Hi,

I'm interested in any reference to discussions, why the
language specification was made so that

"Subtyping does not extend through generic types: T <: U does not imply
that C<T> <: C<U>."
(JLS chapter 4.10)

As seen in this example:

$ cat G.java; javac -Xlint G.java
import java.util.List;
class G {
List<String> ls; List<Object> lo; List l; String s;
public void m() { lo.add(s); lo=ls; lo=(List<Object>)ls; l=ls; lo=l; }
}
G.java:4: incompatible types
found   : java.util.List<java.lang.String>
required: java.util.List<java.lang.Object>
public void m() { lo.add(s); lo=ls; lo=(List<Object>)ls; l=ls; lo=l; }
                                ^
G.java:4: inconvertible types
found   : java.util.List<java.lang.String>
required: java.util.List<java.lang.Object>
public void m() { lo.add(s); lo=ls; lo=(List<Object>)ls; l=ls; lo=l; }
                                                     ^
G.java:4: warning: [unchecked] unchecked conversion
found   : java.util.List
required: java.util.List<java.lang.Object>
public void m() { lo.add(s); lo=ls; lo=(List<Object>)ls; l=ls; lo=l; }
                                                                  ^
2 errors
1 warning

it is no possible to assign the list of strings to the list of objects.

I find this behaviour quite annoying - even more so as the Java language
specification allows the workaround via the non-generic list "for the sake
of compatibility with older code" - that I wonder what kind of expected
problems led the designers of genericity in Java to disallow the direct
assignment of e.g. a list of string to a list of object.

If you can point me to any discussions about this on the web I would
appreciate it.

Thank you very much,
claudio