diff --git a/finalproject/Question6/Borrower.java b/finalproject/Question6/Borrower.java
new file mode 100644
index 0000000..64b700c
--- /dev/null
+++ b/finalproject/Question6/Borrower.java
@@ -0,0 +1,63 @@
+/*
+ * Author: CHEN Yongyuan (Walter) 1930006025 from OOP(1007)
+ * Date: 2022-04-25
+ * Description: This is the borrower class.
+ */
+
+public class Borrower extends User {
+	/**
+	 * Constructor.
+	 *
+	 * @param name Borrower's name
+	 * @param int  book number of books borrowed by user
+	 */
+	public Borrower(String name, int book) throws NotALenderException {
+		super(name, book);
+
+		// check
+		if (book < 0) {
+			throw new NotALenderException("A new borrower cannot lend books.");
+		}
+	}
+
+	/**
+	 * increase the number of books borrowed by the user.
+	 *
+	 * @param number Incrased number of books
+	 * @throws NotALenderException
+	 */
+	public void moreBook(int number) throws NotALenderException {
+		int newBook = getBook() + number;
+		if (number < 0 && newBook < 0) {
+			throw new NotALenderException("A borrower cannot lend " + (-newBook) + " book(s).");
+		}
+		setBook(newBook);
+	}
+
+	/**
+	 * Test.
+	 */
+	public static void testBorrower() {
+		try {
+			Borrower b = new Borrower("Bob", -1);
+		} catch (NotALenderException e) {
+			System.out.println(e.getMessage().equals("A new borrower cannot lend books."));
+		}
+		try {
+			Borrower b = new Borrower("Bob", 10);
+			System.out.println(b.getName() == "Bob");
+			System.out.println(b.getBook() == 10);
+			b.setBook(5);
+			System.out.println(b.getBook() == 5);
+			b.moreBook(2);
+			System.out.println(b.getBook() == 7);
+			b.moreBook(-2);
+			System.out.println(b.getBook() == 5);
+			b.moreBook(-5);
+			System.out.println(b.getBook() == 0);
+			b.moreBook(-1);
+		} catch (NotALenderException e) {
+			System.out.println(e.getMessage().equals("A borrower cannot lend 1 book(s)."));
+		}
+	}
+}
diff --git a/finalproject/Question6/CLI.java b/finalproject/Question6/CLI.java
new file mode 100644
index 0000000..c172ec3
--- /dev/null
+++ b/finalproject/Question6/CLI.java
@@ -0,0 +1,145 @@
+/*
+ * Author: CHEN Yongyuan (Walter) 1930006025 from OOP(1007)
+ * Date: 2022-04-25
+ * Description: This is the CLI class.
+ */
+
+import java.util.InputMismatchException;
+import java.util.Scanner;
+
+public class CLI {
+	private static Scanner input = new Scanner(System.in);
+
+	/**
+	 * Read one line from CLI.
+	 *
+	 * @param message Hint printed to screen
+	 * @return one line input
+	 */
+	private static String readLine(String message) {
+		System.out.print(message);
+		return input.nextLine();
+	}
+
+	/**
+	 * Read one positive integer.
+	 *
+	 * @param message HINT printed to screen
+	 * @return one positive integer
+	 */
+	private static int readPosInt(String message) {
+		while (true) {
+			System.out.print(message);
+			try {
+				int result = input.nextInt();
+				input.nextLine();
+				if (result >= 0) {
+					return result;
+				} else {
+					System.out.println("Positive integers only!");
+				}
+			} catch (InputMismatchException e) {
+				System.out.println("You must type an integer!");
+				input.nextLine();
+			}
+		}
+	}
+
+	public static void main(String[] args) {
+		int book;
+		String username;
+		String menuHint = "Type an action (total:1 add:2 get:3 more:4 less:5 quit:6): ";
+		String userRoleHint = "Type the user role (lender:1 borrower:2): ";
+
+		Library library = new Library("UIC Library");
+		while (true) {
+			switch (readPosInt(menuHint)) {
+				// total
+				case 1:
+					System.out.println("Total number of borrowed books: "
+							+ library.totalBorrowedBooks());
+					break;
+				// add
+				case 2:
+					switch (readPosInt(userRoleHint)) {
+						// lender
+						case 1:
+							username = readLine("Enter the name of the user: ");
+							book = readPosInt(
+									"Enter the initial number of borrowed books: ");
+							Lender lender = new Lender(username, book);
+							library.addUser(lender);
+							System.out.println("Lender \"" + username + "\" lending " + book
+									+ " book(s) has been added.");
+							break;
+						// borrower
+						case 2:
+							username = readLine(
+									"Enter the name of the user: ");
+							book = readPosInt(
+									"Enter the initial number of borrowed books: ");
+							try {
+								library.addUser(new Borrower(username, book));
+								System.out.println("Borrower \""
+										+ username
+										+ "\" borrowing "
+										+ book
