blocks|key|1382542|text|我曾经偶然发现的一个结构看起来像|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1382543|Class<T>+persistentClass+=+(Class<T>)
+++((ParameterizedType)getClass().getGenericSuperclass())
++++++.getActualTypeArguments()[0];|code-block|syntax|javascript|1382544|所以似乎有一些反射魔法，不幸的是我不能完全理解…抱歉的。|1382545|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

One construct, I once stumbled upon looked like

<pre><code>Class&lt;T&gt; persistentClass = (Class&lt;T&gt;)
 ((ParameterizedType)getClass().getGenericSuperclass())
 .getActualTypeArguments()[0];
</code></pre>

So there seems to be some reflection-magic around that I unfortunetly don't fully understand... Sorry.

blocks|key|1382650|text|实际上，我让它起作用了。考虑以下代码片段：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1382651|Method+m;
Type[]+genericParameterTypes+=+m.getGenericParameterTypes();
for+(int+i+=+0;+i+<+genericParameterTypes.length;+i%2B%2B)+{
+++++if(+genericParameterTypes[i]+instanceof+ParameterizedType+)+{
++++++++++++++++Type[]+parameters+=+((ParameterizedType)genericParameterTypes[i]).getActualTypeArguments();
//parameters[0]+contains+java.lang.String+for+method+like+"method(List<String>+value)"

+++++}
+}|code-block|syntax|javascript|1382652|我使用的是jdk+1.6|1382653|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

Actually I got this to work. Consider the following snippet:

<pre><code>Method m;
Type[] genericParameterTypes = m.getGenericParameterTypes();
for (int i = 0; i &lt; genericParameterTypes.length; i++) {
 if( genericParameterTypes[i] instanceof ParameterizedType ) {
 Type[] parameters = ((ParameterizedType)genericParameterTypes[i]).getActualTypeArguments();
//parameters[0] contains java.lang.String for method like "method(List&lt;String&gt; value)"

 }
 }
</code></pre>

I'm using jdk 1.6

blocks|key|1382448|text|不，那不可能。由于向下兼容的问题，Java的泛型是基于type+erasure的。在运行时，您所拥有的只是一个非通用的List对象。在运行时有一些关于类型参数的信息，但它驻留在类定义中(例如，你可以问"what+generic+type+does+this+field's+definition+use?，而不是在对象实例中。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1382449|entityMap|0|LINK|mutability|MUTABLE|url|http://java.sun.com/docs/books/tutorial/java/generics/erasure.html|1|http://java.sun.com/javase/6/docs/api/java/lang/reflect/Field.html#getGenericType(%2529)^0|1N|4|R|C|0|2T|1F|1|0^^$0|@$1|2|3|4|5|6|7|P|8|@$9|Q|A|R|B|C]]|D|@$9|S|A|T|1|U]|$9|V|A|W|1|X]]|E|$]]|$1|F|3|-4|5|6|7|Y|8|@]|D|@]|E|$]]]|G|$H|$5|I|J|K|E|$L|M]]|N|$5|I|J|K|E|$L|O]]]]

Nope, that is not possible. Due to downwards compatibility issues, Java's generics are based on <a href="http://java.sun.com/docs/books/tutorial/java/generics/erasure.html" rel="noreferrer">type erasure</a>, i.a. at runtime, all you have is a non-generic <code>List</code> object. There is some information about type parameters at runtime, but it resides in class definitions (i.e. you can ask "<a href="http://java.sun.com/javase/6/docs/api/java/lang/reflect/Field.html#getGenericType()" rel="noreferrer">what generic type does this field's definition use?</a>"), not in object instances.

blocks|key|1167674|text|由于类型擦除，了解列表类型的唯一方法是将类型作为参数传递给方法：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1167675|public+class+Main+{

++++public+static+void+main(String[]+args)+{
++++++++doStuff(new+LinkedList<String>(),+String.class);

++++}

++++public+static+<E>+void+doStuff(List<E>+list,+Class<E>+clazz)+{

++++}

}|code-block|syntax|javascript|1167676|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Because of type erasure the only way to know the type of the list would be to pass in the type as a parameter to the method:

