你如何创建多个级别的子类以便参数是子类？ [英] How do you create multiple levels of subclasses so that the parameters are of the subclasses?

问题描述

继此 StackOverFlow问题比方说，我想扩大到另一个级别的子类，说马拉德，我该怎么做？

  public abstract class Animal< A extends Animal< A>> 
 {
 public abstract copyAnimal（A animal）; 
} 
 
公共类Duck扩展了Animal< Duck> 
 {
 @Override 
 public Duck copyAnimal（Duck duck）
 {
 return copyOfDuck; 
} 
} 
 
类Mallard延伸Duck< Mallard> 
 {
 public Mallard copyAnimal（Mallard mallard）
 {
 return copyOfMallard; 
 
 
 $ / code $ / pre 
 
 
一切都很适合Duck子类，但是它失败了马拉德子亚类。我怎样才能用不确定数量的子类来设置它？解析方案

你只需要继续使用类型变量：

  class Duck< D extends Duck< D>>延伸动物< D> {} 
  

然后

  class Mallard延伸Duck< Mallard> {
  

是的，这很杂乱！

<我经常看到这种模式与一个扩展的类的构建器一起使用，以创建一个子类的构建器。它使得代码乱七八糟，在我看来最好避免。

---

一个如何使用它的例子：

 抽象类Animal< A extends Animal< A>> {
 abstract A copyAnimal（A animal）; 
} 
 
 class Duck< D延伸Duck< D>>延伸动物< D> {
 void quack（）{} 
 
 copyAnimal（D duck）{
 duck.quack（）; 
归还鸭子; 
} 
} 
 
类Mallard延伸Duck< Mallard> {
野鸭copyAnimal（野鸭）{
 duck.quack（）; 
归还鸭子; 
} 
} 
  

Ideone demo （不做任何事;仅仅演示汇编）

Following this StackOverFlow question let's say I want to expand to another level of subclasses, say Mallards, how do I do it?

public abstract class Animal <A extends Animal<A>>
{
    public abstract A copyAnimal(A animal);
}

public class Duck extends Animal<Duck>
{
    @Override
    public Duck copyAnimal(Duck duck)
    {
        return copyOfDuck;
    }
}

class Mallard extends Duck<Mallard>
{
    public Mallard copyAnimal(Mallard mallard)
    {
        return copyOfMallard;
    }   
}

Everything works great for the Duck subclass but it fails for the Mallard sub-subclass. How can I set this up with an undetermined number of subclasses?

解决方案

You just keep going with the type variables:

class Duck<D extends Duck<D>> extends Animal<D> {}

then

class Mallard extends Duck<Mallard> {}

Yep, it's pretty messy!

I have most often seen this pattern used with a builder of a classes that is extended to create a builders of a subclass. It makes a total mess of the code, and in my opinion is best avoided.

---

An example of how to use it:

abstract class Animal<A extends Animal<A>> {
  abstract A copyAnimal(A animal);
}

class Duck<D extends Duck<D>> extends Animal<D> {
  void quack() {}

  D copyAnimal(D duck) {
    duck.quack();
    return duck;
  }
}

class Mallard extends Duck<Mallard> {
  Mallard copyAnimal(Mallard duck) {
    duck.quack();
    return duck;
  }
}

Ideone demo (does nothing; merely demonstrates compilation)

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