Java通用函数：如何返回泛型类型 [英] Java generic function: how to return Generic type

问题描述

这是一个Java通用模式：

  public< T> T getResultData（Class< T> resultClass，other_args）{
 ... 
 return resultClass.cast（T-thing）; 
 
  

典型的调用如下所示：

  DoubleBuffer缓冲区; 
 buffer = thing.getResultData（DoubleBuffer.class，args）; 
  

我从来没有想过如何在需要的返回类型是干净的时候使用这个模式，本身就是通用的。要成为'具体'，如果像这样的函数想要返回 Map< String，String> ？由于无法为泛型获取类对象，当然，唯一的选择是传递 Map.class ，然后您需要一个转换和一个 @SuppressWarning 毕竟。

最后得到如下的电话：

 映射< String，String> returnedMap; 
 returnedMap = thing.getResultData（Map.class，some_other_args）; 
  

现在我们回到需要强制转换并禁止警告的情况。

我想可以从 java.lang.reflect.Type 系列中取代 Class $ c>，但这些并不是特别容易编制。看起来像：



  class Dummy {
 Map< String，String>领域; 
} 
 
 ... 
 
类型typeObject = Dummy.class.getField（field）。getGenericType（）; 
  

在这种情况下，被调用函数可以提取基类型并将其用于转换或newInstance- ing，但是虚拟领域的业务确实看起来很丑。

请注意，像这样的函数并不总是调用 newInstance 。显然，如果他们这样做，他们不需要调用 resultClass.cast 。

解决方案你不能在标准的Java API中做到这一点。但是，有实用程序类可用于获取 在泛型类中输入 。例如，Google Gson（将JSON转换为完全适用的Javabeans，反之亦然）中，有一个 TypeToken 类。您可以如下使用它：

 列表< Person> fromJson（json，new TypeToken< List< Person>>（）{} .getType（））; 
  

这种结构正是您所需要的。您可以找到这里来源sode，您可能会发现它也很有用。它只需要额外的 getResultData（）方法，它可以接受 Type 。

Here's a Java generic pattern:

public <T> T getResultData(Class<T> resultClass, other_args) { 
   ...
   return resultClass.cast(T-thing);
}

A typical call looks like:

   DoubleBuffer buffer;
   buffer = thing.getResultData(DoubleBuffer.class, args);

I've never been able to figure out how to use this pattern cleanly when the desired return type is, itself, generic. To be 'concrete', what if a function like this wants to return Map<String,String>? Since you can't get a class object for a generic, of course, the only option would be to pass Map.class, and then you need a cast and an @SuppressWarning after all.

One ends up with a call like:

Map<String, String> returnedMap;
returnedMap = thing.getResultData(Map.class, some_other_args);

Now one is back to needing casts and suppressing a warning.

I suppose that one could take something from the java.lang.reflect.Type family instead of the Class, but those aren't especially easy to concoct. That looks like:

class Dummy {
 Map<String, String> field;
}

...

Type typeObject = Dummy.class.getField("field").getGenericType();

Given this, the called function could extract the base type and use that for casting or newInstance-ing, but the dummy field business sure looks ugly.

Note that functions like this are not always calling newInstance. Obviously, if they do, they don't need to call resultClass.cast.

解决方案

You cannot do that in the standard Java API. However, there are utility classes available which can get the Type out of a generic class. In for example Google Gson (which converts JSON to fullworthy Javabeans and vice versa) there's a TypeToken class. You can use it as follows:

List<Person> persons = new Gson().fromJson(json, new TypeToken<List<Person>>() {}.getType());

Such a construct is exactly what you need. You can find here the source sode, you may find it useful as well. It only requires an additional getResultData() method which can accept a Type.

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