public abstract class Entity {
    private int id;

    public Entity() {}
    public Entity(int id);
    public int getId();
    public void setId(int id);
}

// -----------------------------------------------------------------------------------------------------------------------------

public class Book extends Entity {

    private String name;
    private String author;
    private String genre;
    private int amount;  /* кол-во экземпляров */
    private String information;

    public Book() {}
    public Book(int id, String name, String author, String genreID, int amount, String information);

   /* sets & gets */

}

//-------------------------------------------------------------------------------------------------------------------------------

public class Order extends Entity {

    private int idorder;
    private String username;
    private int idbook;

    private int status; /* возвращена или забронированa */
    private String name;
    private String author;
    private String information;
    private String date;
    private String genre;

    public Order() {
    }

    /* sets & gets */

}

//--------------------------------------------------------------------------------------------------------------------------------

public class User extends Entity {

    private String username;
    private String email;
    private String phone;
    private String password;

    public User() {}
    /* sets & gets */

}