Composition in Java: The Essential OOP Concept Every Developer Must Know for Interviews

Composition in Java is a design principle used to model relationships between classes. It is one of the key concepts of object-oriented programming (OOP), and is often referred to as a "has-a" relationship. Composition enables a class to be constructed using objects of other classes, allowing you to design flexible and reusable systems.

Key Concepts of Composition:

"Has-a" Relationship:
- In composition, one class "has-a" reference to another class. This relationship implies that a class contains or is composed of one or more objects from other classes.
- For example, a Car class might "have-a" Engine class because a car contains an engine as part of its structure.
Strong Ownership:
- Composition implies strong ownership, meaning that the composed objects cannot exist without the parent object.
- If the parent object is destroyed, its composed objects are also destroyed.
- In this sense, composition is a tight coupling between objects, as the lifetime of the composed objects depends entirely on the lifetime of the container class (parent).
Different from Inheritance:
- Composition is different from inheritance in that inheritance defines an "is-a" relationship (e.g., Dog is a type of Animal), whereas composition models a "has-a" relationship (e.g., Car has an Engine).
Example: Suppose you want to model a Library class that "has-a" Book class, but a Book can exist independently of a Library. However, if you model a Library class that is composed of Books, the Library object is responsible for the existence of Books in the system. If the library is destroyed, the books should also be destroyed.

Composition vs Aggregation:

Both composition and aggregation represent relationships where one class contains another. The difference lies in the strength of this relationship:
- Composition: The contained objects cannot exist independently from the container object. If the parent object is destroyed, all its components (child objects) are also destroyed.
- Aggregation: The contained objects can exist independently of the container object. If the parent object is destroyed, the child objects still exist.

Composition in Practice:

Here’s an example of composition in Java:

	class Engine {
	    private String type;

	    public Engine(String type) {
	        this.type = type;
	    }

	    public void start() {
	        System.out.println("Engine is starting...");
	    }
	}

	class Car {
	    private Engine engine;  // Car "has-a" Engine

	    public Car(String engineType) {
	        engine = new Engine(engineType); // Composition - Car creates an Engine
	    }

	    public void startCar() {
	        System.out.println("Car is starting...");
	        engine.start();  // Car delegates to Engine to start
	    }
	}

	public class Main {
	    public static void main(String[] args) {
	        Car car = new Car("V8");
	        car.startCar();  // Car is starting... \n Engine is starting...
	    }
	}

Key Characteristics in the Example:

The Car class has a reference to the Engine class. This establishes a "has-a" relationship.
The Car constructor creates an instance of the Engine class, demonstrating that the Car is responsible for the lifetime of the Engine.
When the Car is destroyed, its Engine is also destroyed because of the strong ownership implied by composition.

Advantages of Composition:

Flexibility:
- Composition allows classes to be composed of various components. You can change or replace components without changing the class that uses them.
- For example, you can create different types of Engine objects (e.g., ElectricEngine, GasEngine) and compose them into the Car class without modifying Car.
Reusability:
- You can reuse components (like Engine or Transmission) in different contexts. For example, an Engine might be used in a Car, but also in a Boat.
Decoupling:
- It promotes a more modular design where objects are loosely coupled, as classes can delegate functionality to their contained objects.

When to Use Composition:

When you need to model a part-whole relationship: If one object contains another object as part of its state, composition is a natural fit. Example: A House class might contain rooms, and each room would be composed of doors and windows.
When you want to ensure tight coupling: If the contained object’s lifecycle should be controlled entirely by the parent, composition makes sense.
When you need greater flexibility: Composition allows the parent class to be more flexible. It can change or adapt the components it is composed of without changing the parent class itself.

Disadvantages of Composition:

More complex design: While composition leads to more flexible systems, it can also result in a more complex design. You might end up with a large number of objects that need to be created and managed.
Tight coupling (sometimes): While composition is generally a form of loose coupling, in some cases, the strong ownership in composition can result in tightly coupled objects if not carefully designed.

Conclusion:

Composition is an essential concept in Java for creating flexible, maintainable, and reusable software. It helps in building "has-a" relationships, where one object contains or is composed of other objects. The key advantage is that it promotes modularity, making it easier to modify and extend software without major changes to existing code.

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