blocks|key|782421|text|您使用的代码只在一些非常特殊的情况下工作，在这些情况下，在编译时已知(并存储)实际的类型参数。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|782422|例如，如果您这样做：|782423|class+IntegerList+extends+ArrayList<Integer>+{}

List<Integer>+l+=+new+IntegerList();|code-block|syntax|javascript|782424|在本例中，您显示的代码实际上将返回Integer.class，因为Integer是“烘焙到”IntegerList中的。|offset|length|style|CODE|782425|一些库(Ab)通过使用类型标记来使用这个技巧。例如，请参见TypeToken|782426|表示泛型类型T。可以使用该类获取类的泛型类型。>>例如，要获得Collection<Foo>的泛型类型，可以使用：
类型typeOfCollectionOfFoo+=新的TypeToken(){}.getType()
|blockquote|782427|782428|这是因为这里创建的匿名类已经编译--在其类型参数为Collection<Foo>的信息中。|782429|请注意，这是行不通的(即使TypeToken类不会通过保护其构造函数来阻止它)：|782430|Type+typeOfCollectionOfFoo+=+new+TypeToken<Collection<Foo>>().getType()|782431|entityMap|0|LINK|mutability|MUTABLE|url|http://google-gson.googlecode.com/svn/tags/1.1.1/docs/javadocs/com/google/gson/reflect/TypeToken.html^0|0|0|0|H|D|X|7|1A|B|0|T|9|T|9|0|0|6|1|V|F|0|0|P|F|0|D|9|0|0^^$0|@$1|2|3|4|5|6|7|18|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|19|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|1A|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|1B|8|@$K|1C|L|1D|M|N]|$K|1E|L|1F|M|N]|$K|1G|L|1H|M|N]]|9|@]|A|$]]|$1|O|3|P|5|6|7|1I|8|@$K|1J|L|1K|M|N]]|9|@$K|1L|L|1M|1|1N]]|A|$]]|$1|Q|3|R|5|S|7|1O|8|@$K|1P|L|1Q|M|N]|$K|1R|L|1S|M|N]]|9|@]|A|$]]|$1|T|3|-4|5|6|7|1T|8|@]|9|@]|A|$]]|$1|U|3|V|5|6|7|1U|8|@$K|1V|L|1W|M|N]]|9|@]|A|$]]|$1|W|3|X|5|6|7|1X|8|@$K|1Y|L|1Z|M|N]]|9|@]|A|$]]|$1|Y|3|Z|5|F|7|20|8|@]|9|@]|A|$G|H]]|$1|10|3|-4|5|6|7|21|8|@]|9|@]|A|$]]]|11|$12|$5|13|14|15|A|$16|17]]]]

The code you use only works in some very specific cases, where the actual type parameter is known (and stored) at compile time.

For example if you did this:

<pre><code>class IntegerList extends ArrayList&lt;Integer&gt; {}

List&lt;Integer&gt; l = new IntegerList();
</code></pre>

In this case the code you showed would actually return <code>Integer.class</code>, because <code>Integer</code> is "baked into" the <code>IntegerList</code>.

Some libraries (ab)use this trick via the use of type tokens. See for example the <a href="http://google-gson.googlecode.com/svn/tags/1.1.1/docs/javadocs/com/google/gson/reflect/TypeToken.html" rel="nofollow">GSON class <code>TypeToken</code></a>:

<blockquote>
 Represents a generic type <code>T</code>. You can use this class to get the generic type for a class. > For example, to get the generic type for <code>Collection&lt;Foo&gt;</code>, you can use:

<pre><code>Type typeOfCollectionOfFoo = new TypeToken&lt;Collection&lt;Foo&gt;&gt;(){}.getType()
</code></pre>
</blockquote>

This works because the anonymous class created in here has compiled-in the information that its type parameter is <code>Collection&lt;Foo&gt;</code>.

