我正在研究《Head First Design Patterns》一书中的装饰器模式示例。我以为beverage3
字段描述的值必须是“House Blend Coffee”,但实际上是“Unknown Beverage”。
有人能解释一下吗?我们正在针对接口(interface)进行编码(嗯,在本例中它是一个抽象类),但我仍在实例化 HouseBlend 类,因此该值应该是“House Blend Coffee”,但它不是...
下面的代码不包含示例的所有类。
public class StarbuzzCoffee {
public static void main(String[] args) {
Beverage beverage = new Espresso();
System.out.println(beverage.getDescription()
+ " $" + beverage.cost());
Beverage beverage2 = new DarkRoast();
beverage2 = new Mocha(beverage2);
beverage2 = new Mocha(beverage2);
beverage2 = new Whip(beverage2);
System.out.println(beverage2.getDescription()
+ " $" + beverage2.cost());
Beverage beverage3 = new HouseBlend();
System.out.println("(added by me)return value of getDescription() without condiment: " + beverage3.getDescription());
beverage3 = new Soy(beverage3);
beverage3 = new Mocha(beverage3);
beverage3 = new Whip(beverage3);
System.out.println(beverage3.getDescription()
+ " $" + beverage3.cost());
System.out.println("(added by me)value of field desciption: " + beverage3.description);
}
}
饮料.java
public abstract class Beverage {
String description = "Unknown Beverage";
public String getDescription() {
return description;
}
public abstract double cost();
}
CondimentDecorator.java
public abstract class CondimentDecorator extends Beverage {
public abstract String getDescription();
}
HouseBlend.java
public class HouseBlend extends Beverage{
public HouseBlend() {
description = "House Blend Coffee";
}
@Override
public double cost() {
return .89;
}
}
CondimentDecorator.java
public class Mocha extends CondimentDecorator{
Beverage beverage;
public Mocha(Beverage beverage) {
this.beverage = beverage;
}
@Override
public String getDescription() {
return beverage.getDescription() + ", Mocha";
}
@Override
public double cost() {
return beverage.cost() + .20;
}
}
最佳答案
问题是您正在创建的 Mocha 对象本身就是一个具有描述的饮料。您没有更改其描述。您正在传递另一种饮料并增强对装饰对象的描述。但是,您仍然保留了以前对 Mocha 对象的描述。当您直接访问其描述时(这就是您不使用 getDescription 方法所做的事情),您将访问未更改的字段,因此您将打印“未知饮料”。
您创建的每种饮料都会有其字段描述。如果你不改变它而使用它,你会得到“未知饮料”。
保护类 Beverage 的字符串描述,并且只能通过 Beverage 实现的重写 getDescription 方法来访问它。检查一下:
public class StarbuzzCoffee {
public static void main(String[] args) {
Beverage beverage3 = new HouseBlend();
System.out
.println("(added by me)return value of getDescription() without condiment: " + beverage3.getDescription());
beverage3 = new Mocha(beverage3);
System.out.println(beverage3.getDescription() + " $" + beverage3.cost());
System.out.println("(added by me)value of field desciption: " + beverage3.getDescription());
}
}
abstract class Beverage {
protected String description = "Unknown Beverage";
public String getDescription() {
return description;
}
public abstract double cost();
}
abstract class CondimentDecorator extends Beverage {
public abstract String getDescription();
}
class HouseBlend extends Beverage{
public HouseBlend() {
description = "House Blend Coffee";
}
@Override
public double cost() {
return .89;
}
}
class Mocha extends CondimentDecorator{
Beverage beverage;
public Mocha(Beverage beverage) {
this.beverage = beverage;
}
@Override
public String getDescription() {
return beverage.getDescription() + ", Mocha";
}
@Override
public double cost() {
return beverage.cost() + .20;
}
}
关于设计,将 Beverage 的 getDescription 抽象化并强制每个实现都覆盖它会更安全。这样您将来就可以避免此类问题。检查一下:
public class StarbuzzCoffee {
public static void main(String[] args) {
Beverage beverage3 = new HouseBlend();
System.out
.println("(added by me)return value of getDescription() without condiment: " + beverage3.getDescription());
beverage3 = new Mocha(beverage3);
System.out.println(beverage3.getDescription() + " $" + beverage3.cost());
System.out.println("(added by me)value of field desciption: " + beverage3.description);
}
}
abstract class Beverage {
protected String description = "Unknown Beverage";
public abstract String getDescription();
public abstract double cost();
}
abstract class CondimentDecorator extends Beverage {
// You don't need this code here. Because Beverage already
// provides a getDescription
// public abstract String getDescription();
}
class HouseBlend extends Beverage{
public HouseBlend() {
description = "House Blend Coffee";
}
@Override
public String getDescription() {
return description;
}
@Override
public double cost() {
return .89;
}
}
class Mocha extends CondimentDecorator{
// Also make sure that this guy here is private
// so other objects can't change or access
// its state without going through Mocha
private Beverage beverage;
public Mocha(Beverage beverage) {
this.beverage = beverage;
}
@Override
public String getDescription() {
return beverage.getDescription() + ", Mocha";
}
@Override
public double cost() {
return beverage.cost() + .20;
}
}
关于java - 装饰器模式 : value of field of concrete component is the value of field from component, 但为什么呢?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/47607545/