Aggregation vs Composition

In object-oriented programming, both aggregation and composition are concepts that describe relationships between classes. They both represent ways to establish connections between objects. Let’s explore the differences between aggregation and composition using examples

Aggregation:

Aggregation represents a “has-a” or “contains-a” relationship between classes. In aggregation, one class contains another class as a part, but the contained class can exist independently. If the container class is destroyed, the contained class can still exist. Aggregation is often depicted as a weaker relationship than composition. Aggregation represents a “has-a” relationship where one class contains another class as a part, but the contained class can exist independently.

Example 1: Library and Books

public class Library {
    private List<Book> books;

    public Library(List<Book> books) {
        this.books = books;
    }
}

public class Book {
    private String title;

    public Book(String title) {
        this.title = title;
    }
}

A Library class contains multiple Book objects, but the books can exist outside the library.

Example 2: University and Students

public class University {
    private List<Student> students;

    public University(List<Student> students) {
        this.students = students;
    }
}

public class Student {
    private String name;

    public Student(String name) {
        this.name = name;
    }
}

A University class has multiple Student objects. Students can graduate and continue their lives outside the university.

Example 3: Company and Employees

public class Company {
    private List<Employee> employees;

    public Company(List<Employee> employees) {
        this.employees = employees;
    }
}

public class Employee {
    private String name;

    public Employee(String name) {
        this.name = name;
    }
}

A Company class has multiple Employee objects. If the company is dissolved, employees can still exist independently.

Example 4: Car Dealership and Cars

public class CarDealership {
    private List<Car> cars;

    public CarDealership(List<Car> cars) {
        this.cars = cars;
    }
}

public class Car {
    private String model;

    public Car(String model) {
        this.model = model;
    }
}

A CarDealership class has a collection of Car objects. If the dealership closes, the cars are not necessarily destroyed.

Composition:

Composition represents a stronger relationship where one class is composed of one or more other classes, and the composed classes cannot exist independently without the container class.

Example 1: Computer and Processor

public class Computer {
    private Processor processor;

    public Computer(Processor processor) {
        this.processor = processor;
    }
}

public class Processor {
    // Processor details
}

A Computer class is composed of a Processor class. A computer without a processor is not functional

Example 2: House and Rooms

public class House {
    private List<Room> rooms;

    public House(List<Room> rooms) {
        this.rooms = rooms;
    }
}

public class Room {
    // Room details
}

A House class is composed of multiple Room objects. A house without rooms would not be a valid concept.

Example 3: Car and Engine

public class Car {
    private Engine engine;

    public Car(Engine engine) {
        this.engine = engine;
    }
}

public class Engine {
    // Engine details
}

A Car class is composed of an Engine class. A car without an engine is just a shell.

Example 4: Human Body and Heart

public class HumanBody {
    private Heart heart;

    public HumanBody(Heart heart) {
        this.heart = heart;
    }
}

public class Heart {
    // Heart details
}

A HumanBody class is composed of a Heart class. The body cannot function without the heart.

Example 5: Playlist and Songs

public class Playlist {
    private List<Song> songs;

    public Playlist(List<Song> songs) {
        this.songs = songs;
    }
}

public class Song {
    // Song details
}

A Playlist class is composed of various Song objects. A playlist without songs wouldn't make sense.