I'm learning about the principles of OOP and was doing this exercise question.
We are given a school. The school has classes of students. Each class has
a set of teachers. Each teacher teaches a set of courses. The students
have a name and unique number in the class. Classes have a unique text
identifier. Teachers have names. Courses have a name, count of classes
and count of exercises. The teachers as well as the students are people.
Your task is to model the classes (in terms of OOP) along with their
attributes and operations define the class hierarchy and create a class
diagram with Visual Studio.
I am not sure how to ensure the parts in bold. What I was doing was just normally making the adjectives properties. I am not sure how to ensure that other objects are not able to have the same unique number. I was thinking of using the Dictionary data structure which doesn't allow duplicates. I would use this unique number as the key and the object to be its value. Is my line of thought correct here?
you can have for example static private array property (same for every instance of a class) in class, and on the constructor You can check if this value had been used
Related
I would like to know the proper way to implement UML association class in Java programming code. I have Student and Course classes. Student can attend one or more Courses, Courses can be attended by one or more Student (many-to-many relationship). If I don't have any attribute in association class (despite course and student ids), would it be okey to implement it this way: public class Course{ private List<Student> students ...} public class Student{ private List<Course> course}. Please explain me both situations, because I don't get it if I should have model public CourseEnrollment { private Student; private Course; private LocalDate enrollementDate} and lists of CourseEnrollments in Student and Course classes.
What I tried is explained above with CourseEnrollment class and my doubts.
One easy way to implement an association class is to use a Collection<CourseEnrollment> e.g. an ArrayList<CourseEnrollment>, in both Course and Student, and make sure that CourseEnrollment has a property back to the respective Course and Student.
The navigation would then be indirect: Student -> List of enrolments -> corresponding courses, or Course -> List of enrolments -> corresponding students. The challenge in your code will be to maintain consistency, e.g. if your remove a student from a course, you'll need not only to delete the enrollment from the course's collection, but also from the student's collection.
Another alternative is to use an independent repository of CourseEnrollments, that keeps a list of all the enrolments and provides efficient access for a student and a course, through two HashMap. The advantage is that there's only one place to manage the enrollment, and students or courses can get the relevant and up-to-date links from there. Again, navigation is indirect, this time by querying the repository. The challenge here is to manage decoupling, since every Student or Course would need to know about the repository.
You may have noticed, that in both cases, the trick is to implement the association class as a class, and to decompose the direct association between Student and Course into an indirect association via the association class.
P.S: In the UML semantics, the association class is at the same time a class and an association. But it can have only one name. Chose either chooses or CourseEnrollment, but you should not use different names.
I want to extend the timetable example of the quarkus guide with the entity student. Each student grades all courses (eg. 1-6, more is better). Each student can visit all courses. I am looking for a student - course - allocation, so that the global sum of grades is maximal.
Can I do this with only 1 PlanningEntity or must I have 2?
Is there a trick to add a PlanningVariable List<Student> to Lesson?
I 'd argue this is a different planning problem, with a different Solver configuration (so a different #PlanningSolution and #PlanningEntity class) but which can reuse problem fact classes (Room, Timeslot, etc).
In practice, I'd remove the #PlanningEntity annotation (and the #PlanningVariable annotations) from the Lesson class, because the lesson to room/timeslot assignments will be part of the input, not part of the planning optimization. I am not sure if you even need room/timeslot information at all.
We don't support lists of planning variable (#PlanningVariableCollection) yet and I doubt it would be a good fit here because the order of the students in the list doesn't matter. A set might, but we don't support that either yet. It is being worked on. In any case, there's a much simpler solution:
Create a planning entity StudentToCourseAssignment class. Follow the docs chapter 22 domain modeling guide to decide if the planning variable is on the student field xor the course field.
Also replace the TimeTable class accordingly.
I am new to object oriented design. I am looking forward to some tips on how to model the below mentioned requirement using objects.
