Can I turn a type parameter into a Class in Java?

Question

I have a server that can return a few different kinds of objects, and I'm using GSON's fromJson to try and deal with it. So I want to write a method that sends a GET to the server and parses the result as an object of the right type. Basically I want to be able to do

Foo foo = getStuff("foo_url");

or

Bar[] bars = getStuff("bar_url");

and have it just work. So here's what I got so far (the "get" function is defined elsewhere and sends an HTTP GET request to the given URL and returns the response):

<T> T getStuff(String url) throws IOException {
    CloseableHttpResponse response = get(url);
    String body = EntityUtils.toString(response.getEntity());
    response.close();
    return gson.fromJson(body, /* ???? */);
}

So I need to replace the /* ???? */ with a Class object that represents T so that getStuff will give me back a T. It didn't work to just put T in there, nor did new Class<T>().class, so I'm out of hacks.

Can I do this?

`getStuff("foo_url", Foo.class)`, `getStuff("bar_url", Bar[].class)` — Sotirios Delimanolis, May 23 '15 at 04:28
Yeah but that's ugly and I want to do it with one parameter to getStuff. — Inquisitive Idiot, May 23 '15 at 04:29
That's not possible. You need an extra hint, and you can't get that with generic type arguments. — Sotirios Delimanolis, May 23 '15 at 04:30
That's pretty lame. Hopefully we'll both learn something today. — Inquisitive Idiot, May 23 '15 at 04:31
Sotirios is right, you need the extra argument. The actual value of the generic type parameter is not known at runtime, it only exists at compile time. — Erwin Bolwidt, May 23 '15 at 04:36
Maybe it would help to take a look at how it's done in https://github.com/balanced/balanced-java, specifically around https://github.com/balanced/balanced-java/blob/777c94c5cabe32427f977e94dfa0d67604c38e73/src/main/java/com/balancedpayments/core/Client.java#L197 — Remear, May 23 '15 at 04:38

score 2 · Accepted Answer · edited May 23 '17 at 11:43

2

Nope. By the time the method is executed, the type parameter has been erased. No information about type parameters is stored at runtime or baked into the bytecode at compile time; for example, the compiler does not secretly add an extra argument to your method and pass in Foo.class at the call site. You have to pass the Class yourself.

edited May 23 '17 at 11:43

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answered May 23 '15 at 04:36

user2357112

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Can I turn a type parameter into a Class in Java?

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