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

Question

The following statements:

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


        

Answer 1

Why can't assign I to ?

Because actually is a supertype of ! Let's follow the specification.

4.10.2 Subtyping among Class and Interface Types:

Given a generic type declaration C1,...,F_n>, the direct supertypes of the parameterized type C1,...,T_n> are all of the following:

• C1,...,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 is a supertype of Class.

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 concrete = URLClassLoader.class;
Class wildcard = concrete;