objective c basics

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https://www.tutorialspoint.com/objective_c/index.htm

You should create an iOS application in Xcode, written in Objective-C. It should contain 2 classes: Organization and Employee, both of each are subclasses of NSObject. Organization should have the following properties:

Public “name” of type NSString.
Private “employees” of type NSArray.

And the following methods:

Public initWithName initializer method, which accepts organization name as an input parameter.
Public addEmployeeWithName: (accepts employee name as an input parameter with no return type), which creates a new employee and adds it to the array. The salary is random between 100 and 5000 and is divisible by 10 (e.g. 110, 4670).
Public calculateAverageSalary method, which return the average salary of all employees.
Public employeeWithLowestSalary method, which returns one employee with the lowest salary in the organization.
Public employeesWithSalary: method, which accepts one argument of type int - salary - and returns all employees with given salary as an array.

Employee should have the following properties:

Private “firstName” and “lastName”, which are of type NSString.
Public “salary” of type int.
Public readonly property “fullName”, which returns the conjuction of first name and last name. And the following methods:
Public initWithFirstName:lastName:salary: initializer method, which accepts first name, last name and salary as input parameters.

Application should create one organization and multiple employees, which are added to the organization. Than all of the methods should be demonstrated. Everything should be logged to NSLog.

You should be able to answer the following questions:

How could a class be defined in Objective-C?
What is a property?
What options does a property have?
What is ARC?
What is a category?
What is the difference between instance method and class method?