Association

// ========== Association: Teacher teaches Students ==========

class Student {
    private String name;
    private String id;

    public Student(String name, String id) {
        this.name = name;
        this.id = id;
    }

    public String getName() { return name; }
    public String getId() { return id; }

    public void learn(String subject) {
        System.out.println(name + " is learning " + subject);
    }
}

class Teacher {
    private String name;
    private String subject;
    private List<Student> students;  // Association with Students

    public Teacher(String name, String subject) {
        this.name = name;
        this.subject = subject;
        this.students = new ArrayList<>();
    }

    // Association methods
    public void addStudent(Student student) {
        students.add(student);
    }

    public void removeStudent(Student student) {
        students.remove(student);
    }

    public void teach() {
        System.out.println(name + " is teaching " + subject);
        for (Student student : students) {
            student.learn(subject);
        }
    }
}

// Usage
public class Main {
    public static void main(String[] args) {
        // Create objects independently
        Student alice = new Student("Alice", "S001");
        Student bob = new Student("Bob", "S002");
        Teacher john = new Teacher("John", "Mathematics");

        // Associate them
        john.addStudent(alice);
        john.addStudent(bob);
        john.teach();

        // Objects exist independently
        john.removeStudent(alice);
        // alice still exists - not destroyed
        System.out.println(alice.getName() + " still exists!");

        // Teacher can be destroyed, students survive
        john = null;
        System.out.println(bob.getName() + " is still alive!");
    }
}