Aggregation in Java – HAS-A Relationship Explained

import java.util.ArrayList;
import java.util.List;

public class AggregationDemo {
    // Part: exists independently of any Department
    static class Employee {
        final String name;
        Employee(String name) { this.name = name; }
        public String toString() { return name; }
    }

    // Whole: HAS-A list of employees, but does not own their life cycle
    static class Department {
        final String name;
        private final List<Employee> members = new ArrayList<>();
        Department(String name) { this.name = name; }
        void add(Employee e) { members.add(e); }
        void remove(Employee e) { members.remove(e); }
        public String toString() { return name + " " + members; }
    }

    public static void main(String[] args) {
        Employee asha = new Employee("Asha");      // created outside the department
        Employee ravi = new Employee("Ravi");

        Department backend = new Department("Backend");
        Department platform = new Department("Platform");
        backend.add(asha);
        backend.add(ravi);
        platform.add(asha);                        // the same employee can belong to two departments

        System.out.println(backend);
        System.out.println(platform);

        backend = null;                            // department dissolved...
        System.out.println("Asha still exists: " + asha + ", member of " + platform);
    }
}

import java.util.ArrayList;
import java.util.List;

public class CompositionDemo {
    // Composition: the Order creates and owns its lines; they cannot exist alone
    static class Order {
        private final String id;
        private final List<Line> lines = new ArrayList<>();

        Order(String id) { this.id = id; }

        void addLine(String product, int qty, double price) {
            lines.add(new Line(product, qty, price));   // created by the whole
        }
        double total() { return lines.stream().mapToDouble(Line::amount).sum(); }

        // The part type is private: nobody outside can create or share a Line
        private record Line(String product, int qty, double price) {
            double amount() { return qty * price; }
        }

        public String toString() { return "Order " + id + ": " + lines.size() + " lines, total " + total(); }
    }

    public static void main(String[] args) {
        Order order = new Order("A-17");
        order.addLine("Keyboard", 1, 2499);
        order.addLine("Mouse", 2, 799);
        System.out.println(order);
        // When the order is discarded, its lines go with it
    }
}

public class AssociationDemo {
    // Plain association: objects use each other, no ownership at all
    static class Doctor {
        final String name;
        Doctor(String name) { this.name = name; }
        void treat(Patient p) { System.out.println("Dr. " + name + " treats " + p.name); }
    }
    static class Patient {
        final String name;
        Patient(String name) { this.name = name; }
    }

    public static void main(String[] args) {
        Doctor d = new Doctor("Mehta");
        Patient p1 = new Patient("Asha"), p2 = new Patient("Ravi");
        d.treat(p1);
        d.treat(p2);
    }
}