Question

In C# I can do actually this:

//This is C# static T SomeMethod<T>() where T:new() {   Console.WriteLine('Typeof T: '+typeof(T));   return new T(); }  //And call the method here SomeMethod<SomeClassName>(); 

But for some reason I can’t get it to work in Java.

The thing I want to do is, to create a static method on a superclass, so the subclasses can be converted to XML.

//This is Java, but doesn't work public static T fromXml<T>(String xml) {   try {     JAXBContext context = JAXBContext.newInstance(T.class);     Unmarshaller um = context.createUnmarshaller();     return (T)um.unmarshal(new StringReader(xml));   } catch (JAXBException je) {     throw new RuntimeException('Error interpreting XML response', je);   } }  //Also the call doesn't work... fromXml<SomeSubObject>('<xml/>'); 

Answer 1

public static <T> T fromXml(Class<T> clazz, String xml) { 

Called as:

Thing thing = fromXml(Thing.class, xml); 

or more explicitly:

Thing thing = MyClass.<Thing>fromXml(Thing.class, xml); 

To be even more confusing you can have constructors that both construct a generic type and have a generic parameter themselves. Can’t remember the syntax and have never seen it used in anger (you are probably better off with a static creation method anyway).

The cast (T) is unsafe, and you can’t write T.class. So include the T.class as an argument (as JAXBContext.newInstance does) and throw a relevant exception if the type is wrong.

public static <T> T fromXml(Class<T> clazz, String xml) {     try {         JAXBContext context = JAXBContext.newInstance(clazz);         Unmarshaller um = context.createUnmarshaller();         Object obj = um.unmarshal(new StringReader(xml));         try {             return clazz.cast(obj);         } catch (ClassCastException exc) {              throw new RelevantException(                  'Expected class '+clazz+                   ' but was '+obj.getClass()              );         }     } catch (JAXBException exc) {         throw new RelevantException(             'Error unmarshalling XML response',             exc          );     } } 

I believe the next version of JAXB (in 6u14?) has some convenience methods for this sort of thing in the JAXB class.