How do I build a Java type object at runtime from a generic type definition and runtime type parameters?

Question

Assuming a generic type declaration (Java)

class Foo {
    public T bar;
}

how can I, at runtime, instantiate a Type object that r

Answer 1

The usual way around type erasure is:

class Foo {

    Class clazz;

    public Foo(Class c) {
        clazz = c;
    }

    public T bar {
        return clazz.newInstance();
    }
}

If there's no no-args constructor for your T, you can do something fancier using reflection on the Class object; once you have an instance of Class, you can get an instance.

I faced exactly this problem too with gson. I ended up with this:

public class JsonWrapper {
    private String className;
    private String json; // the output of the gson.toJson() method
}

And when I needed to deserialise, I did a Class.forName(className) then I had all I needed to call the fromJson() method of the gson library.

I couldn't believe gson did not support this natively - it seems like such an obvious thing to want to do... get some json and turn that into an object without knowing which class it is beforehand.