Association vs Aggregation

Association

1. Definition:
   • Association represents a relationship between two classes. It indicates how objects of one class are connected to objects of another class.
2. Types:
   • One-to-One: One instance of a class is associated with one instance of another class.
   • One-to-Many: One instance of a class is associated with multiple instances of another class.
   • Many-to-Many: Multiple instances of one class are associated with multiple instances of another class.
3. Key Points:
   • There is no ownership between the objects.
   • The lifespan of the objects is independent.
4. Example:

class Student {
    private String name;
}

class Course {
    private String title;
}

// A student can be associated with multiple courses, and courses can have multiple students.

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Aggregation(Has-A Relationship)

1. Definition:
   • Aggregation is a specialized form of Association where one class represents a whole and the other represents a part. It defines a “has-a” relationship.
2. Key Points:
   • It is a weak form of association.
   • The part can exist independently of the whole.
   • Objects in an aggregation can have an independent lifecycle.
3. Example:

class Department {
    private String name;
}

class Employee {
    private String name;
}

// A department has employees, but employees can exist even without the department.

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3. Composition (Strong Has-A Relationship)

• Definition: A stronger form of aggregation where the contained object cannot exist independently of the container.
• Example:

class Brain {
    //...
}

class Human {
    private final Brain brain = new Brain(); // composition
}

• Note: If the container object is destroyed, the contained object is also destroyed.
• Real-world analogy: A human has a brain. Without the human, the brain object doesn’t exist.

Key Differences

Aspect	Association	Aggregation
Definition	Describes a relationship between classes.	Represents a whole-part relationship.
Ownership	No ownership between objects.	Whole does not own the part.
Object Lifecycle	Objects have independent lifecycles.	Parts can exist independently of the whole.

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Relationship	Ownership	Lifecycle Dependency	Example
Association	No	Independent	Student ↔ Teacher
Aggregation	Shared	Independent	School ↔ Department
Composition	Strong	Dependent	Human → Brain

Summary

• Use Association to model general relationships between classes.
• Use Aggregation when a “whole-part” relationship exists but the part does not depend on the whole.