Note that this would not work (even if the <code>TypeToken</code> class wouldn't prevent it by making its constructor protected):

<pre><code>Type typeOfCollectionOfFoo = new TypeToken&lt;Collection&lt;Foo&gt;&gt;().getType()
</code></pre>

blocks|key|390024|text|当由于继承而设置泛型类型时，您所描述的方法只起作用，因为它在编译时已知：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|390025|+public+class+SomeClass<T>{

+}

+public+class+SpecificClass+extends+SomeClass<String>{

+}|code-block|syntax|javascript|390026|对于这个例子，您可以使用该方法，然后返回"String.class“。|390027|如果您正在动态地创建实例，它将无法工作：|390028|SomeClass+s+=+new+SomeClass<String>();+//wont+work+here.|390029|一些常见的工作是，将实际的类作为参数传递给以后的引用：|390030|+public+class+SomeClass<T>{
++++Class<T>+clazz

++++public+SomeClass(Class<T>+clazz){
++++++++this.clazz+=+clazz;
++++}

++++public+Clazz<T>+getGenericClass(){
+++++++return+this.clazz;
++++}+
+}|390031|用法：|390032|+SomeClass<String>+someClass=+new+SomeClass<String>(String.class);

+System.out.println(someClass.getGenericClass())+//String.class|390033|实际上，对于这种场景，您甚至不需要泛型类型，因为Java会做同样的事情，就像您将"T“作为Object来处理一样。唯一的优点是，您可以定义getter和Setter的T，而不需要一直键入对象。(因为Java是为您做的)(它被称为类型擦除)|offset|length|style|CODE|390034|entityMap^0|0|0|0|0|0|0|0|0|0|19|6|0^^$0|@$1|2|3|4|5|6|7|12|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|13|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|14|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|15|8|@]|9|@]|A|$]]|$1|K|3|L|5|D|7|16|8|@]|9|@]|A|$E|F]]|$1|M|3|N|5|6|7|17|8|@]|9|@]|A|$]]|$1|O|3|P|5|D|7|18|8|@]|9|@]|A|$E|F]]|$1|Q|3|R|5|6|7|19|8|@]|9|@]|A|$]]|$1|S|3|T|5|D|7|1A|8|@]|9|@]|A|$E|F]]|$1|U|3|V|5|6|7|1B|8|@$W|1C|X|1D|Y|Z]]|9|@]|A|$]]|$1|10|3|-4|5|6|7|1E|8|@]|9|@]|A|$]]]|11|$]]

The method you described does ONLY work, when the Generic Type is Set due to inheritance, because then its known during compile time:

<pre><code> public class SomeClass&lt;T&gt;{

 }

 public class SpecificClass extends SomeClass&lt;String&gt;{

 }
</code></pre>

For this example, you can use the method and you'll get back "String.class". 

If you are creating instances on the fly it won't work:

<pre><code>SomeClass s = new SomeClass&lt;String&gt;(); //wont work here.
</code></pre>

Some common work around is, to pass the actual class as a parameter for later reference:

<pre><code> public class SomeClass&lt;T&gt;{
 Class&lt;T&gt; clazz

 public SomeClass(Class&lt;T&gt; clazz){
 this.clazz = clazz;
 }

 public Clazz&lt;T&gt; getGenericClass(){
 return this.clazz;
 } 
 }
</code></pre>

usage:

<pre><code> SomeClass&lt;String&gt; someClass= new SomeClass&lt;String&gt;(String.class);

 System.out.println(someClass.getGenericClass()) //String.class
</code></pre>

Actually you don't even need the Generic type for such an scenario, because Java would do the same thing, as if you would handle the "T" as <code>Object</code>. Only advantage is, that you can define getter and Setter of T and don't need to typecast Objects all the time. (Because Java is doing that for you)
(It's called Type Erasure)

