Merge pull request #44 from TiagoRG/dev-tiagorg

[POO] aula05 DateYMD update
[POO] added DateYMD to utils
[POO] aula06 ex1 added
[POO] aula06 guide added
[POO] created validations
[POO] aula06 ex2 added
[POO] removed regex for names
[POO] README update
[POO] aula06 ex3 added

1ano/2semestre/poo/src/aula05/DateYMD.java

@@ -71,7 +71,7 @@ public class DateYMD {
     }
 
     public String toString() {
-        return String.format("%04d-%02d-%02d", year, month, day);
+        return String.format("%04d-%02d-%02d", this.year, this.month, this.day);
     }
     static boolean validMonth(int month) {
         return month >= 1 && month <= 12;

1ano/2semestre/poo/src/aula05/README.md

@@ -1,14 +1,16 @@
 # Programação Orientada a Objetos
 ## Aula 05
-### Tópico principal da aula: Inheritance
+### Tópico principal da aula: Classes
 
 * [Guião](https://github.com/TiagoRG/uaveiro-leci/tree/master/1ano/2semestre/poo/guides/POO-2022-aula05.pdf)
 * [Slides](https://github.com/TiagoRG/uaveiro-leci/tree/master/1ano/2semestre/poo/slides/POO_03_Classes.pdf)
 
 ### Exercise List
 | Exercise Number | File Name                                                                                                              |
-|-----------------|----------------------------------------------------------------------------------------------------------------|
+|-----------------|------------------------------------------------------------------------------------------------------------------------|
 | 1               | [DateYMD.java](https://github.com/TiagoRG/uaveiro-leci/blob/master/1ano/2semestre/poo/src/aula05/DateYMD.java)         |
+| 2               | [Calendar.java](https://github.com/TiagoRG/uaveiro-leci/blob/master/1ano/2semestre/poo/src/aula05/Calendar.java)       |
+| 3               | [AuctionDemo.java](https://github.com/TiagoRG/uaveiro-leci/blob/master/1ano/2semestre/poo/src/aula05/AuctionDemo.java) |
 
 ---
 *Pode conter erros, caso encontre algum, crie um* [*ticket*](https://github.com/TiagoRG/uaveiro-leci/issues/new)

1ano/2semestre/poo/src/aula06/Bolser.java

@@ -0,0 +1,43 @@
+package aula06;
+
+import utils.DateYMD;
+
+public class Bolser extends Student{
+    private Professor supervisor;
+    private double monthlyAmount;
+
+    public Bolser(String name, int cc, DateYMD birthDate, DateYMD registrationDate, Professor supervisor, double monthlyAmount) {
+        super(name, cc, birthDate, registrationDate);
+        this.setSupervisor(supervisor);
+        this.setMonthlyAmount(monthlyAmount);
+    }
+
+    public Bolser(String name, int cc, DateYMD birthDate, Professor supervisor, double monthlyAmount) {
+        this(name, cc, birthDate, null, supervisor, monthlyAmount);
+    }
+
+    public Professor getSupervisor() {
+        return this.supervisor;
+    }
+    public void setSupervisor(Professor supervisor) {
+        if (supervisor == null) {
+            throw new IllegalArgumentException("Supervisor cannot be null");
+        }
+        this.supervisor = supervisor;
+    }
+
+    public double getMonthlyAmount() {
+        return this.monthlyAmount;
+    }
+    public void setMonthlyAmount(double monthlyAmount) {
+        if (monthlyAmount < 0) {
+            throw new IllegalArgumentException("Monthly amount cannot be negative");
+        }
+        this.monthlyAmount = monthlyAmount;
+    }
+
+    @Override
+    public String toString() {
+        return String.format("%s; CC: %d; Date de nascimento: %s; Data de matrícula: %s; NMec: %d; Supervisor: %s; Montante mensal: %.2f", this.getName(), this.getCc(), this.getBirthDate(), this.getRegistrationDate(), this.getNMec(), this.supervisor, this.monthlyAmount);
+    }
+}

