如何获得通用类型参数？

如何让代码打印类java.lang.Integer ？

我知道很多stackoverflow线程正在询问（和回答）这个问题。但是他们无法解决这个问题。

• 我不知道为什么有些人需要调用 getGenericSuperclass（） - 因为在这个简单情况下没有继承。
• 我不能将它转换为 ParameterizedType $ b $ p $
$ c> System.out.println（（ParameterizedType）_myObject.getClass（））; //编译错误：无法从Class< capture＃11-of？扩展TreeTest2.MyClass> to ParameterizedType System.out.println（（ParameterizedType）_myObject.getClass（）。getGenericSuperclass（））; //运行时异常：java.lang.ClassCastException

基于@ Thomas的指导，我发现了一种 解决方法 来获取类java.lang.Integer ; T> 在测试代码中。 （这很奇怪，为什么它只支持子类？）

@Test public final void test（） { MyClass< Integer> myObject = new MyClass< Integer>（）{}; //匿名子类 getParamType（myObject）; $ / code>

然后我们可以使用 getGenericSuperclass（） code>方法来获取类型然后将其转换为 ParameterizedType ，之后使用 getActualTypeArguments（）：

private static final< T> void getParamType（final MyClass< T> _myObject）{ System.out.println（（（ParameterizedType）_myObject.getClass（）。getGenericSuperclass（））。getActualTypeArguments（）[0]）; }

完美地打印类java.lang.Integer 。

这是不太好，因为测试代码应该模拟实际情况，用户最有可能将不会继续创建无意义的子类。

这种方法基于 TypeReference类。但我真的不知道如何使用它。我试过 class MyClass< T>扩展了TypeReference< T> 。但我仍然需要创建 MyClass< T> 的子类以使 TypeReference.getType（）打印 class java.lang.Integer 。

请帮忙，谢谢任何输入，因为最好的方法还没有出现。 基于上述解决方法的进一步问题：为什么只有匿名子类有效？

public static class SubMyClass< T>扩展MyClass< T> {} @Test public final void test（）{ MyClass< Integer> myObject = new MyClass< Integer>（）{}; //匿名子类 getParamType（myObject）; // class java.lang.Integer MyClass< Integer> mySubObject = new SubMyClass< Integer>（）; //命名的子类 getParamType（mySubObject）; // T

（ MyClass 和 getParamType（）不变。）

解决方案

因为Java故意无法做到这一点（类型擦除）。

解决方法称为超级类型令牌。还有一些关于此的线索（如这一个或那一个）。

Simply:

public static class MyClass<T> { // i don't want to keep an instance of T, if it is not necessary. // and it is not nice, not neat. // Or, let's say, the member are in the form of : ArrayList<T> mArrayList = new ArrayList<T>(); // the problem of getting the generic type parameter is still present. } @Test public final void test() { MyClass<Integer> myObject = new MyClass<Integer>(); getParamType( myObject ); } private static final <T> void getParamType(final MyClass<T> _myObject) { System.out.println(_myObject.getClass().getTypeParameters()[0]); // T System.out.println(((T) new Object()).getClass()); // class java.lang.Object }

How to let the code print class java.lang.Integer?

i know quite a few of stackoverflow threads are asking (and answering) about this. Yet they couldn't solve this question.

• i don't know why some need to call getGenericSuperclass() - as there is no inheritance involved in this simple case.
• And i can't cast it to ParameterizedType as well.

.

System.out.println((ParameterizedType) _myObject.getClass()); // Compile Error: Cannot cast from Class<capture#11-of ? extends TreeTest2.MyClass> to ParameterizedType System.out.println((ParameterizedType) _myObject.getClass().getGenericSuperclass()); // Runtime Exception: java.lang.ClassCastException

Based on @Thomas's guide, i have found a work-around way to get class java.lang.Integer.

First, we create an anonymous (it need to be anonymous) sub-class of MyClass<T> in the testing code. (Which is weird. Why it only support sub-classes?)

@Test public final void test() { MyClass<Integer> myObject = new MyClass<Integer>() {}; // Anonymous sub-class getParamType( myObject ); }

Then we can use the getGenericSuperclass() method to get a Type then cast it to ParameterizedType and afterwards uses getActualTypeArguments():

private static final <T> void getParamType(final MyClass<T> _myObject) { System.out.println( ((ParameterizedType) _myObject.getClass().getGenericSuperclass()).getActualTypeArguments()[0] ); }

It perfectly prints class java.lang.Integer.

This is not-so-good because the testing codes should simulate the actual situation, where users most likely won't keep creating meaningless sub-classes.

This approach is based on the idea of the TypeReference class. But i don't really know how to use it. I have tried class MyClass<T> extends TypeReference<T>. But i still have to create sub-class of MyClass<T> to have TypeReference.getType() prints class java.lang.Integer.

Please help, and thanks for any inputs, as the best approach is not here yet.

A further question based on the above workaround: Why only anonymous sub-class works?
public static class SubMyClass<T> extends MyClass<T>{} @Test public final void test() { MyClass<Integer> myObject = new MyClass<Integer>() {}; // Anonymous sub-class getParamType( myObject ); // class java.lang.Integer MyClass<Integer> mySubObject = new SubMyClass<Integer>(); // named sub-class getParamType( mySubObject ); // T }

(MyClass and getParamType() unchanged.)

解决方案

This is sort of difficult, because Java deliberately can't do that ("type erasure").

The work-around is called super type tokens. There are also some threads on SO about that (like this one or that one).