Java: how do I get a class literal from a generic type?
You can manage it with a double cast :
@SuppressWarnings("unchecked")
Class<List<Foo>> cls = (Class<List<Foo>>)(Object)List.class
There are no Class literals for parameterized types, however there are Type objects that correctly define these types.
See java.lang.reflect.ParameterizedType - http://java.sun.com/j2se/1.5.0/docs/api/java/lang/reflect/ParameterizedType.html
Google's Gson library defines a TypeToken class that allows to simply generate parameterized types and uses it to spec json objects with complex parameterized types in a generic friendly way. In your example you would use:
Type typeOfListOfFoo = new TypeToken<List<Foo>>(){}.getType()
I intended to post links to the TypeToken and Gson classes javadoc but Stack Overflow won't let me post more than one link since I'm a new user, you can easily find them using Google search
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 literalString.class
denotes theClass
object that represents the typeString
and is identical to theClass
object that is returned when methodgetClass
is invoked on aString
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
andList<?>.class
, since no suchClass
objects exist. Only the raw typeList
has aClass
object that represents its runtime type. It is referred to asList.class
.