What is wrong by inheriting from Object?

Question

Hi and thank you for reading my question,
I have tried the following program:
import java.util.*;
interface I {}
class A1 {}
class B1 extends A1  implements I {
    B1() {}
class B2 extends A1 implements I {
    B2() {}
class Cell<V> 
V  value;
<T extends V> Cell(T a) {value = a;}
V get() { return value; }
class Foo {
    void  test(boolean b){
     Cell<? extends B1> cb1 = new Cell<B1>(new B1());
     Cell<? extends B2> cb2 = new Cell<B2>(new B2());
     Cell<? extends I> c1 = ( b ? cb1 : cb2);
     Cell<? extends A1> c2 = ( b ? cb1 : cb2);
}and I have got the following error:
Test.java:31: incompatible types
found   : Cell<capture of ? extends java.lang.Object&I&A1>
required: Cell<? extends A1>
     Cell<? extends A1> c2 = ( b ? cb1 : cb2);
                                 ^
1 errorWhy c2 is incorrect and c1 is correct?
It seems that there is a bug by inheriting from Object.
I have tried the same code but now class A extends
a class different than Object:
import java.util.*;
interface I {}
class A1 extends Vector<Integer>{}
class B1 extends A1  implements I {
    B1() {}
class B2 extends A1 implements I {
    B2() {}
class Cell<V> 
V  value;
<T extends V> Cell(T a) {value = a;}
V get() { return value; }
class Foo {
    void  test(boolean b){
     Cell<? extends B1> cb1 = new Cell<B1>(new B1());
     Cell<? extends B2> cb2 = new Cell<B2>(new B2());
     Cell<? extends I> c1 = ( b ? cb1 : cb2);
     Cell<? extends A1> c2 = ( b ? cb1 : cb2);
}This program compiles without any error. Both c1 and c2 are correct.

Answer 1

A bug, I've submitted bug 6278798.