1ano/2semestre/poo/src/aula06/Contact.java

@@ -0,0 +1,55 @@
+package aula06;
+
+public class Contact {
+    private final int id;
+    private Person person;
+    private String email;
+    private String phone;
+
+    private static int currentId = 1;
+
+    public Contact(Person person, String email, String phone) {
+        if (email == null || email.isEmpty() || phone == null || phone.isEmpty())
+            throw new IllegalArgumentException("Either email or phone must be provided");
+        this.id = Contact.currentId++;
+        this.setPerson(person);
+        this.setEmail(email);
+        this.setPhone(phone);
+    }
+
+    public int getId() {
+        return id;
+    }
+
+    public Person getPerson() {
+        return person;
+    }
+    public void setPerson(Person person) {
+        if (person == null)
+            throw new IllegalArgumentException("Person must be provided");
+        this.person = person;
+    }
+
+    public String getEmail() {
+        return email;
+    }
+    public void setEmail(String email) {
+        if (!email.matches("^[a-zA-Z_0-9.]+@[a-zA-Z_0-9.]+\\.[a-zA-Z_0-9]+$"))
+            throw new IllegalArgumentException("Invalid email");
+        this.email = email;
+    }
+
+    public String getPhone() {
+        return phone;
+    }
+    public void setPhone(String phone) {
+        if (!phone.matches("^9[0-9]{8}$"))
+            throw new IllegalArgumentException("Invalid phone");
+        this.phone = phone;
+    }
+
+    @Override
+    public String toString() {
+        return String.format("ID: %d%nPerson: %s%nEmail: %s%nPhone: %s", this.id, this.person, this.email, this.phone);
+    }
+}

