diff --git a/finalproject/Question3/Borrower.java b/finalproject/Question3/Borrower.java
new file mode 100644
index 0000000..64b700c
--- /dev/null
+++ b/finalproject/Question3/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/Question3/IUser.java b/finalproject/Question3/IUser.java
new file mode 100644
index 0000000..f2f1b19
--- /dev/null
+++ b/finalproject/Question3/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/Question3/Lender.java b/finalproject/Question3/Lender.java
new file mode 100644
index 0000000..8d719bf
--- /dev/null
+++ b/finalproject/Question3/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/Question3/NotALenderException.java b/finalproject/Question3/NotALenderException.java
new file mode 100644
index 0000000..377f323
--- /dev/null
+++ b/finalproject/Question3/NotALenderException.java
@@ -0,0 +1,5 @@
+public class NotALenderException extends Exception {
+	public NotALenderException(String message) {
+		super(message);
+	}
+}
diff --git a/finalproject/Question3/Test.java b/finalproject/Question3/Test.java
new file mode 100644
index 0000000..636cd79
--- /dev/null
+++ b/finalproject/Question3/Test.java
@@ -0,0 +1,7 @@
+public class Test {
+	public static void main(String[] args) {
+		User.testUser();
+		Lender.testLender();
+		Borrower.testBorrower();
+	}
+}
diff --git a/finalproject/Question3/User.java b/finalproject/Question3/User.java
new file mode 100644
index 0000000..6eec5a9
--- /dev/null
+++ b/finalproject/Question3/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
+	}
+}