Java Generic和Generic Class的继承

Question

似乎通用容器不能使用比通用容器的函数签名更具体的类。

我怎样才能拥有一个容器，该容器使用比传递给它的函数之一更具体的类？这可能吗？我是否误解了 Java 中的泛型？

请看代码，因为它可能更容易理解这个问题。

    package com.demo;

    public class Entity {
        public String baseField;
    }

    public class ChildEntity extends Entity {
        public String extraField;
    }

    public class EntityContainer<T extends Entity> {
        public T instance;
    }

    public class EntityContainerWithMeta<T extends Entity> extends EntityContainer<T> {
        public String childContainerMetaData;
    }

    public class EntityConsumer<T extends Entity> {
        public void consum(EntityContainer<T> container){

        }
    }

    public class DemoGeneric {

        private class ChildChildEntity extends ChildEntity{
            public String extraExtraField;
        }

        // Objective :
        // - using generic
        // - with a data container that hold a more specific Klass ( Klass extends Entity)
        // - pass it to a less specific consumer ( that consum Entity not Klass)

        // real word use case:
        // Entity is the base class for all my Business Specific Object
        // Container are List<T extends Entity>
        // Consumer are ListAdapter for ListView (extends BaseAdapter) (from Android framework)

        public void demo() {

            // compile: (but then container cannot return ChildEntity with its getter)
            //EntityContainerWithMeta<Entity> demoContainer =
            //new EntityContainerWithMeta<Entity>();

            // don't compile:
            EntityContainerWithMeta<ChildEntity> demoContainer = new EntityContainerWithMeta<ChildEntity>();

            demoContainer.instance = new DemoGeneric.ChildChildEntity();

            EntityConsumer<Entity> consumer = new EntityConsumer<Entity>();

            // here is the problem:
            // required: EntityContainer<Entity>
            // found: EntityContainerWithMeta<ChildEntity>
            consumer.consum(demoContainer);

            // if
            // ChildEntity extends Entity
            // and
            // EntityContainerWithMeta extends EntityContainer
            // why is this not compiling ?
        }
    }

编辑:

感谢大家的回答，我找到了这篇文章: http://blog.informatech.cr/2013/03/15/covariance-and-contravariance-in-java/

很好的解释了@kocko和@MarkoTopolnik提到的问题

Answer 1

将您的 consumer 声明更改为:

EntityConsumer<? extends Entity> consumer = new EntityConsumer<ChildEntity>();

If ChildEntity extends Entity and EntityContainerWithMeta extends EntityContainer why is this not compiling ?

因为 generics are not implicitly polimorphic .