Java泛型：获得泛型方法返回类型的类 [英] Java generics: get class of generic method's return type

问题描述

我曾经写过这种方法：

  private < T>的SortedSet< T> createSortedSet（）{
 return new TreeSet< T>（）; 
} 
  

这应该是这样调用的：

 设置< String> set = createSortedSet（）; 
  

这可以正常工作（尽管我在研究当前问题时已经看到了答案，

目前情况

无论如何，现在我正在写下面的代码（在一个类中它扩展了javax.servlet.jsp.tagext.TagSupport）：

  private< T> T评估（字符串表达式）{
 ExpressionEvaluator evaluateator = pageContext.getExpressionEvaluator（）; 
 return evaluateator.evaluate（expression，T，null，pageContext.getVariableResolver（））; 
} 
  

目的是能够像这样调用：

  Integer width = evaluate（widthStr）; 
  

我的评估方法中的代码显然不起作用。 evaluateator.evaluate（）的第二个参数应该是一个Class对象。这导致我：

我的问题

如何获得泛型（返回）类的类？我应该写什么来代替T作为评估的第二个参数？

编辑：结论

Nicolas好像是对的，这是不可能完成的，我需要通过这个类作为我的方法的一个参数。好处在于，由于他的解决方案使参数在泛型上得到了参数化，所以我得到了对该参数的编译检查。

，您必须将您的方法更改为：

 私人< T> T评估（Class< T> clazz，String表达式）
  

然后传递 clazz 作为第二个参数。没有你期望的那么短。

Background

I once wrote this method:

private <T> SortedSet<T> createSortedSet() {
  return new TreeSet<T>();
}

It's supposed to be called like this:

Set<String> set = createSortedSet();

This works fine (although I've seen in answers here when researching the current question that this is error-prone).

The current situation

Anyway, now I am writing the following code (in a class that extends javax.servlet.jsp.tagext.TagSupport):

private <T> T evaluate(String expression) {
  ExpressionEvaluator evaluator = pageContext.getExpressionEvaluator();
  return evaluator.evaluate(expression, T, null, pageContext.getVariableResolver());
}

The purpose is to be able to call this like:

Integer width = evaluate(widthStr);

The code in my evaluate method obviously doesn't work. The second argument to evaluator.evaluate() is supposed to be a Class object. This leads me to:

My question

How can I get the Class of a generic (return) type? What should I write in place of T as the second argument to evaluate?

EDIT: Conclusion

Nicolas seems to be right, it can not be done and I need to pass the class as an argument to my method. The upside is that since his solution makes the argument parametrized on the generic type I get a compilation check on that argument.

解决方案

Unfortunately, you will certainly have to change your method to:

private <T> T evaluate(Class<T> clazz, String expression)

and then pass clazz as your second parameter. Not as short as you expected.

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