1ano/2semestre/poo/src/aula06/ContactList.java

@@ -0,0 +1,177 @@
+package aula06;
+
+import utils.DateYMD;
+
+import java.util.Scanner;
+
+public class ContactList {
+    private static final Scanner sin = new Scanner(System.in);
+    private static Contact[] contacts;
+
+    public static void main(String[] args) {
+
+        while (true) {
+            System.out.println("Selecione uma opção:");
+            System.out.println("1. Inserir contacto");
+            System.out.println("2. Alterar contacto");
+            System.out.println("3. Apagar contacto");
+            System.out.println("4. Procurar contacto");
+            System.out.println("5. Listar contactos");
+            System.out.println("0. Sair");
+            System.out.print("> ");
+            String option = sin.nextLine();
+            switch (option) {
+                case "0" -> {
+                    sin.close();
+                    System.exit(0);
+                }
+                case "1" -> {
+                    System.out.print("Insira o nome: ");
+                    String name = sin.nextLine();
+                    System.out.print("Insira o cc: ");
+                    String ccStr = sin.nextLine();
+                    int cc = Integer.parseInt(ccStr);
+                    System.out.print("Insira a data de nascimento (formado: dd-mm-yyyy): ");
+                    String date = sin.nextLine();
+                    String[] dateParts = date.split("-");
+                    DateYMD birthDate = new DateYMD(Integer.parseInt(dateParts[0]), Integer.parseInt(dateParts[1]), Integer.parseInt(dateParts[2]));
+                    System.out.print("Insira o email: ");
+                    String email = sin.nextLine();
+                    System.out.print("Insira o telefone: ");
+                    String phone = sin.nextLine();
+                    Person person = new Person(name, cc, birthDate);
+
+                    Contact contact = new Contact(person, email, phone);
+
+                    if (contacts == null) {
+                        contacts = new Contact[1];
+                        contacts[0] = contact;
+                    } else {
+                        if (checkIfContactExists(cc)) break;
+                        Contact[] newContacts = new Contact[contacts.length + 1];
+                        System.arraycopy(contacts, 0, newContacts, 0, contacts.length);
+                        newContacts[contacts.length] = contact;
+                        contacts = newContacts;
+                    }
+                }
+                case "2" -> {
+                    System.out.print("Insira o nome, email ou telefone do contacto que pretende alterar: ");
+                    String query = sin.nextLine();
+                    int[] indexes = searchContactsIndex(query);
+                    System.out.printf("Contactos encontrados: %d%n", indexes.length);
+
+                    if (indexes.length == 0) {
+                        System.out.println("Não foram encontrados contactos com esse nome, email ou telefone");
+                    } else if (indexes.length == 1) {
+                        System.out.printf("Alterando contacto: %s%n", contacts[indexes[0]]);
+                        System.out.print("Insira o novo telefone: ");
+                        String phone = sin.nextLine();
+                        System.out.print("Insira o novo email: ");
+                        String email = sin.nextLine();
+                        contacts[indexes[0]].setPhone(phone);
+                        contacts[indexes[0]].setEmail(email);
+                    } else {
+                        Contact contact = selectContact(indexes);
+                        System.out.printf("Alterando contacto: %s%n", contact);
+                        System.out.print("Insira o novo telefone: ");
+                        String phone = sin.nextLine();
+                        System.out.print("Insira o novo email: ");
+                        String email = sin.nextLine();
+                        contact.setPhone(phone);
+                        contact.setEmail(email);
+                    }
+                }
+                case "3" -> {
+                    System.out.print("Insira o nome, email ou telefone do contacto que pretende alterar: ");
+                    String query = sin.nextLine();
+                    int[] indexes = searchContactsIndex(query);
+                    System.out.printf("Contactos encontrados: %d%n", indexes.length);
+
+                    if (indexes.length == 0) {
+                        System.out.println("Não foram encontrados contactos com esse nome, email ou telefone");
+                    } else if (indexes.length == 1) {
+                        System.out.printf("Apagando contacto: %s%n", contacts[indexes[0]]);
+                        Contact[] newContacts = new Contact[contacts.length - 1];
+                        System.arraycopy(contacts, 0, newContacts, 0, indexes[0]);
+                        System.arraycopy(contacts, indexes[0] + 1, newContacts, indexes[0], contacts.length - indexes[0] - 1);
+                        contacts = newContacts;
+                    } else {
+                        Contact contact = selectContact(indexes);
+                        System.out.printf("Apagando contacto: %s%n", contact);
+                        Contact[] newContacts = new Contact[contacts.length - 1];
+                        System.arraycopy(contacts, 0, newContacts, 0, contact.getId());
+                        System.arraycopy(contacts, contact.getId() + 1, newContacts, contact.getId(), contacts.length - contact.getId() - 1);
+                        contacts = newContacts;
+                    }
+                }
+                case "4" -> {
+                    System.out.print("Insira o nome, email ou telefone do contacto que pretende alterar: ");
+                    String query = sin.nextLine();
+                    int[] indexes = searchContactsIndex(query);
+                    System.out.printf("Contactos encontrados: %d%n", indexes.length);
+
+                    if (indexes.length == 0) {
+                        System.out.println("Não foram encontrados contactos com esse nome, email ou telefone");
+                    } else if (indexes.length == 1) {
+                        System.out.println(contacts[indexes[0]]);
+                    } else {
+                        for (int index : indexes)
+                            System.out.printf("%s%n", contacts[index]);
+                    }
+                    System.out.println();
+                }
+                case "5" -> {
+                    for (Contact contact : contacts)
+                        System.out.printf("%s%n", contact);
+                    System.out.println();
+                }
+                default -> System.out.println("Opção inválida!");
+            }
+        }
+    }
+
+    private static boolean checkIfContactExists(int cc) {
+        for (Contact c : contacts) {
+            if (c.getPerson().getCc() == cc) {
+                System.out.print("Já existe um contacto para essa pessoa, pretende criar um novo? (s/n): ");
+                String answer = sin.nextLine();
+                return answer.equals("s");
+            }
+        }
+        return true;
+    }
+
+    private static int[] searchContactsIndex(String query) {
+        int[] indexes = new int[contacts.length];
+        int index = 0;
+        for (int i = 0; i < contacts.length; i++) {
+            if (contacts[i].getPerson().getName().contains(query) || contacts[i].getEmail().contains(query) || contacts[i].getPhone().contains(query)) {
+                indexes[index] = i;
+                index++;
+            }
+        }
+        return indexes;
+    }
+
+    private static Contact selectContact(int[] indexes) {
+        System.out.println("Foram encontrados vários contactos com esse nome, email ou telefone, selecione um:");
+        for (int index : indexes)
+            System.out.printf("%s%n", contacts[index]);
+
+        Contact contact;
+        do {
+            System.out.print("Insira o ID do contacto que pretende alterar: ");
+            int index = sin.nextInt();
+            contact = searchContactById(index);
+        } while (contact == null);
+
+        return contact;
+    }
+
+    private static Contact searchContactById(int id) {
+        for (Contact contact : contacts) {
+            if (contact.getId() == id) return contact;
+        }
+        return null;
+    }
+}

