问题

I am learning Java variance.
我正在学习Java的变异性。

I am getting the Covariant to work but not Contravariance where I can accept a super type.
协变性可以工作，但逆变性无法工作，无法接受超类型。

I have a simple class hierarchy for this:
我有一个简单的类层次结构：

Animal,
Cat,
Lion
动物，猫，狮子

Then I have two classes as Generic holders:
然后我有两个作为泛型持有者的类：

CovariantAnimalHolder
ContraVariantHolder
协变动物持有者，逆变持有者

The Covariant works fine but Contravariance isnt working.
协变性可以正常工作，但逆变性不起作用。

Here is the code:
以下是代码：

public class Animal {
    private String name;

    public Animal(String name) {
        this.name = name;
    }

    public Animal() {

    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

public class Cat extends Animal {
    private boolean isWild;

    public Cat(boolean isWild) {
        this.isWild = isWild;
    }

    public Cat(){

    }

    public boolean isWild() {
        return isWild;
    }

    public void setWild(boolean wild) {
        isWild = wild;
    }
}

public class Lion extends Cat{

    private boolean isExtinct;

    public Lion (boolean isExtinct) {
        this.isExtinct = isExtinct;
    }

    public Lion () {

    }

    public boolean isExtinct() {
        return isExtinct;
    }

    public void setExtinct(boolean extinct) {
        isExtinct = extinct;
    }
}

public class CovariantAnimalHolder<T extends Cat> {
    private T animalHolder;

    public T getAnimalHolder() {
        return animalHolder;
    }

    public void setAnimalHolder(T animalHolder) {
        this.animalHolder = animalHolder;
    }
}

public class VarianceMain {

    public static void main(String[] args) {
        //This works
        CovariantAnimalHolder<Lion> lionCovariantAnimalHolder = new CovariantAnimalHolder<Lion>();

        //This works:
        CovariantAnimalHolder<Cat> catCovariantAnimalHolder = new CovariantAnimalHolder<>();

        //This should not work, Perfect!
        //CovariantAnimalHolder<Animal> animalCovariantAnimalHolder = new CovariantAnimalHolder<Animal>();
    }
}

But when I create the ContraVariant holder I am getting some basic errors which I can't understand:
但是当我创建逆变持有者时，我遇到了一些基本错误，我无法理解：

public class ContravariantAnimalHolder<? super Cat>{
    private T animalHolder;
}

Error:
错误：

public class Animal {
private String name;
public Animal(String name) {
this.name = name;
}
public Animal() {
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
public class Cat extends Animal {
private boolean isWild;
public Cat(boolean isWild) {
this.isWild = isWild;
}
public Cat(){
}
public boolean isWild() {
return isWild;
}
public void setWild(boolean wild) {
isWild = wild;
}
}
public class Lion extends Cat{
private boolean isExtinct;
public Lion (boolean isExtinct) {
this.isExtinct = isExtinct;
}
public Lion () {
}
public boolean isExtinct() {
return isExtinct;
}
public void setExtinct(boolean extinct) {
isExtinct = extinct;
}
}

public class CovariantAnimalHolder<T extends Cat> {
private T animalHolder;
public T getAnimalHolder() {
return animalHolder;
}
public void setAnimalHolder(T animalHolder) {
this.animalHolder = animalHolder;
}
}

public class VarianceMain {
public static void main(String[] args) {
//This works
CovariantAnimalHolder<Lion> lionCovariantAnimalHolder = new CovariantAnimalHolder<Lion>();
//This works:
CovariantAnimalHolder<Cat> catCovariantAnimalHolder = new CovariantAnimalHolder<>();
//This should not work, Perfect!
//CovariantAnimalHolder<Animal> animalCovariantAnimalHolder = new CovariantAnimalHolder<Animal>();
}
}

public class ContravariantAnimalHolder<T super Cat>{
private T animalHolder;
}

答案1

得分: 0

在Java中，无法像在其他语言中那样将泛型类型定义为协变或逆变。例如，在Scala中可以这样做。

在Java中，您定义的是具有上界的泛型类型，这会限制您在处理类型时只能使用 extends，而不能在泛型类型中使用 super，这也是您不能在 ContravariantAnimalHolder 类中使用它的原因。

不过，您可以定义具有下界通配符的方法，这根据您的需求可能适用于逆变。