<pre><code>public class Main {

 public static void main(String[] args) {
 doStuff(new LinkedList&lt;String&gt;(), String.class);

 }

 public static &lt;E&gt; void doStuff(List&lt;E&gt; list, Class&lt;E&gt; clazz) {

 }

}
</code></pre>

blocks|key|1382698|text|正如@bertolami所指出的，我们不可能使用一个变量类型并获得它的未来值(+typeOfList变量的内容)。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1382699|但是，您可以将类作为参数传递给它，如下所示：|1382700|public+final+class+voodoo+{
++++public+static+void+chill(List<T>+aListWithTypeSpiderMan,+Class<T>+clazz)+{
++++++++//+Here+I'd+like+to+get+the+Class-Object+'SpiderMan'
++++++++Class+typeOfTheList+=+clazz;
++++}

++++public+static+void+main(String...+args)+{
++++++++chill(new+List<SpiderMan>(),+Spiderman.class+);
++++}
}|code-block|syntax|javascript|1382701|当你必须将一个类变量传递给ActivityInstrumentationTestCase2的构造函数时，这或多或少就是谷歌所做的。|offset|length|1382702|entityMap|0|LINK|mutability|MUTABLE|url|http://developer.android.com/reference/android/test/ActivityInstrumentationTestCase2.html^0|0|0|0|D|W|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|V|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|W|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|X|8|@]|9|@$K|Y|L|Z|1|10]]|A|$]]|$1|M|3|-4|5|6|7|11|8|@]|9|@]|A|$]]]|N|$O|$5|P|Q|R|A|$S|T]]]]

As pointed out by @bertolami, it's not possible to us a variable type and get its future value (the content of typeOfList variable).

Nevertheless, you can pass the class as parameter on it like this:

<pre><code>public final class voodoo {
 public static void chill(List&lt;T&gt; aListWithTypeSpiderMan, Class&lt;T&gt; clazz) {
 // Here I'd like to get the Class-Object 'SpiderMan'
 Class typeOfTheList = clazz;
 }

 public static void main(String... args) {
 chill(new List&lt;SpiderMan&gt;(), Spiderman.class );
 }
}
</code></pre>

That's more or less what Google does when you have to pass a class variable to the constructor of <a href="http://developer.android.com/reference/android/test/ActivityInstrumentationTestCase2.html" rel="nofollow">ActivityInstrumentationTestCase2</a>.

blocks|key|1382393|text|不不可能。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1382394|如果一个类是该规则的唯一例外，你可以得到一个字段的泛型类型，甚至这也有点麻烦。|1382395|有关这方面的示例，请参阅Knowing+type+of+generic+in+Java。|offset|length|1382396|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/1886677/knowing-type-of-generic-in-java/1886728#1886728^0|0|0|C|V|0|0^^$0|@$1|2|3|4|5|6|7|P|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Q|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|R|8|@]|9|@$F|S|G|T|1|U]]|A|$]]|$1|H|3|-4|5|6|7|V|8|@]|9|@]|A|$]]]|I|$J|$5|K|L|M|A|$N|O]]]]

No it isn't possible.

You can get a generic type of a field given a class is the only exception to that rule and even that's a bit of a hack.

See <a href="https://stackoverflow.com/questions/1886677/knowing-type-of-generic-in-java/1886728#1886728">Knowing type of generic in Java</a> for an example of that.

blocks|key|1167919|text|您可以使用反射来获取泛型参数的类型，如我找到here的示例中所示|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1167920|import+java.lang.reflect.ParameterizedType;
import+java.lang.reflect.Type;

public+class+Home<E>+{
++++@SuppressWarnings+("unchecked")
++++public+Class<E>+getTypeParameterClass(){
++++++++Type+type+=+getClass().getGenericSuperclass();
++++++++ParameterizedType+paramType+=+(ParameterizedType)+type;
++++++++return+(Class<E>)+paramType.getActualTypeArguments()[0];
++++}

