分配Class <>类型的变量以在Java中实现接口

Question

I want to create a variable of type Class , which implements some interface.

Like:

Class<List<String>> b = ArrayList.class;

But when i try something like this, it complains about incompatible types, since the generics is filled with the concrete class.

Is there a way to do this?

I want to do this to be type safe further down the road. I know that it will work when i don't care about type safety, but if i could, I would rather do it in a type safe manner.

Answer 1

You can actually use Class<List<String>> b = (Class)ArrayList.class; . It compiles, doesn't throw an exception and gives type safety.

The reason you can't just use Class<List<String>> b = ArrayList.class; is because ArrayList.class isn't a Class<List<String>> type, but rather a primitive class type. Casting it to Class removes the primitive and wildcard.

Answer 2

You get a type error here Class<List<String>> b = ArrayList.class; 'couse ArrayList.class is not Class>. Why? Let's see, Think about lists, in their implementatios they look like

public interface List<T> ...

So, when you create a List, you MUST tell her wich is his type inside, for example:

List<String> list1;
List<Integer> list2;

the original error is similar to:

List<String> list1 = new String("Hi");
List<Integer> list2 = new Integer(5);

The correct way, is telling both types, that creates a new concrete type:

List<String> list1 = new ArrayList<String>();
List<Integer> list2 = new ArrayList<Integer>();

list1 and list2, don't have the same type, although they both are List.

Finally, for your class "Class", the correct way is to do something like this:

import java.util.ArrayList;
import java.util.List;

public class Class<T> {

    private T v;

    public Class(T t){
        this.v = t;
    }

    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        Class<List<String>> a = new Class<List<String>>(list);
    }

}