Is it possible to obtain an instance of Class>

Question

Is it possible to obtain an instance of Class<SomeGenericClass<SomeType>>?

For example an instance of Class<ArrayList<String>> without instantiating an instance of ArrayList<String>.

@SuppressWarnings("unchecked")
public static <T> T castToGeneric(Object objectToCast, T typeInferrenceClass) {
    return (T)objectToCast;
}

// usage

Object someObject = null /* initialised elsewhere */;
List<String> listOfStrings = castToGeneric(someObject, new ArrayList<String>());

// I want to avoid having to create - new ArrayList<String>();

I know this code is ugly and I shouldn't be doing this. But we are where we are...

Just to be clear, you are obtaining an instance of `Class`, the rest is just annotations for the compiler. — Marko Topolnik, May 16 '12 at 09:54
List is an interface, not a class. You can only get a Class from an ArrayList, for example. — Robert Kovačević, May 16 '12 at 09:58
@RobertKovačević False. Every type in Java has its `Class` instance, even `int.class`. — Marko Topolnik, May 16 '12 at 10:01
@loudsight, sorry it seems that you cant call .class on generic type. I've added an answer for you. — dhblah, May 16 '12 at 10:13
maybe Collections.emptyList(); will help? (you won't instantiate additional objects). But it internally makes a class cast. — dhblah, May 16 '12 at 10:51

Subhrajyoti Majumder · Answer 1 · 2012-05-16T10:16:47.613

1

If want to instantiating Class<SomeGenericClass<SomeType>>

Class<List<Object>> list = (Class<List<Object>>)(Class<?>)List.class;

Type genericclass = new ArrayList<Object>() {}.getClass().getGenericSuperclass();

edited May 16 '12 at 10:16

answered May 16 '12 at 09:55

Subhrajyoti Majumder

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Please illustrate how do you create "theClass" parameter passed into your method. When T is List? – auser May 16 '12 at 09:56
OP specifically asked for a solution **without instantiating** and your first line emits unchecked cast warnings. I kind of think OP is already aware of half-assed solutions and wants a clean one. – Marko Topolnik May 16 '12 at 10:38
2

There can *never* be a clean solution, due to type erasure. There is not really any such thing as a `Class>`; the best thing you can do is cast a `Class` to it. – Louis Wasserman May 16 '12 at 11:15
@LouisWasserman What does this have to do with type erasure? `Class>` is a legitimate type and, for example, the return type of `newInstance` for such a var is `List`. The only issue is how to cast the `List.class` literal to it. – Marko Topolnik May 16 '12 at 16:04
2

A `Class` is a runtime representation of a type, and at runtime, there are no generic types. Any `Class>` *must* satisfy `listStringClass.equals(List.class)`. See e.g. http://stackoverflow.com/questions/71585/how-to-get-parametrized-class-instance – Louis Wasserman May 16 '12 at 18:25

dhblah · Accepted Answer · 2012-05-16T11:09:42.253

1

I found this workaround to your problem:

@SuppressWarnings("unchecked")
public static <T> T castToGeneric(Object objectToCast) {
    return (T)objectToCast;
}

// usage

Object someObject = null /* initialised elsewhere */;
Class<ArrayList<String>> listOfStrings = <Class<ArrayList<String>>>castToGeneric(someObject);

so instead of passing type as a parameter, you're passing it as a type-parameter.

edited May 16 '12 at 11:09

answered May 16 '12 at 10:11

dhblah

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score 0 · Answer 3 · answered May 16 '12 at 10:04

0

Since type erasure in Java generic, result of call "getClass" on instance of "List<String>", would lose type argument "String", in another word, it might be impossible to get "Class<List<String>>". But you can do it in Scala:

scala> List("foo", "bar").getClass
res0: java.lang.Class[_ <: List[java.lang.String]] = class scala.collection.immutable.$colon$colon

answered May 16 '12 at 10:04

xnnyygn

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But say OP wants to assign to a variable `Class> c` – Marko Topolnik May 16 '12 at 10:05

score 0 · Answer 4 · answered May 16 '12 at 18:46

This satisfies your "no-instance" criterion, compiles and runs OK:

Class<ArrayList<String>> c =
    (Class<ArrayList<String>>) Class.forName("java.util.ArrayList");

This works due to type erasure: At runtime, ArrayList<String> is just ArrayList, and further Class<ArrayList<String>> is just Class.

Is it possible to obtain an instance of Class>

4 Answers4