Merge pull request #124 from apurvatrpth/compositon-vs-aggregator

Add README.MD for AggregationVsComposition on code level

Author: ashishps1

oop/java/AggregationVsComposition/README.MD

@@ -0,0 +1,110 @@
+# Aggregation vs Composition in Java (Code-Focused)
+
+In OOP, **Aggregation** and **Composition** both express "has-a" relationships, but they differ in **ownership**, **object lifecycle**, and **instantiation strategy**.
+
+---
+
+## 🔗 Aggregation — External Object Reference
+
+### ✅ Key Code Characteristic:
+> The referenced object is **passed from outside** and can exist independently.
+
+```java
+class Engine {
+    String type;
+    Engine(String type) {
+        this.type = type;
+    }
+}
+
+class Car {
+    Engine engine; // Aggregation - reference passed from outside
+
+    Car(Engine engine) {
+        this.engine = engine;
+    }
+
+    void showEngineType() {
+        System.out.println("Engine: " + engine.type);
+    }
+}
+
+public class AggregationDemo {
+    public static void main(String[] args) {
+        Engine e1 = new Engine("V8");       // Created externally
+        Car car1 = new Car(e1);             // Shared with car1
+
+        Engine e2 = new Engine("V6");
+        Car car2 = new Car(e2);             // Shared with car2
+
+        car1.showEngineType(); // Engine: V8
+        car2.showEngineType(); // Engine: V6
+    }
+}
+```
+
+**🧠 Key Point:**  
+- `Car` does **not own** `Engine`.
+- The same `Engine` can be reused across multiple `Car` objects.
+- `Engine` can outlive or live independently of `Car`.
+
+---
+
+## 🔒 Composition — Object Created Internally
+
+### ✅ Key Code Characteristic:
+> The referenced object is **created inside** the constructor. It **cannot exist without the parent.**
+
+```java
+class Engine {
+    String type;
+    Engine(String type) {
+        this.type = type;
+    }
+}
+
+class Car {
+    private Engine engine; // Composition - created inside
+
+    Car(String engineType) {
+        this.engine = new Engine(engineType); // tightly coupled
+    }
+
+    void showEngineType() {
+        System.out.println("Engine: " + engine.type);
+    }
+}
+
+public class CompositionDemo {
+    public static void main(String[] args) {
+        Car car = new Car("Electric");  // Engine is part of Car
+        car.showEngineType();           // Engine: Electric
+    }
+}
+```
+
+**🧠 Key Point:**  
+- `Car` **fully owns** `Engine`.
+- `Engine` cannot be reused or accessed independently.
+- Lifecycle of `Engine` is strictly bound to `Car`.
+
+---
+
+## 🔍 Code-Level Differences Summary
+
+| Aspect                 | Aggregation                                | Composition                               |
+|------------------------|---------------------------------------------|--------------------------------------------|
+| Object creation        | Object is passed from outside               | Object is created internally               |
+| Ownership              | Parent has a reference                      | Parent owns the object                     |
+| Lifecycle dependency   | Child can live without parent               | Child dies with parent                     |
+| Reusability            | Child can be reused by multiple parents     | Child is bound to one parent only          |
+| Code style             | `this.child = child;`                       | `this.child = new Child();` or similar     |
+
+---
+
+## 💡 When to Use
+
+- **Use Aggregation** when you want to share or reuse the same instance.
+- **Use Composition** when you want a part to be a core, non-detachable component of the object.
+
+---