I ran into a problem with the following code that compiles in eclipse, but fails under the sun jdk: public void testClassCollection() { Collection<Class<?>> c = null; Collection<Class> ce = (Collection<Class>) c; // This is a warning in eclipse, error under sun jdk. This seems like an overly strict check. I can understand it for types that allow mutable operations with respect to the wildcard parameter, such as a Collection of List<?>: public void testCollectionOfUknownlistAdd() { Collection<List<?>> coul = new ArrayList<List<?>>(); List<?> lw = coul.iterator().next(); // This is an error because don't know what ? is lw.add(new Object()); Collection<List> col = (Collection<List>) coul; List l = col.iterator().next(); // This is allowed for legacy usage l.add(new Object()); } but I don't see how I can unsafely use Class in place of Class<?>. Its equivalent to this safe read-only usage of an unknown collection, which eclipse does allow, but the jdk does not: public void testCollectionOfUknownlistGet() { Collection<List<?>> coul = new ArrayList<List<?>>(); List<?> lw = coul.iterator().next(); // This is fine because ? although unknown, is still an Object Object o1 = lw.get(0); Collection<List> col = (Collection<List>) coul; List l = col.iterator().next(); Object o2 = l.get(0); }