Association vs. Aggregation vs. Composition

Association, Aggregation, and Composition are types of relationships that describe the interaction and dependency between objects in Object-Oriented Analysis and Design (OOAD). Each type has its own unique characteristics:

Association

Association represents a general connection or relationship between two classes, where each class is aware of the other. This relationship doesn't imply ownership; both classes are relatively independent of each other. It is typically depicted as a simple line connecting two classes.

Characteristics

• No ownership implied (neither class owns the other).
• The objects involved in the association can exist independently.
• Usually denoted with a simple line between classes in UML diagrams.

Example

A student enrolls in courses, and a course has students enrolled in it, but they don't own each other. The Student and Course classes are associated since they interact, but they can exist independently.

Student ------------ Course

class Student {
    private String name;

    public void enroll(Course course) {
        System.out.println(name + " enrolled in " + course.getName());
    }
}

class Course {
    private String name;

    public String getName() {
        return name;
    }
}

Aggregation

Aggregation is a specialized form of association that represents a "whole-part" relationship between classes. Here, one class is a container or aggregator, but it doesn’t imply strong ownership. The aggregated objects can exist independently outside the lifecycle of the aggregator.

Characteristics

• Represents a whole-part relationship.
• The part (or aggregated) can exist independently of the whole.
• It’s a weaker form of composition, signified by a hollow diamond at the end of the line connecting the aggregator to the part.

Example

A library is composed of books (whole-part), but a book can still exist outside of a library. If a library is closed, the books can still exist in other libraries or as standalone objects.

Library ◇----------- Book

class Library {
    private List<Book> books;

    public void addBook(Book book) {
        books.add(book);
    }
}

class Book {
    private String title;

    public String getTitle() {
        return title;
    }
}

Composition

Composition is a more specific type of aggregation that implies a stronger ownership between classes. In this relationship, the "whole" class cannot exist without its "part" class, and if the whole object is destroyed, its parts are also destroyed.

Characteristics

• Represents a whole-part relationship with strong ownership.
• The lifecycle of the part class depends on the whole.
• Illustrated with a filled diamond on the UML diagram, connecting the whole to the part.

Example

A house is composed of rooms, but rooms cannot exist independently of the house. If the house is demolished, the rooms cease to exist as well.

House ◆----------- Room

class House {
    private List<Room> rooms;

    public House() {
        rooms = new ArrayList<>();
    }

    public void addRoom(Room room) {
        rooms.add(room);
    }
}

class Room {
    private String name;

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

Summary of Differences

Relationship Type	Represents	Lifecycle Dependency	Symbol
Association	Simple relationship	Independent	Simple line
Aggregation	Whole-part	Part can exist independently of the whole	Hollow diamond
Composition	Strong whole-part	Part’s lifecycle depends on the whole	Filled diamond