Requirement: A program has many Students. A program logs in to the application. First page displays a table of all the Students belonging to the program (Table Columns - Id, First name, last name, age, sex, etc. ). The Id is a link. By clicking the Id, the individual student page is displayed. In this page, an individual student related activities can be done - for eg. edit address, change name, add comments etc.
My Solution: Two Classes
Program - Will model a single program, perform all activities related to it and encapsulate all program related db tables.
Student - Will model a single student and perform all activities related to it and encapsulate all student specific db tables.
So far, so good.
In order to fetch details of all students related to a program, from Program object I need to call a method called 'fetch_student_details'.
The Question is where should this method be written? Should it be a Student Class method or Program Class method.
If I write this method in program class, how will the Program class handle it?
(a) First identify the list of student_ids that belong to it and for each id, instantiate a Student Class and get the specific student related information from it. DB: Each instantiated student object will run a query to fetch its information. So 100 students, 100 queries.
OR
(b) Fetch the list of student ids related to the program and fetch the student information directly for all the students. Db: Single query to fetch all the needed information.
In this case, if you can, go for solution (b). Students and Programs, in fact, are two differet entities that can exist independently, and you don't break encapsulation.
With solution (a) you need to pollute Program's interface by adding a method for each possible information that you want to get from the Students.
As final suggestion, if you want to learn OO Design, start by reading the book "Design patterns - Elements of Reusable Object-Oriented Software" by Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides (AKA "The gang of four").
In the question I'm doing at the moment gave two confusing sentences:
1. A property can be either be a house or an apartment. For a house it records ..bula bula
For an apartment, it records .. bula bula
2. A property can be either for sale or rent, or for both. If a property is for sale, it
records .. bula bula. If the property is for rent, it records .. bula bula
These two are in the same question. Do I have to represent it by using subclasses or how?
Many thanks.
Yes, a entity can have multiple subclasses and multiple superclasses. Both faculty staff and student assistants may be subclasses of employees, and a student assistant may be a subclass of both employees and students.
You can create a Property entity with four subclasses, House, Appartment, PropertyForRent and PropertyForSale.
A property may not be both a house and an appartment. Therefore, use a circle with a d in it to indicate that it is disjoint. A property may be both for rent and for sale. Use a circle with an o in it to indicate that it may overlap.
This is described on page 443 in Advanced Data Modelling, and another example can be found on page 30 of this presentation.
Sjoerd's answer is correct.
ER modeling tells you how to diagram subclasses, but it doesn't tell you how to implement them. Nor should it.
If you are interested in designing SQL tables that implement subclasses, look up these topics, or their tags in SO:
Single Table Inheritance
Class Table Inheritance
Shared Primary Key
I understand the difference between aggregation and composition but I am struggling a bit with association. My current understanding is that an association exists between classes when ‘they use each other’, for example, one object is passed to the other during a method call. See also:
http://www.codeproject.com/Articles/330447/Understanding-Association-Aggregation-and-Composit
Both objects exist independently and, in contrast to aggregation, no object is a container class of the other. Does this mean that both objects MUST have a copy of the other(s) (e.g. 1:m relationship) or how else is the association ‘stored’. Any feedback would be very much appreciated.
From the UML Superstructure 2.4.1:
An association declares that there can be links between instances of the associated types. A link is a tuple with one value for each end of the association, where each value is an instance of the type of the end. (UML Superstructure, Page 37)
Nothing more, nothing less. and very vague. Because of this, it is also very hard to understand. What I defined (In a course I teach) is a hierarchy of links from dependency to composition where:
Dependency from A to B means that A uses B but indirectly (say by receiving instances of it and forwarding them to other objects).
Association from A to B means that A uses B directly, (for example by calling methods)
Aggregation from A to B means that B is part of A (semantically) but B can be shared and if A is deleted, B is not deleted. Note that this says nothing about how the "is part" is implemented.
Composition from A to B is like Aggregation, where B cannot be shared and if A is deleted, all of its aggregates (Bs) are deleted also.
