[Building Sakai] Best way to instantiate object in Sakai?

[le tan vu]

Hi all,
I'm concerning which is the best way to instantiate (create) an object from
a class in Sakai?
I often find Sakai uses some service and instantiate an object from that
service.
E.g:
Assignment a = AssignmentService.getAssignment(assignmentID);

How can I create a new class and embed that class into an existing tool,
after that, instantiate some objects for further using if it doesn't belong
to any service as above example?

Many thanks and best regards,
-- 
Vu Tan Le
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