我怎样才能引用当前类的类型?

时间:2013-08-03 21:40:36

标签: java oop generics

这有点难以解释,但我到处寻找,我找不到任何好的答案。

我也看到了Stack Overflow问题 How can I refer to the class type a interface is implementing in Java? How do I return an instance of an object of the same type as the class passed in using Java 6? ,但他们无法回答我的问题。我申请继承时有一个例外。

有一个例子,让它更容易理解:

我们说我有一些名为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。因此,狗是一个&#34; SelfMaker&#34;,就像这样:

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();
    }
}

然而,我有一只DomesticDog,他是一只狗,但它有一个可爱的家庭为他命名。像这样:

public class DomesticDog extends Dog {
    private String name;

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

现在,我有一些课程可以处理一些事情,这些事情是&#34; SelfMaker&#34; s,让我们称这个课程为#34;情侣&#34;。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();
    }
}

EXCEPTION:

当我想创建几个DomesticDog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;
    }
}

这会在<DomesticDog>抱怨Bound mismatch: The type DomesticDog is not a valid substitute for the bounded parameter <T extends SelfMaker<T>> of the type Couple<T>

上引发异常

我已经尝试将通用变量从类Couple更改为:Couple<T extends SelfMaker<?>>但是&#34; son&#34;不会成为DomesticDog(我希望&#34;儿子&#34;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:如果没有演员和概括,有没有办法实现这个目标?

2 个答案:

答案 0 :(得分:7)

如果没有施法,我无法想到这样做。如果您覆盖DomesticDog的procreate和getSelf方法并更改类Couple的声明,那么您的问题将得到解决:

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");
}

这可以保证getSelf()返回的每个值都是this.getClass()的实例。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()

答案 1 :(得分:0)

您必须使Dog通用和抽象,并使用指示getSelf()结果的类型参数。然后,每种类型的Dog都需要将它们作为参数实现:

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

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

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

}

public class DomesticDog extends Dog<DomesticDog> {
    private String name;

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

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