put method: if the key already exists, it will overwrite that object in the map.

get method: will get the copy of the object with the key, so change this object won't impact original object in the map.

1. Declare interface, use interface not specific class. This adds flexibility. This can be combined with java Reflection api to use.

2. Factory pattern has better not to use the simplest version, that is have all the concrete class name in the factory method, but use reflection to instantiate the objects.

3. Object composition or concrete inheritance. Sometimes certain helper code is common for all places, in this case, do not use it as inheritance, but have the helper object composited in the class. This is definitely a better design approach.

4. After fulfilling basic functionality, always refactor the code to extract an interface, and an abstract class for elementary functionalities.

5.