What is the Difference Between Inheritance and Composition in Java?

What is the difference between inheritance and composition in Java, and how can I implement the composition pattern?

// Example of Composition in Java
class Engine {
    void start() {
        System.out.println("Engine starting...");
    }
}

class Car {
    private Engine engine;

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

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

Inheritance and composition are two fundamental concepts in object-oriented programming. They allow you to establish relationships between classes and enable code reusability. However, they differ significantly in their implementation and use cases.

// Example of Composition in Java

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

class Car {
    private Engine engine;

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

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

// Usage
public class Main {
    public static void main(String[] args) {
        Car myCar = new Car();
        myCar.start(); // Outputs engine start and car start messages
    }
}

Causes

• **Inheritance** allows a class to inherit properties and methods from another class, promoting a hierarchical relationship.
• **Composition** entails building a class using references to one or more objects from other classes, allowing for flexible and dynamic relationships.

Solutions

• To implement composition in Java, create one class that contains an instance of another class as a member variable, defining behaviors by delegating responsibilities to composed objects.

Mistake: Assuming that inheritance and composition are interchangeable.

Solution: Understand that inheritance implies an "is-a" relationship while composition implies a "has-a" relationship.

Mistake: Overusing inheritance, leading to a rigid class hierarchy.

Solution: Favor composition for greater flexibility and to reduce tight coupling between classes.