The private properties/methods of the Superclass cannot be accessed by subclass.
The non-private properties/methods of the Superclass can be access by subclass’s objects directly.
总结:
1. “private” can’t be “extends”
2. “private” 物理上已经被继承过来了，只不过逻辑上不能访问他。所以继承必须慎重，否则浪费内存空间。