Command Pattern: Student Enrollment

gistfile1.java

// Client
class Student {

    private int studentId;

    public void requestEnrollment(int courseId) {
        Enroll enroll = new Enroll(studentId, courseId);
        Invoker invoker = new Invoker();
        invoker.request(enroll);
    }

}

// Command
class Enroll {

    private int courseId;
    private int studentId;

    public Enroll(int courseId, int studentId) {
        this.courseId = courseId;
        this.studentId = studentId;
    }

}

// Invoker
class DeskAttendant {

    private Queue<Enroll> enrollmentQueue;
    private Registrar registrar;

    public DeskAttendant() {
        enrollmentQueue = new Queue<Enroll>();
        registrar = new Registrar();
    }

    public boolean request(Enroll enroll) {
        enrollmentQueue.push(enroll);

        Enroll enroll = enrollmentQueue.pop();

        if(registrar.enrollStudent(enroll))
            return "Student successfully enrolled!";
        else
            return "Student does not meet the requirements!";
    }

}

// Receiver
class Registrar {

    public boolean enrollStudent(Enroll enroll) {
        if(canStudentEnroll(enroll))
            return true;
        else
            return false;
    }

    private boolean canStudentEnroll(Enroll enroll) {
        // enrollment logic 
    }

}