Desafio: hora da prática

Eu fiz da seguinte forma:

public class BankAccount {
    private int accountNumber;
    private double balance;
    public String holder;

    public int getAccountNumber() {
        return accountNumber;
    }

    public double getBalance() {
        return balance;
    }

    public void setAccountNumber(int accountNumber) {
        this.accountNumber = accountNumber;
    }

    public void setBalance(double balance) {
        this.balance = balance;
    }
}

public class PersonAge {
    private String name;
    private int age;
    public String systemMessage;

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

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

    public void setAge(int age) {
        this.age = age;
    }

    public void testAge() {
        if (age >= 18) {
            systemMessage = """
                ATENÇÃO: O usuário %s é MAIOR de idade
                """.formatted(name);
            System.out.println(systemMessage);
        } else {
            systemMessage = """
                ATENÇÃO: O usuário %s é MENOR de idade
                """.formatted(name);
            System.out.println(systemMessage);
        }

    }
}

public class Product {
    private String productName;
    private double price;

    public String getProductName () {
        return productName;
    }

    public double getPrice () {
        return price;
    }

    public void setProductName (String productName) {
        this.productName = productName;
    }

    public void setPrice (double price) {
        this.price = price;
    }

    public double discount (double percentage) {
        double discount = price * (percentage / 100);
        price -= discount;
        return discount;
    }

}

public class Student {
    private String studentName;
    private double[] studentNotes;


    public void setStudentName(String studentName) {
        this.studentName = studentName;
    }

    public void setStudentNotes(double[] studentNotes) {
        this.studentNotes = studentNotes;
    }

    public String getStudentName() {
        return studentName;
    }

    public double[] getStudentNotes() {
        return studentNotes;
    }

    public double calculateAverage () {
        if (studentNotes.length == 0) {
            return 0;
        }
        double total = 0;
        for (double notes: studentNotes) {
            total += notes;
        }
        return total / studentNotes.length;
    }

    public int numberOfNotes () {
        if (studentNotes.length == 0) {
            return 0;
        } else {
            return studentNotes.length;
        }
    }


}

public class Book {
    private String titleBook;
    private String authorBook;

    public void setTitleBook(String titleBook) {
        this.titleBook = titleBook;
    }

    public void setAuthorBook(String authorBook) {
        this.authorBook = authorBook;
    }

    public String getTitleBook() {
        return titleBook;
    }

    public String getAuthorBook() {
        return authorBook;
    }

    public void displayDetails () {
        String systemMessage;
        systemMessage = """
                Nome do Livro: %s
                Autor do Livro: %s
                """.formatted(getTitleBook(), getAuthorBook());
        System.out.println(systemMessage);
    }

}