Why can't assign I <? extends Type> to <Type>?

问题

The following statements:

URLClassLoader ucl = (URLClassLoader) ClassLoader.getSystemClassLoader();
Class<URLClassLoader> uclc = ucl.getClass();

fail with error:

Type mismatch: cannot convert from Class<capture#2-of ? extends URLClassLoader> to Class<URLClassLoader>

Why do I need a cast, here?

I found several posts explaining why You can't do the reverse (assign T to a ), but that's (kind of) obvious and understood.

NOTE: I am coding this under eclipse Luna, so I don't know if it's a Luna Quirk or if there's something I really dont understand in generics.

回答1:

Covariance vs contravariance vs invariance

• Class<? extends URLClassLoader> is invariant.

As a result,

Class<? extends URLClassLoader> is not a subtype of Class<URLClassLoader>

---

In Java a variable can hold a reference of an instance of same type or subtype.

Hence,

Class<URLClassLoader> uclc = ucl.getClass();

is invalid.

On the other hand,

Class<? extends URLClassLoader> uclc = ucl.getClass();

would be valid.

回答2:

Why can't assign I <? extends Type> to <Type>?

Because actually <? extends Type> is a supertype of <Type>! Let's follow the specification.

4.10.2 Subtyping among Class and Interface Types:

Given a generic type declaration C<F₁,...,F_n>, the direct supertypes of the parameterized type C<T₁,...,T_n> are all of the following:

• C<S₁,...,S_n>, where S_i contains T_i.

4.5.1. Type Arguments of Parameterized Types:

A type argument T₁ is said to contain another type argument T₂, written T₂ <= T₁, if the set of types denoted by T₂ is provably a subset of the set of types denoted by T₁ under the reflexive and transitive closure of the following rules:

• T <= ? extends T

We therefore know that since ? extends URLClassLoader contains URLClassLoader, Class<? extends URLClassLoader> is a supertype of Class<URLClassLoader>.

Because a narrowing reference conversion is not permitted within an assignment context, a compilation error occurs.

Also note that this means the reverse assignment is permitted:

Class<URLClassLoader> concrete = URLClassLoader.class;
Class<? extends URLClassLoader> wildcard = concrete;

来源：https://stackoverflow.com/questions/24116180/why-cant-assign-i-extends-type-to-type