Composition

1// Composition: House has Rooms
2// Rooms cannot exist without the House
3
4public class Room {
5    private String name;
6    private double area;
7    
8    // Package-private or created by House
9    Room(String name, double area) {
10        this.name = name;
11        this.area = area;
12    }
13    
14    public void display() {
15        System.out.println("  Room: " + name + " (" + area + " sq ft)");
16    }
17}
18
19public class House {
20    private String address;
21    private List<Room> rooms; // Composition - strong ownership
22    
23    public House(String address) {
24        this.address = address;
25        this.rooms = new ArrayList<>();
26        
27        // House creates its own rooms - rooms don't exist outside
28        createDefaultRooms();
29    }
30    
31    private void createDefaultRooms() {
32        rooms.add(new Room("Living Room", 400));
33        rooms.add(new Room("Kitchen", 200));
34        rooms.add(new Room("Bedroom", 300));
35    }
36    
37    public void addRoom(String name, double area) {
38        rooms.add(new Room(name, area));
39    }
40    
41    public void displayHouse() {
42        System.out.println("House at: " + address);
43        for (Room room : rooms) {
44            room.display();
45        }
46    }
47    
48    // When house is destroyed, rooms are destroyed too
49}
50
51// Another example: Car and Engine
52public class Engine {
53    private String type;
54    private int horsepower;
55    
56    Engine(String type, int horsepower) {
57        this.type = type;
58        this.horsepower = horsepower;
59    }
60    
61    public void start() {
62        System.out.println(type + " engine starting...");
63    }
64}
65
66public class Car {
67    private String model;
68    private Engine engine; // Composition - engine is part of car
69    
70    public Car(String model, String engineType, int hp) {
71        this.model = model;
72        // Car creates its own engine
73        this.engine = new Engine(engineType, hp);
74    }
75    
76    public void start() {
77        System.out.println("Starting " + model);
78        engine.start();
79    }
80    
81    // If Car is destroyed, Engine is destroyed too
82}
83
84// Usage
85public class Main {
86    public static void main(String[] args) {
87        House myHouse = new House("123 Main St");
88        myHouse.addRoom("Bathroom", 100);
89        myHouse.displayHouse();
90        
91        // When myHouse goes out of scope, all rooms are destroyed
92        
93        Car myCar = new Car("Tesla Model S", "Electric", 670);
94        myCar.start();
95        
96        // Engine cannot exist without the Car
97    }
98}