类型不匹配：无法从Class< ..>转换等级< ...> [英] Type mismatch: cannot convert from Class <..> to Class <...>

问题描述

我已经继承了一个Eclipse RCP项目，该项目似乎可以在基于3.6的目标平台上正常工作。但是，向前迈进，我们需要更新到该平台的最新版本，但是当我将目标平台更改为3.7（或4.2）时，会遇到一些错误，类似于

I've 'inherited' an Eclipse RCP project which seems to work fine with a target platform set based around 3.6. However, moving forward we need to update to the latest version of the platform, but when I change the target platform to 3.7 (or 4.2) I get a handful of errors along the lines of

Type mismatch: cannot convert from Class<capture#1-of ?> to Class<? extends IDatasetProvider>

任何人都可以建议/提供解释为什么它在3.6中可行，但在3.7中不可行（然后）？关于从哪里开始解决此问题的任何想法也将很棒！

Can anyone suggest/provide an explanation of why this might work fine in 3.6, but not in 3.7 (and later)? Any ideas as to where I would start in resolving this would also be great!

导致此错误的代码片段（出现在b.loadClass部分）：

A snippet of the code causing this error (which appears at the b.loadClass part):

    List<Class<? extends IDatasetProvider>> list = new LinkedList<Class<? extends IDatasetProvider>>();
    ClassMap<IDatasetProvider, List<String>> map = new ClassMap<IDatasetProvider, List<String>>();

    for (IConfigurationElement e : elements)
    {
        try
        {
            Bundle b = Platform.getBundle(e.getContributor().getName());

            String viewId = e.getAttribute("viewId");
            Class<? extends IDatasetProvider> datasetType = b.loadClass(e
                    .getAttribute("datasetProvider"));
            ...
            ...
            ...
        }
     }

还有3个（可能）相关的警告

There are also 3 (possibly) related warnings

 IDatasetProvider is a raw type. References to generic type IDatasetProvider<T> should be parameterized 

如果我改回到我们的3.6平台，那么一切都会再次起作用。

If I change back to our 3.6 platform, it all works again.

编辑：由于Alexy和gzukmin的帮助，已修复。

fixed thanks to the help of Alexy and gzukmin.

我使用以下代码，特别是强制转换为 Class< ;?扩展IDatasetProvider> 而不只是 Class ：

I used the following code, specifically casting to Class<? extends IDatasetProvider> rather than just Class:

Class<? extends IDatasetProvider> datasetType = 
    (Class<? extends IDatasetProvider>) b.loadClass(e.getAttribute("datasetProvider"));

如果有任何原因，我应该考虑转换为更通用的 Class ，请让我知道！

If there's any reason I should consider just casting to the more generic Class, please let me know!

推荐答案

您可以将其转换为原始类型：

You can just cast it to raw type like this:

Class<? extends IDatasetProvider> datasetType = 
    (Class) b.loadClass(e.getAttribute("datasetProvider"));

请注意：

1. 添加有关原始类型和未经检查的强制类型转换的更多警告。

1. Add more warnings about raw types and unchecked casts.

如果您的类实际上没有扩展，则在运行时爆炸 IDatasetProvider 。而不是在转换位置，而是稍后您尝试实际使用该类时。因此，最好使用

Blow up at the runtime if your class actually turns out not to extend IDatasetProvider. And not at the cast location, but later when you try to actually use the class. So it might be a good idea to check this with

IDatasetProvider.class.isAssignableFrom(datasetType)

（请参见 Javadoc ）。

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