Hello, welcome back! Today, we will decode the fundamentals of Revising Basic Design Patterns — Composition! A vital component of software design patterns, composition aids us in creating complex classes using simpler ones. Our journey today includes understanding the concept of composition, its value in software development, and how to practically implement it in Java.

To kick-start our exploration, let's understand Composition. In object-oriented programming (OOP), composition allows a class to include other classes, paving the way for the creation of complex systems out of simpler components. For instance, when building a car, we bring together independent pieces like the Engine, Wheels, and Seats — a perfect reflection of composition in everyday life. Note that in composition, should the parent object (the car) be destroyed, the child objects (the components) also cease to exist.

Now, let's translate the theory into a Java code application. Transforming the previously mentioned car example, a Car class in Java is created by making objects of the Engine, Wheels, and Seats classes. The Car class owns these child objects; their existence is dependent on the Car.

Java
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1class Engine {
2    public void start() {
3        System.out.println("Engine starts");  // Engine start message
4    }
5
6    public void stop() {
7        System.out.println("Engine stops");   // Engine stop message
8    }
9}
10
11class Wheels {
12    public void rotate() {
13        System.out.println("Wheels rotate");  // Wheel rotation message
14    }
15}
16
17class Seats {
18    public void adjust(String position) {
19        System.out.println("Seats adjusted to position " + position); // Seat adjustment message
20    }
21}
22
23class Car {
24    private final Engine engine;
25    private final Wheels wheels;
26    private final Seats seats;
27
28    public Car() {
29        this.engine = new Engine();
30        this.wheels = new Wheels();
31        this.seats = new Seats();
32    }
33
34    public void start() {
35        engine.start();  // Call to start engine
36        seats.adjust("upright");  // Adjust seat position
37        wheels.rotate();  // Get wheels rolling
38    }
39    
40    public static void main(String[] args) {
41        Car myCar = new Car();
42        myCar.start();  // Begin car functions
43    }
44}
45// Prints:
46// Engine starts
47// Seats adjusted to position upright
48// Wheels rotate

In the above code, the Car class encapsulates Engine, Wheels, and Seats objects, which are independent but part of the Car class, forming a composition pattern.

In the composition model, components like Engine, Wheels, and Seats can exist independently. These classes are not subclasses of Car, but are simply used by the Car to achieve its functionality. This is an important feature of composition because it allows for reuse of these components in other contexts. For instance, the Engine class could be used in other vehicles like Truck or Boat, without needing to inherit from a specific Car class.

In OOP, Composition and Inheritance are two significant ways to express relationships between classes. While inheritance implies an "is-a" relationship, composition suggests a "has-a" relationship. For instance, a Car is a Vehicle (inheritance), but a Car has an Engine (composition).

Superb job! You've now decoded the composition and even implemented it in Java! Next, you'll encounter stimulating exercises where you'll gain hands-on experience with composition in Java. Stay curious, and keep practicing to fortify your concepts!