+										+ " book(s) has been added.");
+							} catch (NotALenderException e) {
+								System.out.println("BUG! This must never happen!");
+								System.exit(1);
+							}
+							break;
+						default:
+							System.out.println("Unknown user role!");
+							break;
+					}
+					break;
+				// get
+				case 3:
+					username = readLine("Enter the name of the user: ");
+					try {
+						System.out.println(username + " borrows "
+								+ library.getBook(username) + " book(s).");
+					} catch (UnknownUserException e) {
+						System.out.println("User " + username + " unknown.");
+					}
+					break;
+				case 4:
+					username = readLine("Enter the name of the user: ");
+					book = readPosInt("Enter the number of books: ");
+					try {
+						library.moreBook(username, book);
+					} catch (NotALenderException e) {
+						System.out.println("BUG! This must never happen!");
+					} catch (UnknownUserException e) {
+						System.out.println("User " + username + " unknown.");
+					}
+					break;
+				case 5:
+					username = readLine("Enter the name of the user: ");
+					book = readPosInt("Enter the number of books: ");
+					try {
+						// decrease numbers
+						try {
+							library.moreBook(username, -book);
+						} catch (NotALenderException e) {
+							System.out.println("A borrower cannot lend "
+									+ (-(library.getBook(username) - book))
+									+ " book(s).");
+						}
+					} catch (UnknownUserException e) {
+						System.out.println("User " + username + " unknown.");
+					}
+					break;
+				case 6:
+					System.out.println("Goodbye!");
+					System.exit(0);
+					break;
+				default:
+					System.out.println("Unknown action!");
+					break;
+			}
+		}
+	}
+}
diff --git a/finalproject/Question6/IUser.java b/finalproject/Question6/IUser.java
new file mode 100644
index 0000000..f2f1b19
--- /dev/null
+++ b/finalproject/Question6/IUser.java
@@ -0,0 +1,13 @@
+/*
+ * Author: CHEN Yongyuan (Walter) 1930006025 from OOP(1007)
+ * Date: 2022-04-25
+ * Description: This is the interface of IUser.
+ */
+
+public interface IUser {
+	public String getName();
+
+	public int getBook();
+
+	public void moreBook(int number) throws NotALenderException;
+}
diff --git a/finalproject/Question6/Lender.java b/finalproject/Question6/Lender.java
new file mode 100644
index 0000000..8d719bf
--- /dev/null
+++ b/finalproject/Question6/Lender.java
@@ -0,0 +1,42 @@
+/*
+ * Author: CHEN Yongyuan (Walter) 1930006025 from OOP(1007)
+ * Date: 2022-04-25
+ * Description: This is the Lender class.
+ */
+
+public class Lender extends User {
+	/**
+	 * Constructor.
+	 *
+	 * @param name The user's name.
+	 * @param book number of bookds lent by the user. This will be store in negetive value.
+	 */
+	public Lender(String name, int book) {
+		// lender negetive book value
+		super(name, -book);
+	}
+
+	/**
+	 * Increases the number of bookds.
+	 *
+	 * @param book the number of books.
+	 */
+	public void moreBook(int book) {
+		setBook(getBook() - book);
+	}
+
+	/**
+	 * Test.
+	 */
+	public static void testLender() {
+		Lender l = new Lender("Anna", 5);
+		System.out.println(l.getName() == "Anna");
+		System.out.println(l.getBook() == -5);
+		l.setBook(-6);
+		System.out.println(l.getBook() == -6);
+		l.moreBook(2);
+		System.out.println(l.getBook() == -8);
+		l.moreBook(-9);
+		System.out.println(l.getBook() == 1);
+	}
+}
diff --git a/finalproject/Question6/Library.java b/finalproject/Question6/Library.java
new file mode 100644
index 0000000..09aeb86
--- /dev/null
+++ b/finalproject/Question6/Library.java
@@ -0,0 +1,106 @@
+/*
+ * Author: CHEN Yongyuan (Walter) 1930006025 from OOP(1007)
+ * Date: 2022-04-25
+ * Description: This is the Library class.
+ */
+
+import java.util.ArrayList;
+
+public class Library {
+	private String name;
+	private ArrayList<IUser> users;
+
+	/**
+	 * Constructor.
+	 *
+	 * @param name
+	 */
+	public Library(String name) {
+		this.name = name;
+
+		// init array list
+		users = new ArrayList<IUser>();
+	}
+
+	/**
+	 * Add the user to the array list.
+	 *
+	 * @param user The user to be added
+	 */
+	public void addUser(IUser user) {
+		users.add(user);
+	}
+
+	/**
+	 * The total number of bookds borrowed by all users.
+	 *
+	 * @return the number of books.
+	 */
+	public int totalBorrowedBooks() {
+		int total = 0;
+		for (IUser user : users) {
+			total += user.getBook();
+		}
+		return total;
+	}
+
+	/**
+	 * Get the number of books borrowed by this user.