Aggregation is an Association relationship where the Association can be considered the containing class 'Owning' the contained class, and the lifetime of that relationship is not defined.
Association is an 'Has-A' relationship.
Example:-
public class Person
{
private final Name name;
private Address currentAddress;
//...
}
In this case, the Person Has-A name and Has-A Address, so there is an Association between Person and Name, and Person and Address.
An association describes a relationship between instances of one or more classes. In the words of the UML Reference Manual, "Associations are the glue that holds together a system."
Aggregation is a form of association in which there is a "whole-part" relationship. You may say that if a class Airplane has a class Engine then this forms a "whole-part" relationship.
Aggregation
Let's set the terms. The Aggregation is a metaterm in the UML standard, and means BOTH composition and shared aggregation, simply named shared. Too often it is named incorrectly "aggregation". It is BAD, for composition is an aggregation, too. As I understand, you meant you understand "shared aggregation and composition".
From UML standard:
Precise semantics of shared aggregation varies by application area and
modeler.
I haven't found a word about that aggregation supposed multiplicity, for example.
Association.
A definition from UML 3.4.1 standard:
An association describes a set of tuples whose values refer to typed
instances. An instance of an association is called a link. A link is a
tuple with one value for each end of the association, where each value
is an instance of the type of the end.
Aggregated relationship is a subclass of Association.
Association is based on relationship. IT is the glue for models.
But your feelings didn't lie - as the shared aggregation is not strictly defined, there is also NO any strictly defined boundary between Association and Aggregated association. Authors of tools and modellers have to set it themselves.
Association
It represents a relationship between two or more objects where all objects have their own lifecycle and there is no owner. The name of an association specifies the nature of relationship between objects. This is represented by a solid line.
Let’s take an example of relationship between Teacher and Student. Multiple students can associate with a single teacher and a single student can associate with multiple teachers. But there is no ownership between the objects and both have their own lifecycle. Both can be created and deleted independently.
Aggregation
It is a specialized form of Association where all object have their own lifecycle but there is ownership. This represents “whole-part or a-part-of” relationship. This is represented by a hollow diamond followed by a line.
Let’s take an example of relationship between Department and Teacher. A Teacher may belongs to multiple departments. Hence Teacher is a part of multiple departments. But if we delete a Department, Teacher Object will not destroy.
It depends on the context.
Association: A man drives a car, focus on the caller and callee relationship.
Aggregation: A man has a car, focus on the owner and member relationship.
Composition: A man has a mouth, focus on the owner & member but the owner consists of members, it means that they shared the same life cycle.
Feels like I'm speaking Chinglish.
Association
Association is a relationship where all objects have their own life-cycle and there is no owner. Let’s take the example of Teacher and Student. Multiple students can associate with a single teacher and a single student can associate with multiple teachers but there is no ownership between the objects and both have their own life-cycle. Both can create and delete independently.
Aggregation
the objects in Aggregation have their own life-cycle but there is ownership. Child object can not belong to another parent object. Let’s take an example of Department and teacher. A single teacher can not belongs to multiple departments, but if we delete the department teacher object will not destroy. We can think about the “has-a” relationship.
Composition
It is a strong type of Aggregation. Child object does not have their life-cycle and if parent object deletes all child object will also be deleted. Let’s take again an example of the relationship between House and rooms. House can contain multiple rooms there is no independent life of room and any room can not belongs to two different houses if we delete the house room will automatically delete.
An association between object types classifies relationships between objects of those types. For instance, the association Committee-has-ClubMember-as-chair, which is visualized as a connection line in the class diagram shown below, may classify the relationships FinanceCommittee-has-PeterMiller-as-chair, RecruitmentCommittee-has-SusanSmith-as-chair and AdvisoryCommittee-has-SarahAnderson-as-chair, where the objects PeterMiller, SusanSmith and SarahAnderson are of type ClubMember, and the objects FinanceCommittee, RecruitmentCommittee and AdvisoryCommittee are of type Committee.
See also my alternative CodeProject article.