[java] How to implement a simple scenario the OO way

After starting to read a book on OO programming, I am attempting to make my android app more OO. However I am stumped on a simple scenario.

I have a Book object, which can have many say Chapter objects. I also have a search function which searches across multiple books, 97 of them. I end up with many Chapter objects from the Sqlite table.

I felt that it would be useful to the user to be able to see the title of the book on each result, otherwise it might be confusing if there are say two "chapter 5" results.

For that to happen, I need the book title. Should I make it part of my chapter object, like :

chapter.getBookTitle() 

Which kind of does not seem right, as I have glued the book name onto a chapter... The alternative is to instantiate a book object for each chapter and somehow reference it, which has its own problems including in android with regards to not being able to pass a reference to an in-memory object to another activity.

Also a book may have many other chapters which were not results in the search, and it may seem like they would return if I was to just instantiate the book.

What is the correct OO solution to this seemingly simple issue? Is it just a matter of learning when not to be dogmatic about the whole OO thing?

More Info:

I am using FTS4 in Sqlite, which accounts for over half of my actual DB size of 80mb. What I am storing is text from 97 books, with chapters in 4 languages. So my FTS at the moment stores:

ChapterId, ChapterNo (withinBook), Lang1, Lang2, Lang3, Lang4, Tags, Notes 

The searching is very fast, I retrieve only 50 results. I match any column with a string term, and not one column in particular. So if I type "apple" it will search all the fields above.

Currently as part of my FTS query I am join a join onto Book, fetching the BookId, I later use that to get me the title of the book. However its all in a procedural like style, with no regard to where the information "belongs".

I need the title so I can display it in the results, just for user convenience.

It works well, however I am wanting similar performance or slightly less but with an OO approach as I think that will make more sense to me when I come back to this project after a long pause.

This question is related to java android oop design-patterns

The answer is


You might implement your class model by composition, having the book object have a map of chapter objects contained within it (map chapter number to chapter object). Your search function could be given a list of books into which to search by asking each book to search its chapters. The book object would then iterate over each chapter, invoking the chapter.search() function to look for the desired key and return some kind of index into the chapter. The book's search() would then return some data type which could combine a reference to the book and some way to reference the data that it found for the search. The reference to the book could be used to get the name of the book object that is associated with the collection of chapter search hits.


The Chapter object should have reference to the book it came from so I would suggest something like chapter.getBook().getTitle();

Your database table structure should have a books table and a chapters table with columns like:

books

  • id
  • book specific info
  • etc

chapters

  • id
  • book_id
  • chapter specific info
  • etc

Then to reduce the number of queries use a join table in your search query.


The approach I would take is: when reading the chapters from the database, instead of a collection of chapters, use a collection of books. This will have your chapters organised into books and you'll be able to use information from both classes to present the information to the user (you can even present it in a hierarchical way easily when using this approach).


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