Naming Convention
-----------------
Follows Java naming convention by default.

Only constant number and 'static final' variables whose type can be used as
  'switch' Statements, are represented as captialized name - naming convention
  for constant value(ex. HASH_NUM).
    * Number type
        float, double, byte, char, short, int, long
    * Others
        String(Java 7 or later), Enum.


Terms and concepts
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* Listener / Observer
Callback owner doesn't affected by callback's internal behavior.
And usually, callback owner manages callbacks as list and callback all
    registered object's callback one by one.
In genenral, event is just passed to it's callbacks and caller don't care about
  it.
Usually, return type of callback is 'Void' because caller doens't care about
  callback's internal operation.

* Receiver
Similar with Listener but callback's behavior affects to caller.
So, usually, it's return type is 'boolean'.
In genenral, event may be consumed by one of callback.
For example, 'true' means "Do not pass event to other callbacks behind" and
  'false' means "event is not consumed. Keep passing event others behind".