无法转换为已实现的接口 [英] can't cast to implemented interface

问题描述

我很困惑...

我有一个直接实现接口的类:

I have a class which directly implements an interface:

public class Device implements AutocompleteResult
{...}

以下证明我正在查看正确的变量:

Here is proof that I'm looking at the right variables:

Object match = ...;
log.debug(match.getClass()); // Outputs 'Device'
log.debug(match.getClass().getInterfaces()[0]); // Outputs 'AutocompleteResult'

然而，当我尝试将类的实例转换为接口时:

Yet when I try to cast an instance of the class to the interface:

AutocompleteResult result = (AutocompleteResult) match;

我收到一个 ClassCastException！

I get a ClassCastException!

ClassCastException: Device cannot be cast to AutocompleteResult

此外，isAssignableFrom 返回 false，我不知道为什么:

Also, isAssignableFrom returns false and i'm not sure why:

log.debug(AutocompleteResult.class.isAssignableFrom(Device.class));

来自 doc:

确定此 Class 对象表示的类或接口是否与所表示的类或接口相同，或者是其超类或超接口通过指定的 Class 参数.

Determines if the class or interface represented by this Class object is either the same as, or is a superclass or superinterface of, the class or interface represented by the specified Class parameter.

我不应该总是能够将一个对象转换为它的类实现的接口吗?

Shouldn't I always be able to cast a object to an interface its class implements?

谢谢.

推荐答案

如果两个不同的类加载器加载名为 AutocompleteResult 的类，就会发生这种情况.

This can happen if two different classloaders load a class named AutocompleteResult.

这两个类被视为完全不同的类，即使它们具有相同的包和名称(甚至实现/字段/方法).

These two classes are then treated as entirely different classes, even if they have the same package and name (and even implementation/fields/methods).

造成这种情况的一个常见原因是，如果您使用某种插件系统并且您的基类和插件类提供相同的类.

A common cause for this is if you use some kind of plugin system and both your base classes and the plugin classes provide the same class.

要检查此问题，请打印 Class.getClassLoader() 在两个违规类(即由Device实现的接口的类和<代码>AutocompleteResult.class).

To check for this issue print the value returned by Class.getClassLoader() on both offending classes (i.e. the class of the interface implemented by Device and the result of AutocompleteResult.class).

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