++++private+static+class+StringHome+extends+Home<String>{}
++++private+static+class+StringBuilderHome+extends+Home<StringBuilder>{}
++++private+static+class+StringBufferHome+extends+Home<StringBuffer>{}+++

++++/**
+++++*+This+prints+"String",+"StringBuilder"+and+"StringBuffer"
+++++*/
++++public+static+void+main(String[]+args)+throws+InstantiationException,+IllegalAccessException+{
++++++++Object+object0+=+new+StringHome().getTypeParameterClass().newInstance();
++++++++Object+object1+=+new+StringBuilderHome().getTypeParameterClass().newInstance();
++++++++Object+object2+=+new+StringBufferHome().getTypeParameterClass().newInstance();
++++++++System.out.println(object0.getClass().getSimpleName());
++++++++System.out.println(object1.getClass().getSimpleName());
++++++++System.out.println(object2.getClass().getSimpleName());
++++}
}|code-block|syntax|javascript|1167921|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/a/3635805/3138238^0|M|4|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@$A|R|B|S|1|T]]|C|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|V|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]]]

You can get the type of a generic parameter with reflection like in this example that I found <a href="https://stackoverflow.com/a/3635805/3138238">here</a>:

<pre><code>import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;

public class Home&lt;E&gt; {
 @SuppressWarnings ("unchecked")
 public Class&lt;E&gt; getTypeParameterClass(){
 Type type = getClass().getGenericSuperclass();
 ParameterizedType paramType = (ParameterizedType) type;
 return (Class&lt;E&gt;) paramType.getActualTypeArguments()[0];
 }

 private static class StringHome extends Home&lt;String&gt;{}
 private static class StringBuilderHome extends Home&lt;StringBuilder&gt;{}
 private static class StringBufferHome extends Home&lt;StringBuffer&gt;{} 

 /**
 * This prints "String", "StringBuilder" and "StringBuffer"
 */
 public static void main(String[] args) throws InstantiationException, IllegalAccessException {
 Object object0 = new StringHome().getTypeParameterClass().newInstance();
 Object object1 = new StringBuilderHome().getTypeParameterClass().newInstance();
 Object object2 = new StringBufferHome().getTypeParameterClass().newInstance();
 System.out.println(object0.getClass().getSimpleName());
 System.out.println(object1.getClass().getSimpleName());
 System.out.println(object2.getClass().getSimpleName());
 }
}
</code></pre>

blocks|key|1168003|text|这是另一个窍门。使用泛型vararg数组|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1168004|import+java.util.ArrayList;

class+TypedArrayList<E>+extends+ArrayList<E>
{
++++@SafeVarargs
++++public+TypedArrayList+(E...+typeInfo)
++++{
++++++++//+Get+generic+type+at+runtime+...
++++++++System.out.println+(typeInfo.getClass().getComponentType().getTypeName());
++++}
}

public+class+GenericTest
{
++++public+static+void+main+(String[]+args)
++++{
++++++++//+No+need+to+supply+the+dummy+argument
++++++++ArrayList<Integer>+ar1+=+new+TypedArrayList<>+();
++++++++ArrayList<String>+ar2+=+new+TypedArrayList<>+();
++++++++ArrayList<?>+ar3+=+new+TypedArrayList<>+();
++++}
}|code-block|syntax|javascript|1168005|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Here is another trick. Use a generic vararg array

<pre><code>import java.util.ArrayList;

class TypedArrayList&lt;E&gt; extends ArrayList&lt;E&gt;
{
 @SafeVarargs
 public TypedArrayList (E... typeInfo)
 {
 // Get generic type at runtime ...
 System.out.println (typeInfo.getClass().getComponentType().getTypeName());
 }
}

public class GenericTest
{
 public static void main (String[] args)
 {
 // No need to supply the dummy argument
 ArrayList&lt;Integer&gt; ar1 = new TypedArrayList&lt;&gt; ();
 ArrayList&lt;String&gt; ar2 = new TypedArrayList&lt;&gt; ();
 ArrayList&lt;?&gt; ar3 = new TypedArrayList&lt;&gt; ();
 }
}
</code></pre>

blocks|key|1167447|text|这是不可能的，因为Java中的泛型只在编译时被考虑。因此，Java泛型只是某种预处理器。但是，您可以获得列表成员的实际类。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1167448|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

