How can instantiate Class<T> from T?

Question

I've got following method:

public <T> execute(HttpRequest request) {
   ...
   // in parseAs i have to pass Class<T> how can I instantiate it from T?
   request.execute().parseAs(classT);
}

PS: parseAs is method from google http client library .

Answer 1

You cannot with those parameters.

Java's generics use something called type erasure - basically all those T s become Object at runtime. So if you actually need to know what class this T is, you'll need a Class object to be passed in. This is exactly what parseAs is doing - to invoke parseAs<String> , you'd call parseAs(String.class) .

However, your execute has no Class parameter. As such, it has no idea what specialization it was invoked with, and cannot therefore pass that data on to parseAs .

The solution is to add a Class<T> parameter and punt to the next level up in the call chain, where the concrete type is (hopefully) known:

public <T> execute(Class<T> klass, HttpRequest request) {
   ...
   request.execute().parseAs(klass);
}