java - 有什么方法可以在 Java 中返回多态 this 吗？

Question

在 Typescript 中，有一个多态返回类型 this 的概念。 https://www.typescriptlang.org/docs/handbook/advanced-types.html#polymorphic-this-types示例:

export abstract class Animal {
    private name: string;

    public setName(name: string): this {
        this.name = name;
        return this;
    }
}

export class Dog extends Animal {
    private breed: string;

    public setBreed(breed: string): this {
        this.breed = breed;
        return this;
    }
}

export class FluffyDog extends Dog {
    private fluffiness: number;
    public setFluffiness(fluffiness: number): this {
        this.fluffiness = fluffiness;
        return this;
    }
}

export class Main {
    constructor() {
        const dog: FluffyDog = new FluffyDog()
            .setName('Fluffy')
            .setFluffiness(10)
            .setBreed('Lab');
    }
}

Java 中有什么等价物吗？我想到的最好的是:

public abstract class Animal<T extends Animal<T>> {
    private String name;
    public T setName(String name) {
        this.name = name;
        return (T)this;
    }
}

class Dog extends Animal<Dog> {
    private String breed;
    public Dog setBreed(String breed) {
        this.breed = breed;
        return this;
    }
}


class Main {
    static {
        Dog dog =  new Dog()
                .setName("Fluffy")
                .setBreed("Lab");
    }
}

或者这个:

public abstract class Animal {
    private String name;
    public <T extends Animal> T setName(String name) {
        this.name = name;
        return (T)this;
    }
}

class Dog extends Animal {
    private String breed;
    public <T extends Dog> T setBreed(String breed) {
        this.breed = breed;
        return (T)this;
    }
}

class FluffyDog extends Dog {
    private Long fluffiness;
    public <T extends FluffyDog> T setFluffiness(Long fluffiness) {
        this.fluffiness = fluffiness;
        return (T)this;
    }
}


class Main {
    static {
        FluffyDog dog =  new FluffyDog()
                .<FluffyDog>setName("Fluffy")
                .setFluffiness(10L)
                .setBreed("Lab");
    }
}

第一个似乎只能被子类化一次。
第二个在某些情况下需要显式类型参数。
有没有办法在 Java 中返回多态的 this？

Answer 1

另外两个选项:

1. 由于 setter 方法的顺序并不重要，在功能上，请按自下而上的顺序调用它们。如果您先分配给变量，则无需转换。

   final FluffyDog dog = new FluffyDog();
   dog.setFluffiness(10)
      .setBreed("Lab")
      .setName("Fluffy");
   

2. 覆盖继承的 setter 方法以更改返回类型，使用 super 委托(delegate)给实际实现:

   class Animal {
       private String name;
       public Animal setName(String name) {
           this.name = name;
           return this;
       }
   }
   

   class Dog extends Animal {
       private String breed;
       @Override
       public Dog setName(String name) {
           super.setName(name);
           return this;
       }
       public Dog setBreed(String breed) {
           this.breed = breed;
           return this;
       }
   }
   

   class FluffyDog extends Dog {
       private long fluffiness;
       @Override
       public FluffyDog setName(String name) {
           super.setName(name);
           return this;
       }
       @Override
       public FluffyDog setBreed(String breed) {
           super.setBreed(breed);
           return this;
       }
       public FluffyDog setFluffiness(long fluffiness) {
           this.fluffiness = fluffiness;
           return this;
       }
   }
   

   class Main {
       public static void main(String[] args) throws Exception {
           final FluffyDog dog = new FluffyDog()
                   .setName("Fluffy")
                   .setBreed("Lab")
                   .setFluffiness(10);
       }
   }