Java 如何从泛型类型参数获取`.class` 属性？
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How do I get the `.class` attribute from a generic type parameter?
提问by John Gowers
The accepted answer to this questiondescribes how to create an instance of Tin the Generic<T>class. This involves passing in a Class<T>parameter to the Genericconstructor and callin the newInstancemethod from that.
这个问题的公认答案描述了如何T在Generic<T>类中创建一个实例。这涉及将Class<T>参数传递给Generic构造函数并newInstance从中调用方法。
A new instance of Generic<Bar>is then created, and the parameter Bar.classis passed in.
Generic<Bar>然后创建一个新实例，并Bar.class传入参数。
What do you do if the generic type parameter for the new Genericclass is not some known class like Barbut is itself a generic type parameter? Suppose I had some other class Skeet<J>and I wanted to create a new instance of Generic<J>from inside that class. Then, if I try to pass in J.classI get the following compiler error:
如果新Generic类的泛型类型参数不是某个已知类，Bar但它本身就是泛型类型参数，你会怎么做？假设我有一些其他类，Skeet<J>并且我想Generic<J>从该类内部创建一个新实例。然后，如果我尝试传入，则会J.class出现以下编译器错误：
cannot select from a type variable.
Is there any way around this?
有没有办法解决？
The specific bit of code triggering the error for me is:
对我来说触发错误的特定代码是：
public class InputField<W extends Component & WidgetInterface>
extends InputFieldArray<W>
{
public InputField(String labelText)
{
super(new String[] {labelText}, W.class);
}
/* ... */
}
public class InputFieldArray<W extends Component & WidgetInterface>
extends JPanel
{
/* ... */
public InputFieldArray(String[] labelText, Class<W> clazz)
throws InstantiationException, IllegalAccessException
{
/* ... */
for (int i = 0 ; i < labelText.length ; i++) {
newLabel = new JLabel(labelText[i]);
newWidget = clazz.newInstance();
/* ... */
}
/* ... */
}
/* ... */
}
The error happens, because I can't write W.class. Is there some other way of passing in the same information?
错误发生了，因为我不能写W.class. 是否有其他方式传递相同的信息？
采纳答案by Paul Bellora
Using .classon a type parameter isn't allowed - because of type erasure, Wwill have been erasedto Componentat runtime. InputFieldwill need to also take a Class<W>from the caller, like InputFieldArray:
使用.class一个类型参数是不允许的-因为类型擦除，W将被清除，以Component在运行时。InputField还需要Class<W>从调用者那里获取 a ，例如InputFieldArray：
public InputField(String labelText, Class<W> clazz)
{
super(new String[] {labelText}, clazz);
}
回答by nanofarad
W may not be available due to type erasure. You should require that a Class<W>is passed into the method. You get a class object and its generic ensures that only Wand no subclass is passed in, due to covariance.
由于类型擦除，W 可能不可用。您应该要求将 aClass<W>传递到方法中。你得到一个类对象，它的泛型确保只有W和没有子类被传入，由于协方差。
public InputField(String labelText, Class<W> cls)
{
super(new String[] {labelText}, cls);
}
will take W.classbut not WSubtype.class.
会W.class但不会WSubtype.class。
回答by jdex
If you're using the GSONlibrary, you can get the type of Teasily by using TypeToken. The class documentation is available here:
如果您正在使用该GSON库，则可以T通过使用TypeToken. 类文档可在此处获得：
I did it like this:
我是这样做的：
This is my class definition:
这是我的类定义：
public class ManagerGeneric <T> {}
This is in my method:
这是我的方法：
// Get the type of generic parameter
Type typeOfT = new TypeToken<T>(){}.getType();
// Deserialize
T data = gson.fromJson(json, typeOfT);
