设计一个在线图书阅读器系统（面向对象设计）。

解决方案：假设我们要设计一个提供以下功能的基本在线阅读系统：

• 搜索书籍数据库并阅读书籍。

• 用户成员创建和扩展。

• 一次只有一位活跃用户，并且该用户只有一本活跃图书

OnlineReaderSystem 类代表我们程序的主体。我们可以实现

该类，以便它存储有关用户管理的所有书籍的信息并刷新显示，但这会使该类相当庞大。相反，我们选择将这些组件拆分为 Library、UserManager 和 Display 类。

课程：

1. User

2. Book

3. Library

4. UserManager

5. Display

6. OnlineReaderSystem

完整代码如下：

import java.util.HashMap;

/*

* This class represents the system

*/

class OnlineReaderSystem {

    private Library library;

    private UserManager userManager;

    private Display display;

    private Book activeBook;

    private User activeUser;

    public OnlineReaderSystem()

    {

        userManager = new UserManager();

        library = new Library();

        display = new Display();

    }

    public Library getLibrary()

    {

        return library;

    }

    public UserManager getUserManager()

    {

        return userManager;

    }

    public Display getDisplay()

    {

        return display;

    }

    public Book getActiveBook()

    {

        return activeBook;

    }

    public void setActiveBook(Book book)

    {

        activeBook = book;

        display.displayBook(book);

    }

    public User getActiveUser()

    {

        return activeUser;

    }

    public void setActiveUser(User user)

    {

        activeUser = user;

        display.displayUser(user);

    }

}

/*

* We then implement separate classes to handle the user

* manager, the library, and the display components 

*/

/*

* This class represents the Library which is responsible

* for storing and searching the books.

*/

class Library {

    private HashMap<Integer, Book> books;

    public Library()

    {

        books = new HashMap<Integer, Book>();

    }

    public Boolean addBook(int id, String details, String title)

    {

        if (books.containsKey(id)) {

            return false;

        }

        Book book = new Book(id, details, title);

        books.put(id, book);

        return true;

    }

    public Boolean addBook(Book book)

    {

        if (books.containsKey(book.getId())) {

            return false;

        }

        books.put(book.getId(), book);

        return true;

    }

    public boolean remove(Book b)

    {

        return remove(b.getId());

    }

    public boolean remove(int id)

    {

        if (!books.containsKey(id)) {

            return false;

        }

        books.remove(id);

        return true;

    }

    public Book find(int id)

    {

        return books.get(id);

    }

}

/*

* This class represents the UserManager which is responsible 

* for managing the users, their membership etc.

*/

class UserManager {

    private HashMap<Integer, User> users;

    public UserManager()

    {

        users = new HashMap<Integer, User>();

    }

    public Boolean addUser(int id, String details, String name)

    {

        if (users.containsKey(id)) {

            return false;

        }

        User user = new User(id, details, name);

        users.put(id, user);

        return true;

    }

    public Boolean addUser(User user)

    {

        if (users.containsKey(user.getId())) {

            return false;

        }

        users.put(user.getId(), user);

        return true;

    }

    public boolean remove(User u)

    {

        return remove(u.getId());

    }

    public boolean remove(int id)

    {

        if (users.containsKey(id)) {

            return false;

        }

        users.remove(id);

        return true;

    }

    public User find(int id)

    {

        return users.get(id);

    }

}

/*

* This class represents the Display, which is responsible 

* for displaying the book, it's pages and contents. It also 

* shows the current user. * It provides the method

* turnPageForward, turnPageBackward, refreshPage etc.

*/

class Display {

    private Book activeBook;

    private User activeUser;

    private int pageNumber = 0;

    public void displayUser(User user)

    {

        activeUser = user;

        refreshUsername();

    }

    public void displayBook(Book book)

    {

        pageNumber = 0;

        activeBook = book;

        refreshTitle();

        refreshDetails();

        refreshPage();

    }

    public void turnPageForward()

    {

        pageNumber++;

        System.out.println("Turning forward to page no " +

                   pageNumber + " of book having title " +

                                     activeBook.getTitle());

        refreshPage();

    }

    public void turnPageBackward()

    {

        pageNumber--;

        System.out.println("Turning backward to page no " +

                    pageNumber + " of book having title " + 

                                    activeBook.getTitle());

        refreshPage();

    }

    public void refreshUsername()

    {

        /* updates username display */

        System.out.println("User name " + activeUser.getName() + 

                                             " is refreshed");

    }

    public void refreshTitle()

    {

        /* updates title display */

        System.out.println("Title of the book " +

                        activeBook.getTitle() + " refreshed");

    }

    public void refreshDetails()

    {

        /* updates details display */

        System.out.println("Details of the book " +

                        activeBook.getTitle() + " refreshed");

    }

    public void refreshPage()

    {

        /* updated page display */

        System.out.println("Page no " + pageNumber + " refreshed");

    }

}

/* 

* The classes for User and Book simply hold data and 

* provide little functionality.

* This class represents the Book which is a simple POJO

*/

class Book {

    private int bookId;

    private String details;

    private String title;

    public Book(int id, String details, String title)

    {

        bookId = id;

        this.details = details;

        this.title = title;

    }

    public int getId()

    {

        return bookId;

    }

    public void setId(int id)

    {

        bookId = id;

    }

    public String getDetails()

    {

        return details;

    }

    public void setDetails(String details)

    {

        this.details = details;

    }

    public String getTitle()

    {

        return title;

    }

    public void setTitle(String title)

    {

        this.title = title;

    }

}

/*

* This class represents the User which is a simple POJO

*/

class User {

    private int userId;

    private String name;

    private String details;

    public void renewMembership()

    {

    }

    public User(int id, String details, String name)

    {

        this.userId = id;

        this.details = details;

        this.name = name;

    }

    public int getId()

    {

        return userId;

    }

    public void setId(int id)

    {

        userId = id;

    }

    public String getDetails()

    {

        return details;

    }

    public void setDetails(String details)

    {

        this.details = details;

    }

    public String getName()

    {

        return name;

    }

    public void setName(String name)

    {

        this.name = name;

    }

}

// This class is used to test the Application

public class AppTest {

    public static void main(String[] args)

    {

        OnlineReaderSystem onlineReaderSystem = new OnlineReaderSystem();

        Book dsBook = new Book(1, "It contains Data Structures", "Ds");

        Book algoBook = new Book(2, "It contains Algorithms", "Algo");

        onlineReaderSystem.getLibrary().addBook(dsBook);

        onlineReaderSystem.getLibrary().addBook(algoBook);

        User user1 = new User(1, " ", "Ram");

        User user2 = new User(2, " ", "Gopal");

        onlineReaderSystem.getUserManager().addUser(user1);

        onlineReaderSystem.getUserManager().addUser(user2);

        onlineReaderSystem.setActiveBook(algoBook);

        onlineReaderSystem.setActiveUser(user1);

        onlineReaderSystem.getDisplay().turnPageForward();

        onlineReaderSystem.getDisplay().turnPageForward();

        onlineReaderSystem.getDisplay().turnPageBackward();

    }

}