Java泛型类型问题 [英] Java generic type issue

问题描述

请考虑以下简化示例:

package com.test;

class B<S> {
    B(Class<S> clazz) {}
}

class A<T> {
    class SubB extends B<SubB> {
        SubB() {
            super(SubB.class);
        }
    }
}

尽管IntelliJ没有显示任何错误(通常在存在编译错误时会显示错误)，但是启动程序时的实际编译以super(SubB.class);中的错误结束:

Although IntelliJ is not showing any error (as it usually does when compile errors exist), the actual compilation when starting the program ends with error located in super(SubB.class);:

错误:(8，23)java:不兼容的类型:java.lang.Class<com.test.A.SubB>无法转换为java.lang.Class<com.test.A<T>.SubB>

我很好奇，为什么会这样?那我该怎么解决呢?

I am curious, why is this happening? And how could I solve it?

使用AdoptOpenJDK 11进行编译.

Compilation is done with AdoptOpenJDK 11.

推荐答案

此行为的原因有些复杂.考虑java.util.List.class，其类型为Class<java.util.List>，而不是Class<java.util.List<?>>.这是类文字的限制.

The reason for this behavior is a bit complicated. Consider java.util.List.class, which has the type Class<java.util.List>, not Class<java.util.List<?>>. This is a limitation of the class literal.

在您的示例中，SubB.class具有类型Class<com.test.A.SubB>，同样具有SubB的原始类型.但是构造函数希望使用Class<com.test.A<T>.SubB>类型.

In your example, SubB.class has the type Class<com.test.A.SubB>, again with the raw type of SubB. But the constructor expects some type of Class<com.test.A<T>.SubB>.

这就是为什么我们需要将文字转换为所需的类型:

That's why we need to cast the literal to it's desired type:

super((Class<SubB>) (Class<?>) SubB.class);

这将产生警告，但是快速检查将发现没有什么可担心的.

This will produce a warning, but a quick examination will show that there is nothing to worry about.

这篇关于Java泛型类型问题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！