Well. I found the answer myself. I am putting it here for the benefit of others.

State - districts is a 1-many relation (bi-directional relation).

here In Oops it becomes has a relations. So we have to design a state class which will contain a collection of ditricts as one of the attributes.

This is a bi-directional relation. So we have to design a distric class where a state class is denoted by means of an attribute.

State class will have getDistricts, setDistricts, addDistrict, deleteDistrict methods

District class will have getState, setState method