Question

Is it possible to reflectively instantiate a generic type in Java? Using the technique described here I get an error because class tokens cannot be generic. Take the example below. I want to instantiate some subclass of Creator that implements Creator. The actual class name is passed in as a command line argument. The idea is to be able to specify an implementation of Creator at runtime. Is there another way to accomplish what I’m trying to do here?

public interface Creator<T> {     T create(); } public class StringCreator implements Creator<String> {     public String create() { return new String(); } } public class FancyStringCreator implements Creator<String> {     public String create() { return new StringBuffer().toString(); } } public static void main(String[] args) throws Exception {     Class<?> someClass = Class.forName(args[0]);     /*ERROR*/Class<? extends Creator<String>> creatorClass = someClass.asSubclass(Creator.class);     Constructor<? extends Creator<String>> creatorCtor = creatorClass.getConstructor((Class<?>[]) null);     Creator<String> creator = creatorCtor.newInstance((Object[]) null); } 

Edit: I like Marcus’ approach as being the most simple and pragmatic without circumventing the whole generics thing. I can use it in my situation because I can specify that the class passed must be a subclass of StringCreator. But as Ericson pointed out the generic information is still there at the type level, just not at the runtime level so it is still possible to reflectively examine whether a given class implements the correct generic type.

Answer 1

The generic information is lost in runtime. There is no runtime equivalent of a Creator<String>.class. You could create a type between Creator and StringCreator which fixes the generic type:

public interface Creator<T> {         T create(); } public interface StringCreator extends Creator<String> { } public class StringCreatorImpl implements StringCreator  {         public String create() { return new String(); } } public class FancyStringCreator implements StringCreator  {         public String create() { return new StringBuffer().toString(); } } public static void main(String[] args) throws Exception {         Class<?> someClass = Class.forName(args[0]);         Class<? extends StringCreator> creatorClass = someClass.asSubclass(StringCreator.class);         Constructor<? extends StringCreator> creatorCtor = creatorClass.getConstructor((Class<?>[]) null);         Creator<String> creator = creatorCtor.newInstance((Object[]) null); } 

But of course you lose a bit of flexibility, because you cannot use the following creator class:

public class AnotherCreator implements Creator<String> {     public String create() { return ''; } }