Prototype Pattern

思路类似我们编程，不会每次都重新写代码，而是修改以前的代码。
Instead of creating a new object, clone an exist object, then modify its properties. 
Benefit: when creating process is very complex, such as database read, big network traffic. 


overload clone method.
When the cloned object has sub objects 
shallow clone: only copy references of those sub objects: super.clone()
deep clone: make a new copy of each sub object： new()