java - 方法链接 : How to use getThis() trick in case of multi level inheritance

Question

我的问题是关于 Method chaining + inheritance don’t play well together? 的.但不幸的是，方法链接的所有示例/答案都使用单级继承。我的用例涉及多级继承，例如

abstract class PetBuilder{...}
class  DogBuilder extends PetBuilder{..}
class DogType1Builder extends DogBuilder {...}

要构建 Dog 对象，我将使用 DogBuilder 或 DogType1Builder

如何在上述用例中使用 getThis 技巧？

我想使用构建器模式来构造一个复杂的 Dog 对象(Dog 对象模型)"。DogType1 将添加一些属性。

所以使用getThis Trick 上面类的声明会变成这样

abstract class PetBuilder<T extends PetBuilder<T>>
class DogBuilder<T extends DogBuilder<T>> extends PetBuilder<DogBuilder<T>>
class DogType1Builder extends DogBuilder<DogType1Builder>

现在这会产生两个问题

1.'DogBuilder' 中的 builder 方法看起来像

public T someMethodInDog(String dogName) {
..
return (T)this; ///i dont want type casting and i cant use getThis Trick Here (compiler reports error for conversion from DogBuilder to T)
}

2.由于 DogBuilder 已参数化，因此要创建“DogBuilder”实例，我将不得不使用

DogBuilder<DogBuilder> builder=new DogBuilder(); //passing <DogBuilder> type ...real pain

有没有更好的方法？

Answer 1

你的问题的根源是类设计问题:你试图从一个具体的类继承，这几乎总是一个错误，并且(你的例子)势必会导致无数问题。为了坚持引用线程中给出的示例，您不应该实例化 Dog，因为在这样的宇宙中一般不存在 Dog，Pet - 仅 Poodle、NewFoundland、Spaniel 等。因此，getThis 不应在中级(抽象)类中实现，而应仅在(具体)叶类中实现。并且在所有的中级抽象类中，你应该只引用泛型类型参数T，而不是实际的类名。

这是 the answer to the referred thread 中的示例按照以上规则改写:

public class TestClass {

  static abstract class Pet <T extends Pet<T>> {
    private String name;

    protected abstract T getThis();

    public T setName(String name) {
      this.name = name;
      return getThis(); }
  }

  static class Cat extends Pet<Cat> {
    @Override protected Cat getThis() { return this; }

    public Cat catchMice() {
      System.out.println("I caught a mouse!");
      return getThis();
    }
  }

  // Dog is abstract - only concrete dog breeds can be instantiated
  static abstract class Dog<T extends Dog<T>> extends Pet<T> {
    // getThis is not implemented here - only in concrete subclasses

    // Return the concrete dog breed, not Dog in general
    public T catchFrisbee() {
      System.out.println("I caught a frisbee!");
      return getThis();
    }
  }

  static class Poodle extends Dog<Poodle> {
    @Override protected Poodle getThis() { return this; }

    public Poodle sleep() {
      System.out.println("I am sleeping!");
      return getThis();
    }
  }

  static class NewFoundland extends Dog<NewFoundland> {
    @Override protected NewFoundland getThis() { return this; }

    public NewFoundland swim() {
      System.out.println("I am swimming!");
      return getThis();
    }
  }

  public static void main(String[] args) {
    Cat c = new Cat();
    c.setName("Morris").catchMice();
    Poodle d = new Poodle();
    d.setName("Snoopy").catchFrisbee().sleep();
    NewFoundland f = new NewFoundland();
    f.setName("Snoopy").swim().catchFrisbee();
  }
}