Java泛型错误：来自命令行编译器的不可转换类型 [英] Java Generics Error: inconvertible types from command line compiler

问题描述

我有一些Guice绑定代码使用泛型，从Eclipse的编译器编译和运行良好，但不是来自Java（命令行）编译器。我升级到最新的（1.7.0_01）Java SDK，但仍然会收到以下错误。

  [error] ... \\ 42：错误：不可转换类型
 [错误]（类<？extends ListAdapterDataProvider< Row extends扩展DatabaseItem>>）CategoryDataProvider.class） 
 [error] ^ 
 [error] required：Class<？扩展ListAdapterDataProvider< Row<？扩展DatabaseItem>>>> 
 [error] found：Class< CategoryDataProvider> 
 [错误] 1错误
 [错误] {文件：/.../编译：编译：javac返回非零退出代码
  

相关代码：

  public interface类别扩展DatabaseItem {} 
 public class CategoryDataProvider implements 
 ListAdapterDataProvider< Row< Category>> {} 
 public class BindListViewHandlerWithSpecificProvider extends AbstractModule {
 public BindListViewHandlerWithSpecificProvider（
 Class<？extends ListAdapterDataProvider<？extends DatabaseItem>> 
 dataProviderClass）{} 
 
 
 @SuppressWarnings（unchecked）
 //发生错误：
 final BindListViewHandlerWithSpecificProvider 
 bindListViewHandlerWithSpecificProvider = 
 new BindListViewHandlerWithSpecificProvider（
 （Class<？extends ListAdapterDataProvider< Row<？extends DatabaseItem>>>）
 CategoryDataProvider.class）; 
  

解决方案

自己动手做一个upcast downcast：

  Class< ...> foo =（Class< ...>）（Object）MyClass.class; 
  

问题是 CDP.class 类型为 Class< CDP> ， CDP 为原始类型。而参数化类型 C< T1，...，Tn> 是原始类型的子类型 C （§否则不成立： C 不是 C< T1，...，Tn> 。这仅仅是因为未经检查的转换（§5.1.9）才是真实的。这导致您的问题：您希望 CDP 扩展（如上限><？extends ...> ） LADP< Row <扩展DI>> 。不是这样，因为类型参数包含（§4.5.1.1）是在子类型上定义的，不考虑未经检查的转换。

（或者切换到追逐：javac已经得到了这个权利。）

I have some Guice binding code using generics that compiles and functions fine from Eclipse's compiler, but not from the Java (command-line) compiler. I upgraded to the latest (1.7.0_01) Java SDK but still get the following error.

[error] ...\BindCategorySelectorActivity.java:42: error: inconvertible types
[error]                                 (Class<? extends ListAdapterDataProvider<Row<? extends DatabaseItem>>>) CategoryDataProvider.class);
[error]                                                                                                                             ^
[error]   required: Class<? extends ListAdapterDataProvider<Row<? extends DatabaseItem>>>
[error]   found:    Class<CategoryDataProvider>
[error] 1 error
[error] {file:/.../compile:compile: javac returned nonzero exit code

Relevant code:

public interface Category extends DatabaseItem {}
public class CategoryDataProvider implements 
 ListAdapterDataProvider<Row<Category>> {}
public class BindListViewHandlerWithSpecificProvider extends AbstractModule {
    public BindListViewHandlerWithSpecificProvider(
     Class<? extends ListAdapterDataProvider<Row<? extends DatabaseItem>>>
      dataProviderClass) {}
}

@SuppressWarnings("unchecked")
// Error happens here:
final BindListViewHandlerWithSpecificProvider 
 bindListViewHandlerWithSpecificProvider = 
  new BindListViewHandlerWithSpecificProvider(
   (Class<? extends ListAdapterDataProvider<Row<? extends DatabaseItem>>>)
    CategoryDataProvider.class);

解决方案

Do yourself a favor and do an upcast followed by a downcast:

Class<...> foo = (Class<...>)(Object)MyClass.class;

The issue is that CDP.class is of type Class<CDP>, CDP being a raw type. While a parameterized type C<T1,...,Tn> is the subtype of the raw type C (§4.10.2), the inverse is not true: C is not a subtype of C<T1,...,Tn>. This only appears to be true due to unchecked conversion (§5.1.9). This is causing your issue: You expect CDP to "extend" (as in the upper bound of Class<? extends ...>) LADP<Row<? extends DI>>. This is not the case because type argument containment (§4.5.1.1) is defined over subtyping and does not consider unchecked conversion.

(Or to cut to the chase: javac has got this one right.)

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