Abstract Class and Interface

/* ::class design guidline::
 *
 * cohesion -> single entity + immutable
 * consistency -> choose informative name, similar operation's name, private in constructor
 * encapsulation -> use private modifier to hide info from direct access from clients. getter --> readable, setter --> updateable, if no setter method immutable
 * clarity -> you should n't declare a data field that can be derived from other data field
 * completeness -> class are designed for use by many different customer
 * instance vs static - > it is better use a setter method to change the static data field
 * inheritance vs aggregation -> is a(apple is a fruit), has a(person has a name)
 * abstrac class vs interface  -> what is choose? strong is a relationship(orange is a fruit), weak a relationship(circle, ractangle........ has a area)
 * 
 */