Hidden actually there is no mystery, want to know why, look at the rules of the name Lookup.
Lookup said this: When you need to find a name, you will find things on the same name in the highest priority area, and whether the parameters are met, whether access rights are in line with, just name. (See Namelookup in Exception C )
When the subclass looks on the name, the subclass has higher priority than the parent class, so there is inevitable so-called hidden.
