UML Class Diagrams

// Association  : A has a reference to B, but A does not own B's lifecycle
// Aggregation  : A has-a B (hollow diamond) — B can exist without A
// Composition  : A owns-a B (filled diamond) — B is destroyed when A is destroyed
// Inheritance  : A extends B (is-a relationship)
// Realization  : A implements B (can-do capability)

// ── Inheritance (is-a) ───────────────────────────────────────────────
// UML: Animal <|-- Dog  (open arrowhead, solid line)
public abstract class Animal {
    protected String name;
    public abstract void speak();
}
public class Dog extends Animal {
    @Override public void speak() { System.out.println("Woof!"); }
}

// ── Realization / Interface (can-do) ─────────────────────────────────
// UML: Flyable <|.. Bird  (open arrowhead, dashed line)
public interface Flyable { void fly(); }
public interface Swimmable { void swim(); }
public class Duck extends Animal implements Flyable, Swimmable {
    @Override public void speak() { System.out.println("Quack"); }
    @Override public void fly()   { System.out.println("Duck flying"); }
    @Override public void swim()  { System.out.println("Duck swimming"); }
}

// ── Association (uses-a) ──────────────────────────────────────────────
// UML: Driver --> Car  (open arrowhead, solid line, no diamond)
public class Driver {
    private Car car;  // Driver uses Car, but does not own its lifecycle
    public void drive(Car car) { this.car = car; car.start(); }
}

// ── Aggregation (has-a, hollow diamond) ──────────────────────────────
// UML: Library o-- Book  (hollow diamond on Library side)
// Book can exist independently of Library
public class Book {
    private final String isbn;
    private final String title;
    public Book(String isbn, String title) { this.isbn = isbn; this.title = title; }
    public String getTitle() { return title; }
}

public class Library {
    private final List<Book> books = new ArrayList<>();  // aggregation — books passed in
    public void addBook(Book book) { books.add(book); }
    public void removeBook(Book book) { books.remove(book); }
    // Books are NOT destroyed when Library is destroyed
}

// Usage
Book b1 = new Book("978-0", "Clean Code");
Book b2 = new Book("978-1", "Effective Java");
Library lib = new Library();
lib.addBook(b1);
lib.addBook(b2);
// b1 and b2 still exist after lib is garbage collected

// ── Composition (owns-a, filled diamond) ─────────────────────────────
// UML: House *-- Room  (filled diamond on House side)
// Room CANNOT exist without a House
public class Room {
    private final String name;
    private final int areaSqFt;
    Room(String name, int area) { this.name = name; this.areaSqFt = area; } // package-private
    public String getName() { return name; }
}

public class House {
    private final List<Room> rooms;  // composition — rooms created inside House
    public House(int bedrooms, int bathrooms) {
        rooms = new ArrayList<>();
        for (int i = 0; i < bedrooms;  i++) rooms.add(new Room("Bedroom " + (i+1), 200));
        for (int i = 0; i < bathrooms; i++) rooms.add(new Room("Bathroom " + (i+1), 50));
    }
    public List<Room> getRooms() { return Collections.unmodifiableList(rooms); }
    // When House is garbage collected, all Rooms are too
}

// ── Multiplicity Summary ──────────────────────────────────────────────
// Person (1) --- (0..*) Address   : one person, zero or many addresses
// Order  (1) --- (1..*) OrderItem : one order, one or more items
// Employee (0..1) --- (1) Department : employee may or may not belong to one department