HackerRank Library system design in Java

Solution.java

import java.util.HashSet;
import java.util.Optional;
import java.util.Set;
import java.util.stream.Collectors;

public class Solution {
	public static void main(String args[]) throws Exception {
		/* Enter your code here. Read input from STDIN. Print output to STDOUT */
	}
}

class User {
	private String name;
	private String id;
	private String email;
	private RegistrationStatus status;

	public User(String name, String id, String email, RegistrationStatus status) {
		super();
		this.name = name;
		// Id cannot be changed once user is created.
		this.id = id;
		this.email = email;
		this.status = status;
	}

	/**
	 * ID has to be unique across, if ID is same its the same user
	 */
	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + ((id == null) ? 0 : id.hashCode());
		return result;
	}

	@Override
	public boolean equals(Object obj) {
		if (this == obj) {
			return true;
		}
		if (obj == null) {
			return false;
		}
		if (!(obj instanceof User)) {
			return false;
		}
		User other = (User) obj;
		if (id == null) {
			if (other.id != null) {
				return false;
			}
		} else if (!id.equals(other.id)) {
			return false;
		}
		return true;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public String getEmail() {
		return email;
	}

	public void setEmail(String email) {
		this.email = email;
	}

	public RegistrationStatus getStatus() {
		return status;
	}

	public void setStatus(RegistrationStatus status) {
		this.status = status;
	}

	public String getId() {
		return id;
	}

}

enum RegistrationStatus {
	PENDING, APPROVED, REJECTED
}

class UserUtility {

	private static Library library = Library.getInstance();

	/**
	 * Register new user
	 * 
	 * @param id
	 *            is must field, other values can be default
	 * @return
	 */
	public static boolean registerUser(String id) {
		return registerUser(id, "");
	}

	public static boolean registerUser(String id, String name) {
		return registerUser(id, name, "");
	}

	/**
	 * Register the user
	 * 
	 * @param id
	 * @param name
	 * @param email
	 * @return false if user already exist with same id and does register user.
	 */
	public static boolean registerUser(String id, String name, String email) {
		return library.getUsers().add(new User(id, name, email, RegistrationStatus.PENDING));
	}

	public static Set<User> getApprovedUsers() {
		return getUsersByStatus(RegistrationStatus.APPROVED);

	}

	public static Set<User> getPendingUsers() {
		return getUsersByStatus(RegistrationStatus.PENDING);
	}

	public static Set<User> getRejectedUsers() {
		return getUsersByStatus(RegistrationStatus.REJECTED);

	}

	private static Set<User> getUsersByStatus(RegistrationStatus status) {
		return library.getUsers().stream().filter(user -> user.getStatus() == status).collect(Collectors.toSet());
	}

	public void approveUser(String id) {
		setUserStatus(id, RegistrationStatus.APPROVED);
	}

	private static void setUserStatus(String id, RegistrationStatus status) {
		library.getUsers().stream().filter(user -> user.getId() == id).findFirst().get().setStatus(status);
	}

	public void rejectUser(String id) {
		setUserStatus(id, RegistrationStatus.REJECTED);
	}

}

class BooksUtility {

	private static Library library = Library.getInstance();

	public Set<Book> findBook(String author, String category) {
		return library.getBooks().stream().filter(book -> book.getAuthor() == author || book.getCategory() == category)
				.collect(Collectors.toSet());
	}

	public static void loanBook(Book book) {
		Optional<Book> bookFound = library.getBooks().stream().filter(b -> b.equals(book)).findFirst();
		synchronized (library) {
			Book found = bookFound.get();
			if (found.getStatus() == LoanStatus.AVAILABLE) {
				bookFound.get().setStatus(LoanStatus.LOANED);
			} else {
				throw new IllegalStateException("Book already loaned");
			}

		}
	}

}

enum LoanStatus {
	LOANED, AVAILABLE
}

class Book {
	private String name;
	private String author;
	private String category;

	private LoanStatus status;

	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + ((author == null) ? 0 : author.hashCode());
		result = prime * result + ((category == null) ? 0 : category.hashCode());
		result = prime * result + ((name == null) ? 0 : name.hashCode());
		return result;
	}

	@Override
	public boolean equals(Object obj) {
		if (this == obj) {
			return true;
		}
		if (obj == null) {
			return false;
		}
		if (!(obj instanceof Book)) {
			return false;
		}
		Book other = (Book) obj;
		if (author == null) {
			if (other.author != null) {
				return false;
			}
		} else if (!author.equals(other.author)) {
			return false;
		}
		if (category == null) {
			if (other.category != null) {
				return false;
			}
		} else if (!category.equals(other.category)) {
			return false;
		}
		if (name == null) {
			if (other.name != null) {
				return false;
			}
		} else if (!name.equals(other.name)) {
			return false;
		}
		return true;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public String getAuthor() {
		return author;
	}

	public void setAuthor(String author) {
		this.author = author;
	}

	public String getCategory() {
		return category;
	}

	public void setCategory(String category) {
		this.category = category;
	}

	public LoanStatus getStatus() {
		return status;
	}

	public void setStatus(LoanStatus status) {
		this.status = status;
	}
}

class Library {
	private static Library instance = new Library();

	private Set<User> users = new HashSet<>();
	// Assuming there is only one book of each type.
	private Set<Book> books = new HashSet<>();

	private Library() {
	}

	public static Library getInstance() {
		return instance;
	}

	public Set<User> getUsers() {
		return users;
	}

	public Set<Book> getBooks() {
		return books;
	}

}