+	 *
+	 * @param name the name of the query user
+	 * @return the number of books
+	 * @throws UnknownUserException if user not found
+	 */
+	public int getBook(String name) throws UnknownUserException {
+		for (IUser user : users) {
+			if (user.getName().equals(name)) {
+				return user.getBook();
+			}
+		}
+		// user not found
+		throw new UnknownUserException("User " + name + " unknown.");
+	}
+
+	/**
+	 * Change the number of books currently borrowed by that user.
+	 *
+	 * @param name   the user's name
+	 * @param number the number of books
+	 * @throws UnknownUserException user not found
+	 * @throws NotALenderException  can not lend book
+	 */
+	public void moreBook(String name, int number) throws UnknownUserException, NotALenderException {
+		for (IUser user : users) {
+			if (user.getName().equals(name)) {
+				user.moreBook(number);
+				return;
+			}
+		}
+		// user not found
+		throw new UnknownUserException("User " + name + " unknown.");
+	}
+
+	/**
+	 * Test.
+	 */
+	public static void testLibrary() {
+		Library li = new Library("UIC Library");
+		System.out.println(li.totalBorrowedBooks() == 0);
+		li.addUser(new Lender("L1", 10));
+		try {
+			System.out.println(li.getBook("L1") == -10);
+			System.out.println(li.totalBorrowedBooks() == -10);
+			li.addUser(new Borrower("B1", 20));
+			System.out.println(li.getBook("L1") == -10);
+			System.out.println(li.getBook("B1") == 20);
+			System.out.println(li.totalBorrowedBooks() == 10);
+			li.getBook("B2");
+		} catch (UnknownUserException ex) {
+			System.out.println(ex.getMessage().equals("User B2 unknown."));
+		} catch (NotALenderException ex) {
+			// This should never happen!
+			System.out.println(false);
+		}
+		// More test cases are needed
+	}
+}
diff --git a/finalproject/Question6/NotALenderException.java b/finalproject/Question6/NotALenderException.java
new file mode 100644
index 0000000..377f323
--- /dev/null
+++ b/finalproject/Question6/NotALenderException.java
@@ -0,0 +1,5 @@
+public class NotALenderException extends Exception {
+	public NotALenderException(String message) {
+		super(message);
+	}
+}
diff --git a/finalproject/Question6/Test.java b/finalproject/Question6/Test.java
new file mode 100644
index 0000000..d7fc547
--- /dev/null
+++ b/finalproject/Question6/Test.java
@@ -0,0 +1,8 @@
+public class Test {
+	public static void main(String[] args) {
+		User.testUser();
+		Lender.testLender();
+		Borrower.testBorrower();
+		Library.testLibrary();
+	}
+}
diff --git a/finalproject/Question6/UnknownUserException.java b/finalproject/Question6/UnknownUserException.java
new file mode 100644
index 0000000..9f9f04a
--- /dev/null
+++ b/finalproject/Question6/UnknownUserException.java
@@ -0,0 +1,5 @@
+public class UnknownUserException extends Exception {
+	public UnknownUserException(String message) {
+		super(message);
+	}
+}
diff --git a/finalproject/Question6/User.java b/finalproject/Question6/User.java
new file mode 100644
index 0000000..6eec5a9
--- /dev/null
+++ b/finalproject/Question6/User.java
@@ -0,0 +1,69 @@
+/*
+ * Author: CHEN Yongyuan (Walter) 1930006025 from OOP(1007)
+ * Date: 2022-04-25
+ * Description: This is the User class.
+ */
+
+public abstract class User implements IUser {
+	/**
+	 * The user's name.
+	 */
+	private String name;
+
+	/**
+	 * The number of books borrowed by the user.
+	 */
+	private int book;
+
+	/**
+	 * Constructor.
+	 *
+	 * @param name The user's name.
+	 * @param book The number of books borrowed by the user.
+	 */
+	public User(String name, int book) {
+		this.name = name;
+		this.book = book;
+	}
+
+	/**
+	 * Getter of the user's name.
+	 *
+	 * @return The user's name.
+	 */
+	public String getName() {
+		return name;
+	}
+
+	/**
+	 * Getter of the number of books borrowed by the user.
+	 *
+	 * @return The number of books borrowed by the user.
+	 */
+	public int getBook() {
+		return book;
+	}
+
+	/**
+	 * Change the number of books borrowed by the user.
+	 *
+	 * @param book The number of books borrowed by the user.
+	 */
+	protected void setBook(int book) {
+		this.book = book;
+	}
+
+	/**
+	 * Increase the number of books borrowed by the user.
+	 *
+	 * @param book The number of books borrowed by the user.
+	 */
+	public abstract void moreBook(int number) throws NotALenderException;
+
+	/**
+	 * Test.
+	 */
+	public static void testUser() {
+		// abstract class not testing
+	}
+}