This is impossible because generics in Java are only considered at compile time. Thus, the Java generics are just some kind of pre-processor. However you can get the actual class of the members of the list.

blocks|key|1167793|text|不能从变量中获取泛型参数。但您可以通过方法或字段声明：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1167794|Method+method+=+getClass().getDeclaredMethod("chill",+List.class);
Type[]+params+=+method.getGenericParameterTypes();
ParameterizedType+firstParam+=+(ParameterizedType)+params[0];
Type[]+paramsOfFirstGeneric+=+firstParam.getActualTypeArguments();|code-block|syntax|javascript|1167795|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

You cannot get a generic parameter from a variable. But you can from a method or field declaration:

<pre><code>Method method = getClass().getDeclaredMethod("chill", List.class);
Type[] params = method.getGenericParameterTypes();
ParameterizedType firstParam = (ParameterizedType) params[0];
Type[] paramsOfFirstGeneric = firstParam.getActualTypeArguments();
</code></pre>

blocks|key|1382801|text|对于我来说，阅读这段代码很难，我只是把它分成了两行：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1382802|//+assuming+that+the+Generic+Type+parameter+is+of+type+"T"
ParameterizedType+p+=+(ParameterizedType)+getClass().getGenericSuperclass();
Class<T>+c+=(Class<T>)p.getActualTypeArguments()[0];|code-block|syntax|javascript|1382803|我想创建一个Type参数的实例，但我的方法没有任何参数：|1382804|publc+T+getNewTypeInstance(){
++++ParameterizedType+p+=+(ParameterizedType)+getClass().getGenericSuperclass();
++++Class<T>+c+=(Class<T>)p.getActualTypeArguments()[0];

++++//+for+me+i+wanted+to+get+the+type+to+create+an+instance
++++//+from+the+no-args+default+constructor
++++T+t+=+null;
++++try{
++++++++t+=+c.newInstance();
++++}catch(Exception+e){
++++++++//+no+default+constructor+available
++++}
++++return+t;
}|1382805|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

Just for me reading this snippet of code was hard, I just divided it into 2 readable lines :

<pre><code>// assuming that the Generic Type parameter is of type "T"
ParameterizedType p = (ParameterizedType) getClass().getGenericSuperclass();
Class&lt;T&gt; c =(Class&lt;T&gt;)p.getActualTypeArguments()[0];
</code></pre>

I wanted to create an instance of the Type parameter without having any parameters to my method :

<pre><code>publc T getNewTypeInstance(){
 ParameterizedType p = (ParameterizedType) getClass().getGenericSuperclass();
 Class&lt;T&gt; c =(Class&lt;T&gt;)p.getActualTypeArguments()[0];

 // for me i wanted to get the type to create an instance
 // from the no-args default constructor
 T t = null;
 try{
 t = c.newInstance();
 }catch(Exception e){
 // no default constructor available
 }
 return t;
}
</code></pre>

blocks|key|1382920|text|我已经为期望接受或返回Iterable<?...>的方法编写了这个代码。代码如下：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1382921|/**
+*+Assuming+the+given+method+returns+or+takes+an+Iterable<T>,+this+determines+the+type+T.
+*+T+may+or+may+not+extend+WindupVertexFrame.
+*/
private+static+Class+typeOfIterable(Method+method,+boolean+setter)
{
++++Type+type;
++++if+(setter)+{
++++++++Type[]+types+=+method.getGenericParameterTypes();
++++++++//+The+first+parameter+to+the+method+expected+to+be+Iterable<...>+.
++++++++if+(types.length+==+0)
++++++++++++throw+new+IllegalArgumentException("Given+method+has+0+params:+"+%2B+method);
++++++++type+=+types[0];
++++}
++++else+{
++++++++type+=+method.getGenericReturnType();
++++}

