Tom Tom - 23 days ago 16
Java Question

Java: how do I get a class literal from a generic type?

Typically, I've seen people use the class literal like this:

Class<Foo> cls = Foo.class;


But what if the type is generic, e.g. List? This works fine, but has a warning since List should be parameterized:

Class<List> cls = List.class


So why not add a
<?>
? Well, this causes a type mismatch error:

Class<List<?>> cls = List.class


I figured something like this would work, but this is just a plain ol' a syntax error:

Class<List<Foo>> cls = List<Foo>.class


How can I get a
Class<List<Foo>>
statically, e.g. using the class literal?

I could use
@SuppressWarnings("unchecked")
to get rid of the warnings caused by the non-parameterized use of List in the first example,
Class<List> cls = List.class
, but I'd rather not.

Any suggestions?

Answer

You can't due to type erasure.

Java generics are little more than syntactic sugar for Object casts. To demonstrate:

List<Integer> list1 = new ArrayList<Integer>();
List<String> list2 = (List<String>)list1;
list2.add("foo"); // perfectly legal

The only instance where generic type information is retained at runtime is with Field.getGenericType() if interrogating a class's members via reflection.

All of this is why Object.getClass() has this signature:

public final native Class<?> getClass();

The important part being Class<?>.

To put it another way, from the Java Generics FAQ:

Why is there no class literal for concrete parameterized types?

Because parameterized type has no exact runtime type representation.

A class literal denotes a Class object that represents a given type. For instance, the class literal String.class denotes the Class object that represents the type String and is identical to the Class object that is returned when method getClass is invoked on a String object. A class literal can be used for runtime type checks and for reflection.

Parameterized types lose their type arguments when they are translated to byte code during compilation in a process called type erasure . As a side effect of type erasure, all instantiations of a generic type share the same runtime representation, namely that of the corresponding raw type . In other words, parameterized types do not have type representation of their own. Consequently, there is no point in forming class literals such as List<String>.class , List<Long>.class and List<?>.class , since no such Class objects exist. Only the raw type List has a Class object that represents its runtime type. It is referred to as List.class.

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