1ano/2semestre/poo/src/aula06/Person.java

@@ -0,0 +1,47 @@
+package aula06;
+
+import utils.DateYMD;
+
+public class Person {
+    private String name;
+    private int cc;
+    private DateYMD birthDate;
+
+    public Person(String name, int cc, DateYMD birthDate) {
+        this.setName(name);
+        this.setCc(cc);
+        this.setBirthDate(birthDate);
+    }
+
+    public String getName() {
+        return this.name;
+    }
+    public void setName(String name) {
+        if (name == null || name.isEmpty())
+            throw new IllegalArgumentException("Name cannot be null or empty");
+        this.name = name;
+    }
+
+    public int getCc() {
+        return this.cc;
+    }
+    public void setCc(int cc) {
+        if (String.valueOf(cc).length() != 8)
+            throw new IllegalArgumentException("CC must have 8 digits");
+        this.cc = cc;
+    }
+
+    public DateYMD getBirthDate() {
+        return this.birthDate;
+    }
+    public void setBirthDate(DateYMD birthDate) {
+        if (birthDate == null)
+            throw new IllegalArgumentException("Birth date cannot be null");
+        this.birthDate = birthDate;
+    }
+
+    @Override
+    public String toString() {
+        return String.format("%s; CC: %d; Data de nascimento: %s", this.name, this.cc, this.birthDate);
+    }
+}

1ano/2semestre/poo/src/aula06/PersonTest.java

@@ -0,0 +1,21 @@
+package aula06;
+
+import java.util.Scanner;
+import utils.DateYMD;
+
+public class PersonTest {
+    public static void main(String[] args) {
+        Scanner sin = new Scanner(System.in);
+
+        Student al = new Student ("Andreia Melo", 98556781,new DateYMD(18, 7, 1990), new DateYMD(1, 9, 2018));
+        Professor p1 = new Professor("Jorge Almeida", 34672215, new DateYMD(13, 3, 1967), "Associado", "Informática");
+        Bolser bls = new Bolser ("Igor Santos", 89765431, new DateYMD(11, 5, 1985), p1, 900);
+        bls.setMonthlyAmount(1050);
+        System.out.println("Student:"+ al.getName());
+        System.out.println(al);
+        System.out.println("Bolser:"+ bls.getName() + ", NMec: " + bls.getNMec() + ", Bolsa:" +  bls.getMonthlyAmount()+ ", Orientador:" +  bls.getSupervisor());
+        System.out.println(bls);
+
+        sin.close();
+    }
+}

1ano/2semestre/poo/src/aula06/Professor.java

@@ -0,0 +1,39 @@
+package aula06;
+
+import utils.DateYMD;
+
+public class Professor extends Person {
+    private String category;
+    private String department;
+
+    public Professor(String name, int cc, DateYMD birthDate, String category, String department) {
+        super(name, cc, birthDate);
+        this.setCategory(category);
+        this.setDepartment(department);
+    }
+
+    public String getCategory() {
+        return this.category;
+    }
+    public void setCategory(String category) {
+        if (category == null || category.isEmpty())
+            throw new IllegalArgumentException("Category cannot be null or empty");
+        if (!(category.equals("Auxiliar") || category.equals("Associado") || category.equals("Catedrático")))
+            throw new IllegalArgumentException("Invalid category");
+        this.category = category;
+    }
+
+    public String getDepartment() {
+        return this.department;
+    }
+    public void setDepartment(String department) {
+        if (department == null || department.isEmpty())
+            throw new IllegalArgumentException("Department cannot be null or empty");
+        this.department = department;
+    }
+
+    @Override
+    public String toString() {
+        return String.format("%s; CC: %d; Date de nascimento: %s; Categoria: %s; Departamento: %s", this.getName(), this.getCc(), this.getBirthDate(), this.category, this.department);
+    }
+}

