英文:
In Java, how can I implement a non-generic interface which has methods that use generics using concrete types?
问题
我有一个接口...
public interface ApiClient {
<T> void updateData(List<T> data);
}
和该接口的两个实现...
public class Version1ApiClient implements ApiClient {
public <Version1DataContract> void updateData(List<Version1DataContract> data) {
// ...
}
}
public class Version2ApiClient implements ApiClient {
public <Version2DataContract> void updateData(List<Version2DataContract> data) {
// ...
}
}
我在这两个实现中都得到了这个错误...
类型参数 'Version#DataContract' 隐藏了可见类型 'com.company.package.Version#DataContract'
...其中 # 是相应实现的数字。
我做错了什么？另外，在这种情况下，接口不能直接有一个类型参数，它只能存在于方法中。
英文:
I have an interface...
public interface ApiClient {
<T> void updateData(List<T> data);
}
and two implementations of that interface...
public class Version1ApiClient implements ApiClient {
public <Version1DataContract> void updateData(List<Version1DataContract> data) {
// ...
}
}
public class Version2ApiClient implements ApiClient {
public <Version2DataContract> void updateData(List<Version2DataContract> data) {
// ...
}
}
I get this error in both of the implementations...
> Type parameter 'Version#DataContract' hides visible type 'com.company.package.Version#DataContract'
...where the # is the number for that respective implementation.
What have I done wrong? Also, in this use case the interface cannot have a type parameter on it directly. It can only be on the method.
答案1
得分: 2
你没有做太多错误的事情；但你跳过了一项命名约定——但这可能掩盖了一个更重要的误解
你认为在这里 <Version1DataContract> 是什么意思？
public <Version1DataContract> void updateData(List<Version1DataContract> data) {
// ...
}
你认为它让 updateData 只与 Version1DataContract 一起工作，而 Version1DataContract 是你的类路径上的一个类，并且在 Version1ApiClient 类的源文件中被导入吗？如果你的答案是肯定的，那么你是错的。这段代码等同于：
public <T> void updateData(List<T> data) {
// ...
}
<Version1DataContract> 只是一个奇怪命名的类型变量。
现在，关于你的问题... 问题在于你在方法中声明了 <Version1DataContract> 作为类型变量，而已经有一个叫做 Version1DataContract 的导入类型，已经是可见的了。这是一个警告，提醒你可能会出现错误。
为了解决你似乎主要的问题，如果你希望 Version1ApiClient 限制 <T> void updateData(List<T> data); 为 T=Version1DataContract，你不能在不改变接口的情况下做到这一点。你必须更改接口并扩大 T 的范围，以便子类可以“冻结”它：
interface ApiClient<T> {
void updateData(List<T> data);
}
class Version1ApiClient implements ApiClient<Version1DataContract> {
public void updateData(List<Version1DataContract> data) {
// ...
}
}
英文:
You didn't do much that's wrong; but you skipped a naming convention - but that could be hiding a more important misunderstanding
What do you think <Version1DataContract> does here?
public <Version1DataContract> void updateData(List<Version1DataContract> data) {
// ...
}
Do you think it makes updateData work just with Version1DataContract, which is a class on your classpath and imported in the source file of your Version1ApiClient class? If your answer is yes, then you're mistaken. That code is equivalent to:
public <T> void updateData(List<T> data) {
// ...
}
<Version1DataContract> is just a weirdly named type variable.
Now, about your question... The problem is that you have declared <Version1DataContract> as a type variable on a method, while there is an imported type called Version1DataContract, and that is already visible. This is a warning alerting you to potential bugs.
To address what seems to be your main issue, if you need Version1ApiClient to limit <T> void updateData(List<T> data); to T=Version1DataContract, you can't do it without changing the interface. You'd have to change your interface and broaden the scope of T so that the subclass can "freeze" it:
interface ApiClient<T> {
void updateData(List<T> data);
}
class Version1ApiClient implements ApiClient<Version1DataContract> {
public void updateData(List<Version1DataContract> data) {
// ...
}
}
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