Java 不兼容类型无法转换为通配符参数 HashMap<> 里面

发布于 2025-01-19 14:49:40 字数 649 浏览 0 评论 0 原文

最近，我在编译过程中遇到了以下错误：

error: incompatible types: HashMap<Integer, HashMap<Object, Consumer<A>>> cannot be converted
to HashMap<Integer, HashMap<Object, Consumer<? extends B>>>

在此代码中，类 a 是 b 的直接子类。

此错误发生在以下片段中：

HashMap<Integer, HashMap<Object, Consumer<A>>> item = new HashMap<>();
HashSet<HashMap<Integer, HashMap<Object, Consumer<? extends B>>>> set = new HashSet();
set.add(item);

试图解决错误，我已经尝试删除？从 hashmap 扩展位 hashset 的内部，但是编译器不断丢弃错误。

原文

Recently I have come across the following error during compilation:

error: incompatible types: HashMap<Integer, HashMap<Object, Consumer<A>>> cannot be converted
to HashMap<Integer, HashMap<Object, Consumer<? extends B>>>

In this code, class A is a direct subclass of B.

This error occurred in the following snippet:

HashMap<Integer, HashMap<Object, Consumer<A>>> item = new HashMap<>();
HashSet<HashMap<Integer, HashMap<Object, Consumer<? extends B>>>> set = new HashSet();
set.add(item);

In attempting to resolve the error I have already tried removing the ? extends bit from the HashMap inside of the HashSet, but the compiler kept throwing the error.

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染墨丶若流云 2025-01-26 14:49:40

消费者＆lt; a＆gt; 和消费者＆lt;扩展B＆gt; 是完全不同的，实际上消费者？扩展b＆gt; 可以是消费者＆lt; c＆gt; 其中 c 是另一个类的类，

与 consemer＆lt; a＆gt; 相同。 消费者＆lt; b＆gt; 它们不是同一回事， b 类的实例不是 a 类的实例，因此请删除<<<<<代码>？扩展仍然会给您编译时间错误。

import java.util.ArrayList;
import java.util.List;

public class Test {
    static class Lion extends Animal {
    }
    static class Animal {
    }
    public static void main(String[] args) {
        List<? extends Animal> animals = new ArrayList<Animal>();
        animals .add(new Lion()); // DOES NOT COMPILE 

        // at compile time , our List(reference) may be a List of Animal or List of Lion , not a List of both of them
        
        // using ? extends Animal to refer the ArrayList instance 
        // we cannot add a Lion to an ArrayList of Animal

        animals .add(new Animal()); // DOES NOT COMPILE

        // the same using the ? extends Animal as reference 
       // we cannot add an animal to an ArrayList of Lion
    }
}

Consumer<A> and Consumer<? extends B> are totally different , in fact Consumer<? extends B> can be Consumer<C> where C is another class who extends B.

The same with Consumer<A> and Consumer<B> they are not the same thing, an instance of B class is not an instance of A class, so deleting the ? extends still give you a compile time error.

import java.util.ArrayList;
import java.util.List;

public class Test {
    static class Lion extends Animal {
    }
    static class Animal {
    }
    public static void main(String[] args) {
        List<? extends Animal> animals = new ArrayList<Animal>();
        animals .add(new Lion()); // DOES NOT COMPILE 

        // at compile time , our List(reference) may be a List of Animal or List of Lion , not a List of both of them
        
        // using ? extends Animal to refer the ArrayList instance 
        // we cannot add a Lion to an ArrayList of Animal

        animals .add(new Animal()); // DOES NOT COMPILE

        // the same using the ? extends Animal as reference 
       // we cannot add an animal to an ArrayList of Lion
    }
}

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