COI Principle(Composition Over Inheritance)

Definition:

Composition Over Inheritance is a design principle that suggests building complex objects by combining simpler, independent components (composition) rather than creating rigid parent-child class hierarchies (inheritance). It promotes flexibility, reusability, and easier maintenance by favoring “has-a” relationships over “is-a” relationships.

Key Benefits

• Flexibility: Easy to change behavior at runtime
• Loose coupling: Components are independent
• Better testing: Test components separately
• Avoid inheritance problems: No deep, complex hierarchies

Example:

• Car Manufacturing Inheritance (Rigid):

Vehicle → Car → SedanCar → LuxurySedanCar → ElectricLuxurySedanCar

Composition (Flexible):

class Car {
    private Engine engine;           // Has an engine
    private TransmissionSystem transmission; // Has a transmission
    private EntertainmentSystem entertainment; // Has entertainment

    // Can swap components easily
    public void upgradeEngine(Engine newEngine) {
        this.engine = newEngine;
    }

    public void addEntertainment(EntertainmentSystem system) {
        this.entertainment = system;
    }
}

// Mix and match components
Car economyCar = new Car(new EconomyEngine(), new ManualTransmission());
Car luxuryCar = new Car(new V8Engine(), new AutomaticTransmission(), new PremiumAudio());