to the underlying classes
• pitfall
  • sometimes it could lead to a lot of new classes

4. 具体实现

• One highlight

  • the condimentDecorator is a beverage, and also need to has a beverage

  • we are using inheritance to achieve the type matching, but we aren’t using inheritance to get behavior

    

4.1 接口和数据对象定义

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

    public abstract double cost();

    public String getDescription() {
        return description;
    }
}

public abstract class CondimentDecorator extends Beverage{

    Beverage beverage;

    public abstract String getDescription();
}

4.2 具体实现类的定义

public class DecafCoffee extends Beverage {
    @Override
    public double cost() {
        return 2.5f;
    }

    String description() {
        return "Decat coffee";
    }
}

public class Espresso extends Beverage{

    public Espresso() {
        description = "Espresso";
    }
    @Override
    public double cost() {
        return 4;
    }
}

public class HouseBlend extends Beverage{

    public HouseBlend() {
        description = "House Blend Coffee";
    }

    @Override
    public double cost() {
        return 0.99;
    }
}

public class MilkDecorator extends CondimentDecorator{

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

    @Override
    public double cost() {
        return beverage.cost() + 0.8;
    }

    @Override
    public String getDescription() {
        return beverage.getDescription() + "with milk";
    }
}

public class MochaDecorator extends CondimentDecorator{

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

    @Override
    public double cost() {
        return beverage.cost() + 0.5;
    }

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

4.3 测试

@GetMapping("/decorator")
    public String decorator() {
        Beverage decafCoffee = new DecafCoffee();
        log.info("before decoration" + decafCoffee.cost());

        Beverage decafWithMocha = new MochaDecorator(decafCoffee);
        log.info("after decoration" + decafWithMocha.cost());
        log.info("after decoration 2" + new MochaDecorator(new MochaDecorator(decafCoffee)).cost());

        return "check the log";
    }

Reference

1. https://learning.oreilly.com/library/view/head-first-design/9781492077992/ch02.html

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