java - 有没有办法在 Java 中返回多态 this ？

Question

在 Typescript 中，有一个多态返回类型的概念this。 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中有什么等价的吗？我想出的最好的是：

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 this ？

score 3 · Accepted Answer

另外两个选项：

由于 setter 方法的顺序无关紧要，因此在功能上以自下而上的顺序调用它们。如果您首先分配给变量，则不需要强制转换。
```
final FluffyDog dog = new FluffyDog();
dog.setFluffiness(10)
   .setBreed("Lab")
   .setName("Fluffy");
```

覆盖继承的 setter 方法以更改返回类型，super用于委托给实际实现：

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);
    }
}

score 2 · Accepted Answer

使用您的第一个替代方案，您仍然可以进行两个级别的继承。诚然，它的可读性和可理解性不是很高。不幸的是，泛型是 1.5 中添加的 Java 的补充，许多 Java Bean 模式并不适合这些类型的链式方法。（您会注意到大多数 setter 方法void在传统的 Java 类中返回）。

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

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

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

如果您想允许FluffyDog被覆盖并提供相同的模式，那么您将不得不再次使用类中的泛型类型做同样的事情Dog。

java - 有没有办法在 Java 中返回多态 this ？

2 回答 2

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