java：反射获得枚举 [英] java: reflection to obtain an Enum
问题描述
我有一个 Map< Enum<？>，FooHandler> 映射枚举 s（我不在乎哪个类型，即使它们是同一类型，只要它们是枚举常量）到我的 FooHandler class。
I have a Map<Enum<?>, FooHandler> that I want to use to map Enums (I don't care which type or even if they are the same type, just as long as they are enum constants) to my FooHandler class.
我想使用我阅读的文本文件填充此地图。我可以得到它的工作，但我有两个警告我想要解决：
I would like to populate this map using a text file that I read. I can get it to work, but I have two warnings I would like to get around:
static private <E extends Enum<E>> E getEnum(String enumFullName) {
// see https://stackoverflow.com/questions/4545937/
String[] x = enumFullName.split("\\.(?=[^\\.]+$)");
if (x.length == 2)
{
String enumClassName = x[0];
String enumName = x[1];
try {
Class<E> cl = (Class<E>)Class.forName(enumClassName);
// #1
return Enum.valueOf(cl, enumName);
}
catch (ClassNotFoundException e) {
e.printStackTrace();
}
}
return null;
}
public void someMethod(String enumName, String fooHandlerName)
{
FooHandler fooHandler = getFooHandler(fooHandlerName);
Enum e = getEnum(enumName);
// #2
map.put(e, fooHandler);
}
警告＃1：取消选中cast
警告＃2：枚举是一个原始的类型。
Warning #1: unchecked cast Warning #2: Enum is a raw type.
我得到＃1，可以只是一个警告我想，但我似乎不能打2号警告＃我试过枚举<？> ，这只是给我一个关于泛型类型捕获绑定不匹配的错误。
I get #1 and could just put a warning I suppose, but I can't seem to beat warning #2; I've tried Enum<?> and that just gives me an error about generic type capture bound mismatch.
更糟糕的替代实现：
在我的之前，E扩展枚举< E> 通用返回值我尝试返回Enum，它没有工作;我收到这些警告/错误：
Alternative implementations that are worse:
Before my <E extends Enum<E>> generic return value, I tried returning Enum and it didn't work; I got these warnings/errors:
static private Enum<?> getEnum(String enumFullName) {
...
Class<?> cl = (Class<?>)Class.forName(enumClassName);
// 1
return Enum.valueOf(cl, enumName);
// 2
}
-
警告：
warnings:
- Type safety: Unchecked cast from Class<capture#3-of ?> to Class<Enum>
- Enum is a raw type. References to generic type Enum<E> should be parameterized
- Enum is a raw type. References to generic type Enum<E> should be parameterized
- Unnecessary cast from Class<capture#3-of ?> to Class<?>
错误：
errors:
- Type mismatch: cannot convert from capture#5-of ? to Enum<?>
- Type safety: Unchecked invocation valueOf(Class<Enum>, String) of the generic method
valueOf(Class<T>, String) of type Enum
- Bound mismatch: The generic method valueOf(Class<T>, String) of type Enum<E> is not
applicable for the arguments (Class<capture#5-of ?>, String). The inferred type capture#5-of ? is not
a valid substitute for the bounded parameter <T extends Enum<T>>
而这个：
static private Enum<?> getEnum(String enumFullName) {
...
Class<Enum<?>> cl = (Class<Enum<?>>)Class.forName(enumClassName);
// 1
return Enum.valueOf(cl, enumName);
// 2
- 警告：
类型安全性：来自类< capture＃3-of？>的未经检查的投射类 - 错误：
绑定不匹配：通用方法valueOf（Class< T>，String ）类型为Enum< E>不适用于参数（类< Enum<？>>，String）。推断类型Enum<>不是有限参数的有效替代< T extends Enum< T>
- warning:
Type safety: Unchecked cast from Class<capture#3-of ?> to Class<Enum<?>> - error:
Bound mismatch: The generic method valueOf(Class<T>, String) of type Enum<E> is not applicable for the arguments (Class<Enum<?>>, String). The inferred type Enum<?> is not a valid substitute for the bounded parameter <T extends Enum<T>>
推荐答案
对于＃1，除 SuppressWarnings（unchecked）之外，没有解决方案。
For #1 there's no solution except SuppressWarnings("unchecked").
对于＃2，声明有一个问题：
For #2 there's a problem with the declaration:
static private <E extends Enum<E>> E getEnum(String enumFullName)
您可以返回 E ，但编译器无法确定 E 。没有类型 E 或 Class< E> 或任何可允许的参数。你可以写它，但是在某个地方会有一个未被检查的演员，当你调用它时，你可能会得到一个 ClassCastException 。所以不要这样做。
You can return E, but there's no way for the compiler to determine E. There's no argument of type E or Class<E> or whatever, which would allow it. You can write it, but there'll be an unchecked cast somewhere and when you call it, you may get a ClassCastException. So don't do it.
只需将其更改为
static private Enum<?> getEnum(String enumFullName)
因为这将工作，更公平。您会在每个通话网站上收到警告，这是正确的，因为有一些警告。
as this will work and is more fair. You'll get a warning on each call site and that's correct as there is something to warn of.
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