Most major websites collect feedback from their customers in order to improve the quality of their services. The most common form of user feedback is...

Most major websites collect feedback from their customers in order to improve the quality of their services. The most common form of user feedback is ratings. For example, Amazon.com collects users' ratings of books, Imdb.com collects movie ratings and Booking.com gathers users' rating of hotels.

A rating consists mainly of three important pieces of information:

1. The user ID: this is the ID of the user who gave the rating.

2. The item ID: this is the ID of the item being rated. For example, the item can be a book, a movie or a hotel.

3. The rating value: this is a numerical value that is usually an integer ranging from 1 to 5 (or 1 to 10), where 1 is the lowest rating (user not satised) and 5 (or 10) is

the highest rating (user very satised).

The goal in this project is to write a program that reads a list of ratings from a text file, stores them efficiently in memory (you should choose the appropriate data structure for this) and answers a number of queries as efficiently as possible. Examples of queries are:

What is the rating given by user i to item j?

What is the average rating of item j?

What is (are) the highest rated item(s)?

Requirements:

In this phase, you are required to implement the following classes:

public class Rating {

private int userId;

private int itemId;

private int value; // The value of the rating

// Constructor

public Rating(int userId, int itemId, int value);

// Getters... (No setters. This class is immutable)

}

public class RatingManager {

// Constructor

public RatingManager();

// Read ratings from a file and create a RatingManager object that stores these ratings

public static RatingManager read(String fileName);

// Add a rating

public void addRating(Rating rating);

// Return all ratings given by user i. Search should be efficient.

public LinkedList<Rating> getUserRatings(int i);

// Return all ratings given to item j. Search should be efficient.

public LinkedList<Rating> getItemRatings(int j);

// Return the average rating of item j. If i has no ratings, -1 is returned

public double getAverageItemRating(int j);

// Return the average rating given by user i. If i has no ratings, -1 is returned

public double getAverageUserRating(int i);

// Return the list of all items having the highest average rating (for example if the highest average rating is 4.9, the method should return all items

with average rating 4.9)

public LinkedList<Integer> getHighestRatedItems();

}

Let n denote the number of users, m the number of items and k the number of ratings.

Usually k is much smaller than nm, because a user does not usually rate all items.

Your memory requirement must be O(k) and not O(nm).

Note ( The use of Java collections or any other library is strictly forbidden.)