如何在Java中實(shí)現(xiàn)裝飾器設(shè)計(jì)模式？可通過(guò)定義組件接口、創(chuàng)建具體組件、構(gòu)建基礎(chǔ)裝飾器類(lèi)、添加具體裝飾器、使用裝飾器組合擴(kuò)展對(duì)像功能。 1. 定義組件接口Coffee，包含cost()和description()方法；2. 創(chuàng)建具體組件SimpleCoffee實(shí)現(xiàn)基本功能；3. 構(gòu)建抽象裝飾器類(lèi)CoffeeDecorator實(shí)現(xiàn)相同接口並持有組件引用；4. 添加MilkDecorator和SugarDecorator等具體裝飾器，在原有行為基礎(chǔ)上增加新功能；5. 在運(yùn)行時(shí)通過(guò)嵌套調(diào)用組合裝飾器實(shí)現(xiàn)動(dòng)態(tài)功能擴(kuò)展，例如new SugarDecorator(new MilkDecorator(new SimpleCoffee()))。

Sure, here's how to implement the Decorator design pattern in Java in a practical and clear way.

What is the Decorator Pattern?

The Decorator pattern lets you add behavior or responsibilities to objects dynamically, without affecting the behavior of other objects of the same class. It's a flexible alternative to subclassing for extending functionality.

In Java, this pattern is commonly used in I/O streams like InputStream , OutputStream , Reader , and Writer .

When Should You Use It?

You should reach for the Decorator pattern when:

• You want to add features to an object dynamically.
• Subclassing would lead to too many combinations or become unmanageable.
• You need to follow the Open/Closed Principle : open for extension, closed for modification.

This often comes up when dealing with things like formatting, logging, encryption, compression, etc.

How to Implement the Decorator Pattern in Java

Let's walk through a simple example using coffee and toppings (like milk, sugar) to demonstrate how decorators work.

1. Define the Component Interface

Start by defining the base interface that both the concrete component and decorators will implement.

 public interface Coffee {
    double cost();
    String description();
}

2. Create the Concrete Component

This is the basic object you'll be decorating.

 public class SimpleCoffee implements Coffee {
    public double cost() {
        return 2.0;
    }

    public String description() {
        return "Simple Coffee";
    }
}

3. Create the Base Decorator Class

This abstract class holds a reference to the component and implements the same interface.

 public abstract class CoffeeDecorator implements Coffee {
    protected Coffee decoratedCoffee;

    public CoffeeDecorator(Coffee coffee) {
        this.decoratedCoffee = coffee;
    }

    public double cost() {
        return decoratedCoffee.cost();
    }

    public String description() {
        return decoratedCoffee.description();
    }
}

4. Add Concrete Decorators

Each decorator adds its own behavior on top of the existing one.

For example, a milk decorator:

 public class MilkDecorator extends CoffeeDecorator {
    public MilkDecorator(Coffee coffee) {
        super(coffee);
    }

    public double cost() {
        return super.cost() 0.5;
    }

    public String description() {
        return super.description() ", Milk";
    }
}

And a sugar decorator:

 public class SugarDecorator extends CoffeeDecorator {
    public SugarDecorator(Coffee coffee) {
        super(coffee);
    }

    public double cost() {
        return super.cost() 0.2;
    }

    public String description() {
        return super.description() ", Sugar";
    }
}

5. Use the Decorators

Now you can combine decorators as needed at runtime:

 public class Main {
    public static void main(String[] args) {
        Coffee coffee = new SimpleCoffee();
        System.out.println(coffee.cost()); // Output: 2.0
        System.out.println(coffee.description()); // Output: Simple Coffee

        Coffee milkCoffee = new MilkDecorator(new SimpleCoffee());
        System.out.println(milkCoffee.cost()); // Output: 2.5
        System.out.println(milkCoffee.description()); // Output: Simple Coffee, Milk

        Coffee milkAndSugarCoffee = new SugarDecorator(new MilkDecorator(new SimpleCoffee()));
        System.out.println(milkAndSugarCoffee.cost()); // Output: 2.7
        System.out.println(milkAndSugarCoffee.description()); // Output: Simple Coffee, Milk, Sugar
    }
}

Key Points to Remember

• Each decorator wraps another instance of the same interface.
• The client code doesn't need to know whether it's dealing with a base component or a decorated one.
• Keep each decorator focused on a single responsibility.
• Avoid deep nesting of decorators — it can get hard to read and debug.

If you're working with streams in Java, think about how BufferedInputStream wraps any InputStream to add buffering. That's the exact same idea!

Wrap-Up

Implementing the Decorator pattern in Java isn't complicated once you understand the structure. You define a common interface, create a base implementation, then build reusable wrappers that enhance behavior without modifying existing classes. It's clean, scalable, and fits well into systems where flexibility matters.

That's basically it.

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