++++//+Now+get+the+parametrized+type+of+the+generic.
++++if+(!(type+instanceof+ParameterizedType))
++++++++throw+new+IllegalArgumentException("Given+method's+1st+param+type+is+not+parametrized+generic:+"+%2B+method);
++++ParameterizedType+pType+=+(ParameterizedType)+type;
++++final+Type[]+actualArgs+=+pType.getActualTypeArguments();
++++if+(actualArgs.length+==+0)
++++++++throw+new+IllegalArgumentException("Given+method's+1st+param+type+is+not+parametrized+generic:+"+%2B+method);

++++Type+t+=+actualArgs[0];
++++if+(t+instanceof+Class)
++++++++return+(Class<?>)+t;

++++if+(t+instanceof+TypeVariable){
++++++++TypeVariable+tv+=++(TypeVariable)+actualArgs[0];
++++++++AnnotatedType[]+annotatedBounds+=+tv.getAnnotatedBounds();///
++++++++GenericDeclaration+genericDeclaration+=+tv.getGenericDeclaration();///
++++++++return+(Class)+tv.getAnnotatedBounds()[0].getType();
++++}

++++throw+new+IllegalArgumentException("Unknown+kind+of+type:+"+%2B+t.getTypeName());
}|code-block|syntax|javascript|1382922|entityMap^0|B|E|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|P|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|L|$]]

I've coded this for methods which expect to accept or return <code>Iterable&lt;?...&gt;</code>. Here is the code:

<pre><code>/**
 * Assuming the given method returns or takes an Iterable&lt;T&gt;, this determines the type T.
 * T may or may not extend WindupVertexFrame.
 */
private static Class typeOfIterable(Method method, boolean setter)
{
 Type type;
 if (setter) {
 Type[] types = method.getGenericParameterTypes();
 // The first parameter to the method expected to be Iterable&lt;...&gt; .
 if (types.length == 0)
 throw new IllegalArgumentException("Given method has 0 params: " + method);
 type = types[0];
 }
 else {
 type = method.getGenericReturnType();
 }

 // Now get the parametrized type of the generic.
 if (!(type instanceof ParameterizedType))
 throw new IllegalArgumentException("Given method's 1st param type is not parametrized generic: " + method);
 ParameterizedType pType = (ParameterizedType) type;
 final Type[] actualArgs = pType.getActualTypeArguments();
 if (actualArgs.length == 0)
 throw new IllegalArgumentException("Given method's 1st param type is not parametrized generic: " + method);

 Type t = actualArgs[0];
 if (t instanceof Class)
 return (Class&lt;?&gt;) t;

 if (t instanceof TypeVariable){
 TypeVariable tv = (TypeVariable) actualArgs[0];
 AnnotatedType[] annotatedBounds = tv.getAnnotatedBounds();///
 GenericDeclaration genericDeclaration = tv.getGenericDeclaration();///
 return (Class) tv.getAnnotatedBounds()[0].getType();
 }

 throw new IllegalArgumentException("Unknown kind of type: " + t.getTypeName());
}
</code></pre>

blocks|key|1382758|text|使用：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1382759|Class<?>+typeOfTheList+=+aListWithTypeSpiderMan.toArray().getClass().getComponentType();|code-block|syntax|javascript|1382760|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Use:

<pre><code>Class&lt;?&gt; typeOfTheList = aListWithTypeSpiderMan.toArray().getClass().getComponentType();
</code></pre>

Is it possible to get the type of a generic parameter?

An example:

<pre><code>public final class Voodoo {
 public static void chill(List&lt;?&gt; aListWithTypeSpiderMan) {
 // Here I'd like to get the Class-Object 'SpiderMan'
 Class typeOfTheList = ???;
 }

 public static void main(String... args) {
 chill(new ArrayList&lt;SpiderMan&gt;());
 }
}
</code></pre>

Get type of a generic parameter in Java with reflection

可以获得泛型参数的类型吗？举个例子：public final class Voodoo { public static void chill(List<?> aListWithTypeSpiderMan) { // Here I'd like to get the Class-Object 'SpiderMan' Class typeOfTheList = ???

问在带有反射的Java中获取泛型参数的类型
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回答 13

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