This is one of the most common areas of confusion when learning about object-oriented relationships. Let me break it down for you in a clear and practical way.

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🛠️ Both are “has-a” Relationships

Both composition and aggregation describe a “has-a” relationship between two classes — meaning one class contains or owns another class. The difference is the strength and nature of ownership.

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📦 1️⃣ Composition (Strong Ownership)

What is Composition?

• One object is a part of another object.
• The child object’s lifecycle is fully dependent on the parent.
• If the parent dies, the child dies too.
• Tight coupling between parent and child.

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Real-World Example

A Car has an Engine.

• The engine cannot exist independently — it’s part of the car.
• When you destroy the car, the engine is gone too.

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Code Example

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

class Car {
    private final Engine engine;

    public Car() {
        this.engine = new Engine(); // Car creates its own engine
    }

    public void start() {
        engine.start();
        System.out.println("Car started");
    }
}

✅ Car owns and manages the Engine — strong lifecycle dependency.

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🏘️ 2️⃣ Aggregation (Weak Ownership)

What is Aggregation?

• One object has a reference to another object.
• The child object can exist independently of the parent.
• If the parent dies, the child can continue to exist.
• Looser coupling than composition.

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Real-World Example

A Library has Books.

• The books exist independently — they aren’t destroyed if the library closes.
• Books can also belong to other libraries.

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Code Example

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

class Library {
    private List<Book> books;

    public Library(List<Book> books) {
        this.books = books; // Library gets books from outside
    }
}

✅ Library aggregates Book, but does not own their lifecycle — books can exist without the library.

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🔥 Quick Comparison Table

Aspect	Composition	Aggregation
Ownership	Strong (parent owns child fully)	Weak (parent just holds reference)
Lifecycle	Parent & child live/die together	Child can outlive parent
Example	Car & Engine	Library & Book
Object Creation	Parent usually creates the child	Child is created outside and passed in
Coupling	Tighter	Looser

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💡 Rule of Thumb

Question	Which to use?
Does the part only exist inside the whole?	Use composition.
Can the part exist independently and be reused elsewhere?	Use aggregation.