Java泛型的双通配符参数化(嵌套通配符) [英] Double wildcard parameterization (nested wildcards) with Java generics

问题描述

我正在使用第三方库( Reflections )中的一种方法查找给定类型的子类型，看起来像

I am using a method from a third party library (Reflections) which is supposed to find subtypes of the given type and looks like

public <T> Set<Class<? extends T>> getSubTypesOf(final Class<T> type) {
...

当呼叫者代码看起来像

Class<?> type = ...
Set<Class<?>> subTypes = reflections.getSubTypesOf(type);

我收到一个编译错误:"cannot convert from Set<Class<? extends capture#19-of ?>> to Set<Class<?>>".以下解决了这种情况:

I am getting a compile error: "cannot convert from Set<Class<? extends capture#19-of ?>> to Set<Class<?>>". The following fixes the situation:

Class<?> type = ...
Set<?> subTypes = reflections.getSubTypesOf(ht);

，因此看来对于Set<Class<? extends ?>>错误的唯一可能的补救方法是Set<?>，而不是Set<Class<?>>.为什么会这样呢? 感谢您对此的任何解释.

so it looks like the only possible remedy for incorrect Set<Class<? extends ?>> would be Set<?> but not Set<Class<?>>. Why is it so? Thanks for any explanation on this.

推荐答案

请改用以下内容:

Set<? extends Class<?>> subTypes = reflections.getSubTypesOf(type);

问题是嵌套通配符不执行Set<Class<? extends capture#19-of ?>>，这意味着一组从特定类型扩展的任何类型的类未知类型".在这种情况下，特定的未知类型"是从T的type参数派生的，该参数从type推断为无限制的通配符捕获(Class<?>中的?).

The problem is that nested wildcards don't perform type capture. Declaring a Set<Class<?>> means "a set of classes of any type", while what's being returned is a Set<Class<? extends capture#19-of ?>>, which means "a set of classes of any type extending from some specific unknown type". In this case, that "specific unknown type" is derived from the type argument to T, which is inferred from type to be an unbounded wildcard capture (the ? in Class<?>).

例如，假设特定的未知类型"为Number:

For example, pretend that "specific unknown type" is Number:

Class<Number> type = ...
Set<Class<?>> subTypes = reflections.getSubTypesOf(type);

此处，getSubTypesOf返回Set<Class<? extends Number>>.该类型不能分配给Set<Class<?>>，因为

Here, getSubTypesOf returns a Set<Class<? extends Number>>. That type is not assignable to Set<Class<?>> because generic types aren't covariant. But, wildcard capture helps us express covariance, allowing types like Set<? extends Class<?>>. The caveat is that we can't add anything but null to such a set, since we don't know its specific type.

相关帖子:

• Java Generic List< List< ;?扩展Number>>
• 在多个通配符上通用方法使Java编译器(和我自己)非常困惑
• 什么是PECS(生产者扩展了超级消费者)?

• Java Generic List<List<? extends Number>>
• Multiple wildcards on a generic methods makes Java compiler (and me!) very confused
• What is PECS (Producer Extends Consumer Super)?

类似的帖子:

• Java:通配符类型不匹配会导致编译错误
• Map声明的问题< String，Class< ;?扩展了Serializable>>
• 有界通配符相关的编译器错误

• Java: Wildcard Types Mismatch Results in Compilation Error
• Issue with declaration of Map<String,Class<? extends Serializable>>
• Bounded-wildcard related compiler error

这篇关于Java泛型的双通配符参数化(嵌套通配符)的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！