我怎样才能引用当前类的类型？

Question

这有点难以解释,但我到处都看,我找不到任何好的答案.

我也看过Stack Overflow问题如何引用接口在Java中实现的类类型？和如何返回相同类型的对象的实例作为类中使用Java 6过去了？,但他们无法回答我的问题.应用继承时有一个例外.

有一个例子,让它更容易理解:

假设我有一些名为SelfMaker的界面:

public interface SelfMaker <SELF>{
    public SELF getSelf();
}

A有一只狗,可以与另一只狗生育.所以狗是一个"SelfMaker",像这样:

public class Dog implements SelfMaker<Dog> {
    String color;

    public String toString() {
        return "some " + color + " dog";
    }

    public Dog procreate(Dog anotherDog) {
        Dog son = getSelf();
        son.color = color;
        return son;
    }

    @Override
    public Dog getSelf() {
        return new Dog();
    }
}

但是,我有一只国内狗,他是一只狗,但它有一个可爱的家庭,他给他起了个名字.像这样:

public class DomesticDog extends Dog {
    private String name;

    public String toString() {
        return super.toString() + " named " + name;
    }
}

现在,我有一些课程处理"SelfMaker"的事情,让我们把这个课称为"情侣".像这样:

public class Couple<T extends SelfMaker<T>> {
    private T first;
    private T second;

    public String toString() {
        return first.toString() + " and " + second.toString();
    }
}

除外:

当我想创建几个DomesticDogs 时会出现异常.像这样:

public class CoupleOfDomesticDogs extends Couple<DomesticDog>{
    public DomesticDog procreate(){
        DomesticDog son = first.procreate(second);
        return son;
    }
}

这将引发<DomesticDog>抱怨的例外:Bound mismatch: The type DomesticDog is not a valid substitute for the bounded parameter <T extends SelfMaker<T>> of the type Couple<T>

我已经尝试将类中的通用变量更改为:Couple<T extends SelfMaker<?>>但是"son"不会是DomesticDog(我希望"son"成为DomesticDog).如果我添加一些演员,那么它将编译,但它将不太清晰.

所以...这里有一个问题:有没有办法在没有铸造和概括的情况下实现这一目标？

Answer 1

没有铸造,我无法想到这样做.如果覆盖DomesticDog的procreate和getSelf方法并更改类Couple的声明,则问题将解决:

public class DomesticDog extends Dog {
    private String name;

    public DomesticDog procreate(Dog anotherDog) {
        return (DomesticDog)super.procreate(anotherDog);
    }

    public Dog getSelf() {
        return new DomesticDog();
    }

    public String toString() {
        return super.toString() + " named " + name;
    }
}

public class Couple<T extends SelfMaker<? super T>> {
    protected T first;
    protected T second;

    public String toString() {
        return first.toString() + " and " + second.toString();
    }
}

如果你不想在Dog的每个子类中覆盖getSelf(),你可以在类Dog中进行以下更改:

public Dog getSelf() {
    Class<? extends Dog> thisClass = this.getClass();
    try {
        return thisClass.newInstance();
    } catch (InstantiationException e) {
    } catch (IllegalAccessException e) {
    }
    throw new UnsupportedOperationException(thisClass 
                         + " does not supply a public no-arg constructor");
}

这保证了getSelf()返回的每个值都是this.getClass().但是你仍然需要为子类强制转换procreate()的返回值.无法将返回类型明确指定为this.getClass().