Association vs Aggregation vs Composition in Java & OOP - Complete Guide with Code and Examples

• 1️⃣ What is Association?
  • ✅ Key Points:
  • 🛠️ Example:
• 2️⃣ What is Aggregation?
  • ✅ Key Points:
  • 🛠️ Java Example:
  • 🧠 Explanation:
• 3️⃣ What is Composition?
  • ✅ Key Points:
  • 🛠️ Java Example:
  • 🧠 Explanation:
• 🌍 Real-World Analogies
• 📊 Comparison Table
• 🔄 Diagrammatic Representation
• 🧠 When to Use What?
• 🎯 Key Interview Questions & Answers
• ✅ Summary

In Object-Oriented Programming, relationships between classes define how objects interact and depend on each other. The three most important types are:

• Association
• Aggregation
• Composition

Understanding these distinctions is vital for object modeling, class design, and interview success.

1️⃣ What is Association?

Association is a general relationship between two classes where one object uses or interacts with another.

✅ Key Points:

• Represents “uses-a” or “knows-a” relationship
• Objects are loosely coupled
• No ownership implied
• Can be unidirectional or bidirectional

🛠️ Example:

class Employee {
    String name;
}

class Department {
    Employee employee; // Association
}

2️⃣ What is Aggregation?

Aggregation is a weaker form of Composition. It represents a “has-a” relationship where the contained object can exist independently of the container.

✅ Key Points:

• Special case of Association
• Denotes partial ownership
• Object lifetime is independent
• Weak coupling between classes

🛠️ Java Example:

class Student {
    String name;
}

class School {
    List<Student> students; // Aggregation
}

🧠 Explanation:

• A Student can exist outside of the School
• Removing the School does not destroy the Student objects

3️⃣ What is Composition?

Composition is a stronger form of Aggregation. It implies ownership where the contained object cannot exist without the container.

✅ Key Points:

• Represents “has-a” with strong association
• Contained object is fully dependent
• When the container is destroyed, so is the contained object
• Denotes tight coupling

🛠️ Java Example:

class Engine {
    void start() {
        System.out.println("Engine started");
    }
}

class Car {
    private Engine engine = new Engine(); // Composition

    void startCar() {
        engine.start();
    }
}

🧠 Explanation:

• The Engine is part of the Car
• If the Car is destroyed, so is the Engine

🌍 Real-World Analogies

📊 Comparison Table

🔄 Diagrammatic Representation

Association:
[Doctor] → [Patient]

Aggregation:
[University] ◇── [Student]

Composition:
[Car] ◆── [Engine]

• Arrow ----->: Association
• Diamond <>: Aggregation
• Filled diamond ◼️: Composition

🧠 When to Use What?

🎯 Key Interview Questions & Answers

Q1. What’s the difference between Aggregation and Composition? A: Aggregation allows independent existence of the contained object. Composition implies strong ownership — the contained object can't exist without the container.

Q2. Can an object have multiple relationships? A: Yes. For example, a Person may be associated with a Company (Association), own a Car (Composition), and borrow a Book from a Library (Aggregation).

Q3. Which is tighter: Aggregation or Composition? A: Composition is tighter. Aggregation is a weaker association.

Q4. Is inheritance a type of Association? A: No. Inheritance (is-a) defines hierarchy, while Association (uses-a) defines object interaction.

Q5. Can Composition exist without a constructor? A: Typically, Composition is initialized in the constructor or directly inline, but technically it can exist outside constructor too.

✅ Summary

Understanding the nuances of object relationships is key to writing modular, scalable, and maintainable object-oriented code.

Whether you're preparing for interviews or designing a real-world application — mastering these concepts is essential for any Java or OOP developer.