什么是装饰器模式？

• 装饰器类和原始类继承同样的父类，这样我们可以对原始类“嵌套”多个装饰器类。
• 装饰器类是对功能的增强，这也是装饰器模式应用场景的一个重要特点。

为什么用装饰器模式？

• 装饰器模式主要解决继承关系过于复杂的问题，通过组合来替代继承。
• 它主要的作用是给原始类添加增强功能。这也是判断是否该用装饰器模式的一个重要的依据。
• 可以对原始类嵌套使用多个装饰器。为了满足这个应用场景，在设计的时候，装饰器类需要跟原始类继承相同的抽象类或者接口。

如何使用装饰器模式？

public interface BakeryComponent {
  public String getName();

  public double getPrice();
}

@Data
public class CakeBase implements BakeryComponent {

  private String name = "Cake Base";

  private double price = 200.0;

  @Override
  public String getName() {
    return this.name;
  }

  @Override
  public double getPrice() {
    return this.price;
  }
}

@Data
public class PastryBase implements BakeryComponent {

  private String name = "Pastry Base";

  private double price = 200.0;


  @Override
  public String getName() {
    return this.name;
  }

  @Override
  public double getPrice() {
    return this.price;
  }
}

public abstract class Decorator implements BakeryComponent {

  private BakeryComponent bakeryComponent = null;

  protected String name = "Undefined Decorator";
  protected double price = 0.0;

  public Decorator(BakeryComponent bakeryComponent) {
    this.bakeryComponent = bakeryComponent;
  }

  @Override
  public String getName() {
    return this.bakeryComponent.getName() + ", " + this.name;
  }

  @Override
  public double getPrice() {
    return this.price + this.bakeryComponent.getPrice();
  }
}

public class ArtificialScentDecorator extends Decorator {
  public ArtificialScentDecorator(BakeryComponent bakeryComponent) {
    super(bakeryComponent);
    this.name = "ArtificialScent";
    this.price = 3.0;
  }
}

public class CherryDecorator extends Decorator {

  public CherryDecorator(BakeryComponent bakeryComponent) {
    super(bakeryComponent);
    this.name = "Cherry";
    this.price = 2.0;
  }

}

public class CreamDecorator extends Decorator {
  public CreamDecorator(BakeryComponent bakeryComponent) {
    super(bakeryComponent);
    this.name = "Cream";
    this.price = 1.0;
  }
}


public class NameCardDecorator extends Decorator {
  public NameCardDecorator(BakeryComponent bakeryComponent) {
    super(bakeryComponent);
    this.name = "NameCard";
    this.price = 4.0;
  }
}

public class App {

  private static void printProductDetails(BakeryComponent bakeryComponent) {
    String out = "Item: " + bakeryComponent.getName() + ", Price: " + bakeryComponent.getPrice();
    System.out.println(out);
  }

  public static void main(String[] args) {
    System.out.println("Hello World!");

    CakeBase cakeBase = new CakeBase();
    printProductDetails(cakeBase);

    CreamDecorator creamDecorator = new CreamDecorator(cakeBase);
    printProductDetails(creamDecorator);

    CherryDecorator cherryDecorator = new CherryDecorator(creamDecorator);
    printProductDetails(cherryDecorator);

    ArtificialScentDecorator artificialScentDecorator = new ArtificialScentDecorator(cherryDecorator);
    printProductDetails(artificialScentDecorator);

    NameCardDecorator nameCardDecorator = new NameCardDecorator(artificialScentDecorator);
    printProductDetails(nameCardDecorator);

    PastryBase pastry = new PastryBase();
    printProductDetails(pastry);

    CreamDecorator creamDecorator1 = new CreamDecorator(pastry);
    printProductDetails(creamDecorator1);

  }
}

装饰器模式使用场景

• Java 中的 I/O 类库符合这一特征