Java类类作为泛型无法正常工作

Question

A simple question about Class and its generic semantics. Why doesn't the following code work?

Class<Serializable> s = String.class;

Java tells me that Class<Serializable> and Class<String> are incompatible. How can they be, when String is implementing Serializable ?

Shouldn't generics allow exactly this type of things?

Answer 1

No, generics explicitly don't allow this kind of thing. The classic example is a collection class of some kind. If ArrayList<String> was a subclass of ArrayList<Serializable> , then you could write

ArrayList<String> astr = new ArrayList<String>();
ArrayList<Serializable> aser = astr;
aser.add(new Integer());
// This will throw ClassCastException!
String str = astr.get(0);

After this code, you have an ArrayList<String>() containing an Integer object -- clearly this is not good.

Answer 2

because Class<String> is not Class<Serializable> . However Class<String> is Class<? extends Serializable> Class<? extends Serializable>