具有泛型类的构造函数未定义

Question

I'm stuck with a stupid problem I do not understand.

class Foo<T extends Collection<E>, E> {
  private Class<T> collectionClass;
  private Class<E> elementClass;

  public Foo(Class<T> collectionClass, Class<E> elementClass) {
       this.collectionClass = collectionClass;
       this.elementClass = elementClass;
  }
} 

When I try to run this

Foo<Collection<String>, String> foo = 
    new Foo<Collection<String>, String>(
        Collection.class,
        String.class);

I get a compiler error

java.lang.Error: Unresolved compilation problem: 
The constructor Foo<Collection<String>,String>(Class<Collection>, Class<String>) is undefined

Why ? If I erase generics, it's ok

Foo foo = 
    new Foo(
        Collection.class,
        String.class);

If somebody has an idea, it will be great and stop me to bang my head on wall.

Answer 1

There is no such thing as a Collection<String>.class , is the problem. Your alternatives include

• don't use reflection; why are you using reflection?
• accept the need for an unsafe cast
• use one of the various tools for referring to generic types at runtime, such as Guava's TypeToken

Answer 2

There's not much you can do about it. Just cast around it

new Foo<Collection<String>, String>(
    (Class<Collection<String>>)(Class<?>)Collection.class,
    String.class);

Answer 3

No luck, because of type erasure, see this question .

However a raw collection class and an element class may be used to generate a type safe Collection<String> object. So remove the redundance of E in Collection, as Collection.

Answer 4

For your use case, you can probably relax constructor parameter type

public Foo(Class<?> collectionClass, Class<E> elementClass) 
{
    this.collectionClass = (Class<T>)collectionClass;
    this.elementClass = elementClass;
}

Foo<Collection<String>, String> foo = new Foo<>(Collection.class, String.class);

There's less type checking, so the user must make sure a wrong class is not passed in, like

Foo<Set<String>, String> foo = new Foo<>(List.class, String.class);
    ^^^                                  ^^^^

Answer 5

Ok,

I have looked at other peoples' answers and not being the best at this...

In the Oreilly Tiger book (for version 5) (p167), they make a very explicit statement that a constructor CAN NOT have a wildcard in it's signature...