如何在Java中覆盖通用列表返回类型 [英] How to override generic list return type in java

问题描述

我正在尝试重写将类型返回为子类列表的方法，如下例所示，由于泛型问题，我无法在我的子类中做到这一点.我无法更改我的超类代码，那么如何解决该问题?任何人都可以指导我...在此先感谢.

I am trying to override the method which return type as list in sub class like as below example, due to generics issue i can not able to do it in my sub class. I can not able to change my super class code, so how i can resolve the issue? any one can please guide me...many thanks in advance.

无法更新的超级类:

public class Animal {
           private String legs;
    }

public class TestSuper {

    public List<Animal> addAnimals() {
        return animals;
    }
}

子类:

public class Dog extends Animal {
         private String sound;
    }

public class TestSub extends TestSuper {

    public List<Dog> addAnimals() { --> I need to override this method but want return type as List of dogs
        return null;
    }
}

推荐答案

如果真的无法更新超级类，恐怕您根本无法更新.

If the super classes really can't be updated, I'm afraid you simply can't.

如果您能够更新超级类:

在 TestSuper 中，您将使用 public List< ;?扩展Animal>addAnimals()而不是 public List< Animal>addAnimals().

In TestSuper, you would have use public List<? extends Animal> addAnimals() instead of public List<Animal> addAnimals().

您还有什么其他解决方案:

您可以使用这样的实用程序方法:

You could use such a utility method:

@SuppressWarnings("unchecked")
public static <T extends Animal> List<T> cast(List<Animal> animals, Class<T> subclass) {
    List<T> out = new ArrayList<T>();
    for (Animal animal : animals) {
        if (!subclass.isAssignableFrom(animal.getClass())) {
            // the "animal" entry isn't an instance of "subclass"
            // manage this case however you want ;)
        } else {
            out.add((T) animal);
        }
    }
    return out;
}

然后致电:

List<Dog> dogs = TheClassNameYouChose.cast(animals, Dog.class);

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