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Java Composition | The Strong 'HAS-A' Relationship

Java Composition (Strong HAS-A) 🏠¶

Mentor's Note: Composition is about Ownership. It means an object is made up of smaller parts. If you destroy the main object, the parts are destroyed too. They are "bound" together forever. 💡

🌟 The Scenario: The Modern House 🏠¶

Think about your home. - The Whole: The House. - The Parts: The Kitchen, the Bedroom, and the Bathroom. - The Bond: If the house is demolished, the kitchen doesn't survive on its own in the middle of the street. It is part of the house. ✅

🎨 Visual Logic: Strong Parts¶

classDiagram
    House *-- Kitchen
    House *-- Bedroom

    Car *-- Engine
    Car *-- Steering

    Computer *-- CPU
    Computer *-- RAM

(-- represents strong ownership)*

💻 Java Implementation: The Ownership Lab¶

1. The Car & Engine 🚗¶

Logic: A Car has an Engine. The engine belongs to that car.

class Engine {
    void start() { System.out.println("Engine Vroom! ⚙️"); }
}

class Car {
    // Composition: The Engine is created INSIDE the Car
    private final Engine engine;

    Car() {
        this.engine = new Engine(); 
    }

    void drive() {
        engine.start();
        System.out.println("Car is moving... 🏎️");
    }
}

2. The Personal Computer 💻¶

Logic: A Computer has a CPU.

class CPU {
    String model = "Intel i9";
}

class Computer {
    private CPU cpu = new CPU(); // Strong bond

    void showSpecs() {
        System.out.println("Powered by: " + cpu.model);
    }
}

3. The Human Body 🧠¶

Logic: A Human has a Heart.

class Heart {
    void beat() { System.out.println("Lubb-Dupp... ❤️"); }
}

class Human {
    private final Heart heart = new Heart();
}

📖 Composition vs. Inheritance¶

Feature	Inheritance (IS-A)	Composition (HAS-A)
Bond	Permanent Family	Assembly of Parts
Visibility	Shares code with child	Keeps parts Private
Flexibility	Rigid	Flexible (can swap parts)
Rule	A Dog is an Animal	A Car has an Engine

💡 Interview Tip 👔¶

"Interviewers often say: 'Favor Composition over Inheritance.' Why? Because Composition allows you to change the parts easily without breaking the whole system. It makes your code more modular!"

💡 Pro Tip: "Composition is like LEGO. You build complex things by snapping together smaller, specialized bricks."

📈 Learning Path¶