1ano/2semestre/poo/src/aula06/README.md

@@ -0,0 +1,16 @@
+# Programação Orientada a Objetos
+## Aula 06
+### Tópico principal da aula: Inheritance
+
+* [Guião](https://github.com/TiagoRG/uaveiro-leci/tree/master/1ano/2semestre/poo/guides/POO-2022-aula06.pdf)
+* [Slides](https://github.com/TiagoRG/uaveiro-leci/tree/master/1ano/2semestre/poo/slides/POO_04_Herança.pdf)
+
+### Exercise List
+| Exercise Number | Files Name                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
+|-----------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| 1               | [PersonTest.java](https://github.com/TiagoRG/uaveiro-leci/blob/master/1ano/2semestre/poo/src/aula06/PersonTest.java)<br/>[Person.java](https://github.com/TiagoRG/uaveiro-leci/blob/master/1ano/2semestre/poo/src/aula06/Person.java)<br/>[Student.java](https://github.com/TiagoRG/uaveiro-leci/blob/master/1ano/2semestre/poo/src/aula06/Student.java)<br/>[Professor.java](https://github.com/TiagoRG/uaveiro-leci/blob/master/1ano/2semestre/poo/src/aula06/Professor.java)<br/>[Bolser.java](https://github.com/TiagoRG/uaveiro-leci/blob/master/1ano/2semestre/poo/src/aula06/Bolser.java) |
+| 2               | [Contact.java](https://github.com/TiagoRG/uaveiro-leci/blob/master/1ano/2semestre/poo/src/aula06/Contact.java)<br/>[ContactList.java](https://github.com/TiagoRG/uaveiro-leci/blob/master/1ano/2semestre/poo/src/aula06/ContactList.java)                                                                                                                                                                                                                                                                                                                                                        |
+| 3               | [Vector.java](https://github.com/TiagoRG/uaveiro-leci/blob/master/1ano/2semestre/poo/src/aula06/Vector.java)<br/>[VectorTest.java](https://github.com/TiagoRG/uaveiro-leci/blob/master/1ano/2semestre/poo/src/aula06/VectorTest.java)                                                                                                                                                                                                                                                                                                                                                            |
+
+---
+*Pode conter erros, caso encontre algum, crie um* [*ticket*](https://github.com/TiagoRG/uaveiro-leci/issues/new)

1ano/2semestre/poo/src/aula06/Student.java

@@ -0,0 +1,40 @@
+package aula06;
+
+import utils.DateYMD;
+import java.time.LocalDate;
+
+public class Student extends Person {
+    private DateYMD registrationDate;
+    private int nMec;
+    public static int currentNMec = 100;
+
+    public Student(String name, int cc, DateYMD birthDate, DateYMD registrationDate) {
+        super(name, cc, birthDate);
+        this.setRegistrationDate(registrationDate);
+        this.setNMec(Student.currentNMec++);
+    }
+
+    public Student(String name, int age, DateYMD birthDate) {
+        this(name, age, birthDate, null);
+    }
+
+    public int getNMec() {
+        return this.nMec;
+    }
+    public void setNMec(int nMec) {
+        this.nMec = nMec;
+    }
+
+    public DateYMD getRegistrationDate() {
+        return this.registrationDate;
+    }
+    public void setRegistrationDate(DateYMD registrationDate) {
+        LocalDate now = LocalDate.now();
+        this.registrationDate = registrationDate == null ? new DateYMD(now.getDayOfMonth(), now.getMonthValue(), now.getYear()) : registrationDate;
+    }
+
+    @Override
+    public String toString() {
+        return String.format("%s; CC: %d; Date de nascimento: %s; Data de matrícula: %s; NMec: %d", this.getName(), this.getCc(), this.getBirthDate(), this.registrationDate, this.nMec);
+    }
+}

1ano/2semestre/poo/src/aula06/Vector.java

@@ -0,0 +1,95 @@
+package aula06;
+
+public class Vector {
+    private int[] vector;
+
+    // Constructors and main getter
+    public Vector() {
+        this.vector = new int[0];
+    }
+    public Vector(int size) {
+        this.vector = new int[size];
+    }
+    public Vector(int[] vector) {
+        this.vector = vector;
+    }
+    public int[] getVector() {
+        return vector;
+    }
+
+    // Getters
+    public int size() {
+        return this.vector.length;
+    }
+    public boolean contains(int value) {
+        for (int n : this.vector)
+            if (n == value)
+                return true;
+        return false;
+    }
+    public int count(int value) {
+        int count = 0;
+        for (int n : this.vector)
+            if (n == value)
+                count++;
+        return count;
+    }
+
+    // Method to change values
+    public void insert(int value) {
+        if (this.contains(value)) return;
+        int[] aux = new int[this.size()+1];
+        System.arraycopy(this.vector, 0, aux, 0, this.size());
+        aux[this.size()] = value;
+        this.vector = aux;
+    }
+    public void remove(int value) {
+        int[] aux = new int[this.size()-this.count(value)];
+        int i = 0;
+        for (int n : this.vector) {
+            if (n == value)
+                continue;
+            aux[i] = n;
+            i++;
+        }
+        this.vector = aux;
+    }
+    public void empty() {
+        this.vector = new int[0];
+    }
+
+    // Interact with other vectors
+    public Vector add(Vector secondVector) {
+        Vector result = new Vector();
+        for (int n : this.vector)
+            if (!result.contains(n))
+                result.insert(n);
+        for (int n : secondVector.vector)
+            if (!result.contains(n))
+                result.insert(n);
+        return result;
+    }
+    public Vector subtract(Vector secondVector) {
+        Vector result = new Vector();
+        for (int n : this.vector)
+            if (!result.contains(n) && !secondVector.contains(n))
+                result.insert(n);
+        return result;
+    }
+    public Vector intersect(Vector secondVector) {
+        Vector result = new Vector();
+        for (int n : this.vector)
+            if (!result.contains(n) && secondVector.contains(n))
+                result.insert(n);
+        return result;
+    }
+
+    // ToString
+    @Override
+    public String toString() {
+        StringBuilder result = new StringBuilder();
+        for (int n : this.vector)
+            result.append(String.format("%d ", n));
+        return this.size() > 0 ? result.substring(0, result.length()-1) : result.toString();
+    }
+}

