Object Oriented Design for stackoverflow - object

Question

I am trying to implement an object-oriented design for stack overflow. I have Member, Question, Answer, Comment object. (not considering tags, bounty other objects for simplicity). I am not able to figure out the storage of data in design.
The user creates a question, Answer, Comment with a title and description. I am not able to figure out where I should save the data because these objects belong to the user and global search also. Answer and comment can also happen on any question but they belong to the user also. so the problem is to store the data user-specific and global.
I have a thought process to create a repository that will keep data for questions, answers, and comments. I can store questionsIds, answerIds, commentIds in member object. similarly, anserIds and comments in Question object. I am not sure it's good practice for object-oriented design or not.
public class Member {
private long memberId;
// few more properties
Member(){
}
public long createQuestion(String title, String description){ return 1;}
public long createAnswer(long questionId, String description){ return 1;}
public long createQuestionComment(long questionId, String description){ return 1;}
public long createAnswerComment(long answerId, String description){ return 1;}
}
public class Question {
private long id;
private String title;
private String description;
Question(String title, String description){
}
}
In case, you want more details. I will create a UML dig for more explanation. If someone knows about any good article please refer me there. If you know about any good book where I can learn more about the object-oriented approach please let me know that also.

Answer 1

The storage is a separate issue from how you interact with it.
In a classic SQL setup the member, answer, comment, etc are all separate database tables with numeric id keys. The relationships are likely one-directional where the comment has a memberId field, but the member would not know anything about the comments. You would find a member's comments by querying the comment table for comments with the correct memberId. Here's your StackOverflow comments, which we find through the query SELECT * FROM Comments WHERE UserId = 8906489.
These classes that you are designing would be used to interact with those database tables and create a local (per page load) cache system. In terms of the Model-View-Controller pattern the database tables are the Model and these classes are the Controller. Your Member class knows its memberId so it can create and execute a comments query that selects comments for that id, or that inserts a comment with that member id, etc.
You don't want to have the same data in lots of places so you probably have some sort of central Repository class. When you are calling createQuestion on the Member that needs to be linked to the Repository. You would either design it so that instances of Member get the Repository as a constructor argument, or so that all interactions are initiated by the Repository, which can pass itself as a function argument.

Answer 2

I have created an OO solution with a repository structure. I have considered simple functionality only. I think we can break the repository into separate repositories like AnswerRepository, QuestionRepository.
Main:
public class StackOverflow {
public static void main(String[] args) {
StackOverflow app= new StackOverflow();
StackOverflowRepository repository = new StackOverflowRepository();
String memberName = "Vimit"; // assume this as a input from application
long memberId = app.registerMember(memberName, repository);
long questionId = app.createQuestion(repository,memberId, "OO design stackoverflow", "How to do OO design for stack overflow questions"); // when anymmeber will call we have only member id
app.createAnswer(repository, "I dont know about OO design", questionId, memberId); // whe someone call this function, it will be meber and on some squestion so we always have member Id
app.searchQuestions( "ood", repository);
}
public long registerMember(String memberName, StackOverflowRepository repository){
Member member = new Member(memberName);
repository.addMember(member);
return member.getId();
}
public long createQuestion(StackOverflowRepository repository, long memberId, String title, String description){
Member member = repository.getMember(memberId);
return member.createQuestion(repository, title, description);
}
public long createAnswer(StackOverflowRepository repository, String description, long questionId, long memberId){
Member member = repository.getMember(memberId);
return member.createAnswer(repository, description,questionId);
}
public List<Question> searchQuestions(String searchText, StackOverflowRepository repository){
return repository.searchQuestion(searchText);
}
}
Repository:
public class StackOverflowRepository {
private AtomicLong memberId;
private AtomicLong questionId;
private AtomicLong answerId;
private Map<Long, Member> members;
private Map<Long, Question> questions;
private Map<Long, Answer> answers;
private Map<String, List<Long>> searchQuestions; // reverseIndex for question search
StackOverflowRepository() {
memberId = new AtomicLong(0);
questionId = new AtomicLong(0);
members = new HashMap<>();
questions = new HashMap<>();
}
public Question addQuestion(long memberId, Question question){
question.setId(questionId.incrementAndGet());
questions.put(questionId.get(), question);
members.get(memberId).addQuestionId(questionId.get());
createReverseIndex(question);
return question;
}
public long addMember(Member member) {
member.setId(memberId.incrementAndGet());
members.put(memberId.get(), member);
return memberId.get();
}
public Member getMember(long memberId){
return members.get(memberId);
}
public List<Question> searchQuestion(String search){
String[] words = search.split("//s");
List<Long> questionIds = new ArrayList<>();
for(String word: words){
questionIds.addAll(searchQuestions.get(word));
}
return questionIds.stream().distinct().map(id -> questions.get(id)).collect(Collectors.toList());
}
public long addAnswer(Answer answer, long memberId, long questionId){
answer.setId(answerId.incrementAndGet());
answers.put(answerId.get(), answer);
members.get(memberId).addAnswerId(answerId.get());
questions.get(memberId).addAnswerId(answerId.get());
return answer.getId();
}
private void createReverseIndex(Question question){
String[] words = question.getDescription().split("//s");
for(String word: words){
List<Long> ids = searchQuestions.getOrDefault(word, new ArrayList<>());
ids.add(question.getId());
searchQuestions.put(word, ids);
}
}
}
Member:
public class Member {
private long id;
private String memberName;
private List<Long> questionIds;
private List<Long> answerIds;
Member(String memberName){
this.memberName = memberName;
}
public long createQuestion(StackOverflowRepository stackOverflowRepository, String title, String description){
Question question = new Question(title, description);
stackOverflowRepository.addQuestion(this.id,question);
return question.getId();
}
public long createAnswer(StackOverflowRepository stackOverflowRepository, String description, long questionId){
Answer answer = new Answer(description);
return stackOverflowRepository.addAnswer(answer, this.id, questionId);
}
public void addQuestionId(long id){
this.questionIds.add(id);
}
public void setId(long id){
this.id = id;
}
public long getId(){
return id;
}
public void addAnswerId(long id) {
answerIds.add(id);
}
}
Question:
public class Question {
private Long id;
private String title;
private String description;
private List<Long> answerIds;
Question(String title, String description){
this.title = title;
this.description = description;
}
public void setId(Long id){
this.id = id;
}
public long getId(){
return this.id;
}
public void addAnswerId(long id) {
this.answerIds.add(id);
}
public String getDescription() {
return this.description;
}
}
Answer:
public class Answer {
private Long id;
private String description;
Answer(String description) {
this.description = description;
}
public void setId(Long id){
this.id = id;
}
public long getId(){
return this.id;
}
}