Computational Thinking & Object-Oriented Design

9. Practice Problems

Try each on your own first, then expand the answer.

1. Read the statement "A student at NSU registers for a course taught by a teacher." List the classes, fields, and methods.
   উপরের বাক্য থেকে class, field এবং method খুঁজে বের করুন।
   ✨ Show Answer (উত্তর দেখুন)

   Classes: Student, Course, Teacher. Fields: Student{id, name}, Course{code, credits}, Teacher{name, department}. Methods: Student.register(Course), Course.addStudent(Student), Teacher.teach(Course).

   Class: Student, Course, Teacher। Field: id, name, code, credits, department। Method: register, addStudent, teach।

2. Create a Student class with fields name and cgpa, and a method describe(). Create two students and print them.
   Student class তৈরি করে দুটি object বানান এবং তাদের তথ্য print করুন।
   ✨ Show Answer (উত্তর দেখুন)

   Main.java

   class Student {
       String name;
       double cgpa;
       Student(String n, double c) { name = n; cgpa = c; }
       void describe() {
           System.out.println(name + " — CGPA " + cgpa);
       }
   }
   class Main {
       public static void main(String[] args) {
           new Student("Arif", 3.72).describe();
           new Student("Nila", 3.91).describe();
       }
   }

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3. Extend the Wallet example to reject deposits of zero or negative amount. Run it.
   Wallet-এ শূন্য বা ঋণাত্মক deposit বন্ধ করুন।
   ✨ Show Answer (উত্তর দেখুন)

   Main.java

   class Wallet {
       String owner; double balance;
       Wallet(String o, double b) { owner = o; balance = b; }
       void deposit(double amt) {
           if (amt <= 0) { System.out.println("Invalid amount"); return; }
           balance += amt;
           System.out.println(owner + " balance: ৳" + balance);
       }
   }
   class Main {
       public static void main(String[] args) {
           Wallet w = new Wallet("Arif", 100);
           w.deposit(50);
           w.deposit(0);
           w.deposit(-10);
       }
   }

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4. Explain in 3 sentences why abstraction is essential when designing a ride-hailing app (like Pathao).
   Pathao-এর মতো অ্যাপে abstraction কেন জরুরি — ৩ বাক্যে লিখুন।
   ✨ Show Answer (উত্তর দেখুন)

   Answer: (1) A ride-hailing app must track vehicles, drivers, riders, and trips — without abstraction the code quickly becomes unmanageable. (2) Abstraction lets us treat every Vehicle the same way (get location, compute ETA) regardless of whether it is a bike, car, or CNG — the caller doesn't care. (3) That simplifies matching, pricing, and UI code, and lets a new vehicle type be added without rewriting the rest of the system.

5. Design a class Transaction with from, to, amount, timestamp and a summary() method. Create one and print it.
   Transaction class design করে একটি object তৈরি ও print করুন।
   ✨ Show Answer (উত্তর দেখুন)

   Main.java

   class Transaction {
       String from, to; double amount; long ts;
       Transaction(String f, String t, double a, long s) {
           from = f; to = t; amount = a; ts = s;
       }
       void summary() {
           System.out.println("[#" + ts + "] " + from + " → " + to + " : ৳" + amount);
       }
   }
   class Main {
       public static void main(String[] args) {
           new Transaction("Arif", "Nila", 250.0, System.currentTimeMillis()).summary();
       }
   }

   Run Copy