1ano/2semestre/poo/src/aula06/VectorTest.java

@@ -0,0 +1,31 @@
+package aula06;
+
+
+public class VectorTest {
+    public static void main(String[] args) {
+        Vector c1 = new Vector();
+        c1.insert(4); c1.insert(7); c1.insert(6); c1.insert(5);
+
+        Vector c2 = new Vector();
+        int[] test = { 7, 3, 2, 5, 4, 6, 7};
+        for (int el : test) c2.insert(el);
+        c2.remove(3); c2.remove(5); c2.remove(6);
+
+        System.out.println(c1);
+        System.out.println(c2);
+
+        System.out.println("Número de elementos em c1: " + c1.size());
+        System.out.println("Número de elementos em c2: " + c2.size());
+        
+        System.out.println("c1 contém 6?: " + ((c1.contains(6) ? "sim" : "não")));
+        System.out.println("c2 contém 6?: " + ((c2.contains(6) ? "sim" : "não")));
+
+        System.out.println("União:" + c1.add(c2));
+        System.out.println("Interseção:" + c1.intersect(c2));
+        System.out.println("Diferença:" + c1.subtract(c2));
+
+        c1.empty();
+        System.out.println("c1:" + c1);
+
+    }
+}

1ano/2semestre/poo/src/utils/DateYMD.java

@@ -0,0 +1,94 @@
+package utils;
+
+public class DateYMD {
+    private int day;
+    private int month;
+    private int year;
+
+    public DateYMD(int day, int month, int year) {
+        if (!validDate(day, month, year))
+            throw new IllegalArgumentException("Invalid date");
+        this.day = day;
+        this.month = month;
+        this.year = year;
+    }
+
+    public void set(int day, int month, int year) {
+        if (!validDate(day, month, year))
+            throw new IllegalArgumentException("Invalid date");
+        this.day = day;
+        this.month = month;
+        this.year = year;
+    }
+
+    public int[] get() {
+        return new int[]{this.day, this.month, this.year};
+    }
+
+    public int getDay() {
+        return day;
+    }
+
+    public int getMonth() {
+        return month;
+    }
+
+    public int getYear() {
+        return year;
+    }
+
+    public void increment() {
+        if (this.day < monthDays(this.month, this.year))
+            this.day++;
+        else if (this.month < 12) {
+            this.day = 1;
+            this.month++;
+        } else {
+            this.day = 1;
+            this.month = 1;
+            this.year++;
+        }
+    }
+
+    public void addDays(int days) {
+        for (int i = 0; i < days; i++)
+            this.increment();
+    }
+
+    public void decrement() {
+        if (this.day > 1)
+            this.day--;
+        else if (this.month > 1) {
+            this.day = monthDays(this.month - 1, this.year);
+            this.month--;
+        } else {
+            this.day = 31;
+            this.month = 12;
+            this.year--;
+        }
+    }
+
+    public String toString() {
+        return String.format("%04d-%02d-%02d", this.year, this.month, this.day);
+    }
+    static boolean validMonth(int month) {
+        return month >= 1 && month <= 12;
+    }
+
+    static int monthDays(int month, int year) {
+        if (!validMonth(month))
+            return -1;
+        int[] daysPerMonth = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
+        if (month == 2 && isLeapYear(year))
+            return 29;
+        return daysPerMonth[month - 1];
+    }
+
+    static boolean isLeapYear(int year) {
+        return year % 100 == 0 ? year % 400 == 0 : year % 4 == 0;
+    }
+
+    public static boolean validDate(int day, int month, int year) {
+        return day >= 1 && day <= monthDays(month, year);
+    }
+}