blocks|key|782505|text|javadoc会告诉您您在做什么。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|782506|Class#getGenericSuperclass()态|offset|length|style|CODE|782507|返回表示由该类表示的实体(类、接口、原始类型或空)的直接超类的类型。
如果超类是参数化类型，则返回的type对象必须准确地反映源代码中使用的实际类型参数。..。
|blockquote|782508|782509|ArrayList的直接超类是AbstractList。声明在源代码中是这样的。|782510|public+class+ArrayList<E>+extends+AbstractList<E>
++++implements+List<E>,+RandomAccess,+Cloneable,+java.io.Serializable|code-block|syntax|javascript|782511|因此，如果打印出它返回的Type对象，您将看到|782512|java.util.AbstractList<E>|782513|因此，ParameterizedType#getActualTypeArguments()表示|782514|返回表示此类型的实际类型参数的type对象数组。
|782515|782516|将返回Type|782517|E|782518|因为E是ArrayList类定义中使用的实际类型参数。|782519|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.oracle.com/javase/7/docs/api/java/lang/Class.html#getGenericSuperclass%2528%2529|1|http://docs.oracle.com/javase/7/docs/api/java/lang/reflect/ParameterizedType.html#getActualTypeArguments%2528%2529^0|0|0|S|0|S|0|0|0|0|0|9|F|C|0|0|C|4|0|0|3|16|3|16|1|0|0|0|3|4|0|0|2|1|4|9|0^^$0|@$1|2|3|4|5|6|7|1H|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|1I|8|@$D|1J|E|1K|F|G]]|9|@$D|1L|E|1M|1|1N]]|A|$]]|$1|H|3|I|5|J|7|1O|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|1P|8|@]|9|@]|A|$]]|$1|L|3|M|5|6|7|1Q|8|@$D|1R|E|1S|F|G]|$D|1T|E|1U|F|G]]|9|@]|A|$]]|$1|N|3|O|5|P|7|1V|8|@]|9|@]|A|$Q|R]]|$1|S|3|T|5|6|7|1W|8|@$D|1X|E|1Y|F|G]]|9|@]|A|$]]|$1|U|3|V|5|P|7|1Z|8|@]|9|@]|A|$Q|R]]|$1|W|3|X|5|6|7|20|8|@$D|21|E|22|F|G]]|9|@$D|23|E|24|1|25]]|A|$]]|$1|Y|3|Z|5|J|7|26|8|@]|9|@]|A|$]]|$1|10|3|-4|5|6|7|27|8|@]|9|@]|A|$]]|$1|11|3|12|5|6|7|28|8|@$D|29|E|2A|F|G]]|9|@]|A|$]]|$1|13|3|14|5|P|7|2B|8|@]|9|@]|A|$Q|R]]|$1|15|3|16|5|6|7|2C|8|@$D|2D|E|2E|F|G]|$D|2F|E|2G|F|G]]|9|@]|A|$]]|$1|17|3|-4|5|6|7|2H|8|@]|9|@]|A|$]]]|18|$19|$5|1A|1B|1C|A|$1D|1E]]|1F|$5|1A|1B|1C|A|$1D|1G]]]]

The javadoc will tell you what you are doing.

<a href="http://docs.oracle.com/javase/7/docs/api/java/lang/Class.html#getGenericSuperclass%28%29" rel="nofollow"><code>Class#getGenericSuperclass()</code></a> states

<blockquote>
 Returns the Type representing the direct superclass of the entity
 (class, interface, primitive type or void) represented by this Class. 
 
 If the superclass is a parameterized type, the Type object returned
 must accurately reflect the actual type parameters used in the source
 code. [...]
</blockquote>

The direct superclass of <code>ArrayList</code> is <code>AbstractList</code>. The declaration is as such in the source code

<pre><code>public class ArrayList&lt;E&gt; extends AbstractList&lt;E&gt;
 implements List&lt;E&gt;, RandomAccess, Cloneable, java.io.Serializable
</code></pre>

So if you print out the <code>Type</code> object returned by it, you will see 

<pre><code>java.util.AbstractList&lt;E&gt;
</code></pre>

and therefore <a href="http://docs.oracle.com/javase/7/docs/api/java/lang/reflect/ParameterizedType.html#getActualTypeArguments%28%29" rel="nofollow"><code>ParameterizedType#getActualTypeArguments()</code></a> which states

<blockquote>
 Returns an array of Type objects representing the actual type
 arguments to this type.
</blockquote>

will return the <code>Type</code>

<pre><code>E
</code></pre>

since <code>E</code> is the actual type argument used in the <code>ArrayList</code> class definition.

I have read <a href="https://stackoverflow.com/questions/1901164/get-type-of-a-generic-parameter-in-java-with-reflection">Get type of a generic parameter in Java with reflection</a> post and it made me wonder how that would be possible. I used the solution that someone posted and using the code

<pre><code>List&lt;Integer&gt; l = new ArrayList&lt;&gt;();
Class actualTypeArguments = (Class) ((ParameterizedType) l.getClass().getGenericSuperclass()).getActualTypeArguments()[0];
</code></pre>

This, however does not work for me, resulting in 

<pre><code>java.lang.ClassCastException: sun.reflect.generics.reflectiveObjects.TypeVariableImpl cannot be cast to java.lang.Class
</code></pre>

If I remove the class cast, the type of the actual argument is <code>E</code>, which is the type definition from List interface.

My question is, therefore, am I doing something wrong here? This behaviour is something I would have expected anyway, since the types are supposed to be erased during compile time, correct?

Java generics - obtaining actual type of generic parameter

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我读过的帖子，这让我想知道这怎么可能。我使用了有人发布的解决方案，并使用了代码List<Integer> l = new ArrayList<>();Class actualTypeArguments = (Class) ((ParameterizedType) l.getClass().getGenericSuper...

问Java泛型-获取实际类型的泛型参数
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