Java泛型 - Java是否需要支持本地定义的类型？ [英] Java generics - Does Java need support for locally defined types?

问题描述

我希望能在这里与Java仿制药专家联系。假设你有一些类型的类：

I am hoping to reach the Java generics experts here. Let's say you have some typed class:

public interface SomeClass<T> {
    void doSomething(final T t);
}

还有一个函数可以获得 T 给定一个的实例SomeClass< T> ：

There is also a function which gets you an instance of T given an instance of SomeClass<T>:

public static class Retriever {
    public <T> T get(final SomeClass<T> c) {
        return null; // actual implementation left out
    }
}

现在让我们说你有一个 SomeClass<？> 和一个检索器的集合：

Now let's say you have a collection of SomeClass<?> and a retriever:

final List<SomeClass<?>> myClasses   = null; // actual implementation left out
final Retriever          myRetriever = null; // actual implementation left out

我们无法执行以下操作：

We are not able to do the following:

for (final SomeClass<?> myClass : myClasses) {
    myClass.doSomething(myRetriever.get(myClass));
}

现在我的问题：Java是否需要支持才能在本地定义类型？类似于：

Now my question: does Java need support to be able to locally define a type? Something like:

<T> for (final SomeClass<T> myClass : myClasses) {
    myClass.doSomething(myRetriever.get(myClass));
}

此处，类型 T 作用于for循环。我们定义 T 来摆脱通配符？。而已。引入 T 应该使我们能够编写如上所述的所需for循环。

Here, the type T is scoped to the for-loop. We are defining T to get rid of the wildcard ?. That's it. The introduction of T should enable us to write the desired for loop as expressed above.

FWIW，以下代码是一种解决方法。我们正在推出一个功能，仅用于将？转换为 T 。

FWIW, the following code is a workaround. We are introducing a function, solely for the conversion of ? to T.

for (final SomeClass<?> myClass : myClasses) {
    workAround(myRetriever, myClass);
}

public static <T> void workAround(final Retriever myRetriever, final SomeClass<T> myClass) {
    myClass.doSomething(myRetriever.get(myClass));
}

本地定义的用户类型可能是更优雅的解决方案吗？

A locally defined user type might be a more elegant solution?

推荐答案

现在我的问题：Java是否需要支持才能在本地定义类型？

Now my question: does Java need support to be able to locally define a type?

不。类型参数的最小范围是方法，即为了使for循环可以使用类型 T ，您必须将封闭方法定义为通用或封闭的类。例如：

No. The minimal scope of a type-parameter is the method, i.e. in order to have the type T available for your for loop, you will have to either defined the enclosing method a generic or the enclosing class. For example:

<T> void method(List<SomeClass<T> myClasses) {
    for (final SomeClass<T> myClass : myClasses) {
        myClass.doSomething(myRetriever.get(myClass));
    }
}

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