Encapsulation & Access Modifiers — Hide, Protect, Expose

7. Practice Problems

1. Write an Employee class with a private salary field and a public raise(double percent) method that rejects any negative raise.
   Employee class লিখুন — private salary, এবং raise(double percent) method যেটি negative raise reject করবে।
   ✨ Show Answer

   Main.java

   class Employee {
       private String name;
       private double salary;
       Employee(String n, double s) { name = n; salary = s; }
       public void raise(double pct) {
           if (pct < 0) throw new IllegalArgumentException("no pay cuts here");
           salary *= (1 + pct / 100);
       }
       public double getSalary() { return salary; }
   }
   class Main {
       public static void main(String[] args) {
           Employee e = new Employee("Sumi", 40000);
           e.raise(10);
           System.out.println("New salary: " + e.getSalary());
       }
   }

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2. In two sentences, why is a public field worse than a public getter that returns the same value?
   দুই বাক্যে — একটি public field-এর চেয়ে একই মান ফেরত দেওয়া public getter কেন ভালো?
   ✨ Show Answer

   Answer: A public field forever binds callers to that exact name, type, and representation — changing any of those is a breaking change. A getter is a method, so you can later change the field (rename it, compute it, back it with a cache) without touching a single caller.

   Public field callers-কে field-এর নাম, type ও representation-এর সাথে চিরতরে বেঁধে ফেলে — পরে বদলালে সব caller ভাঙবে। Getter method হওয়ায় ভেতরের field আপনি ইচ্ছামতো বদলাতে পারবেন, caller-এ ধাক্কা লাগবে না।

3. Make an immutable Money class with amount (long paisa) and currency (String). Add a method plus(Money other) that returns a new Money when currencies match, else throws.
   একটি immutable Money class বানান (amount: long paisa, currency: String)। plus(Money other) method মুদ্রা মিললে নতুন Money দেবে, না মিললে throw করবে।
   ✨ Show Answer

   Main.java

   final class Money {
       private final long amount;
       private final String currency;
       Money(long a, String c) { amount = a; currency = c; }
       public Money plus(Money o) {
           if (!currency.equals(o.currency)) throw new IllegalArgumentException("currency mismatch");
           return new Money(amount + o.amount, currency);
       }
       public String toString() { return amount + " " + currency; }
   }
   class Main {
       public static void main(String[] args) {
           Money a = new Money(5000, "BDT");
           Money b = new Money(2500, "BDT");
           System.out.println(a.plus(b));
       }
   }

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4. Show a Temperature class that stores Celsius privately but exposes getters for both Celsius and Fahrenheit — no setter.
   একটি Temperature class বানান — private-এ Celsius রাখবে, getter দেবে Celsius ও Fahrenheit দুইয়ের, কোনো setter থাকবে না।
   ✨ Show Answer

   Main.java

   final class Temperature {
       private final double celsius;
       Temperature(double c) { celsius = c; }
       public double getCelsius()    { return celsius; }
       public double getFahrenheit() { return celsius * 9 / 5 + 32; }
   }
   class Main {
       public static void main(String[] args) {
           Temperature t = new Temperature(30);
           System.out.println(t.getCelsius() + "C = " + t.getFahrenheit() + "F");
       }
   }

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5. In 2–3 sentences, explain what "package-private" means and when you might prefer it over public.
   দুই-তিন বাক্যে — package-private কী এবং কখন একে public-এর চেয়ে ভালো মনে হয়?
   ✨ Show Answer

   Answer: Package-private (no modifier) means the class or member is visible only to other classes in the same package — the outside world cannot see it at all. Prefer it for helper classes and methods that are implementation details of a module: other packages cannot even depend on them, so you are free to refactor them without breaking anyone.

   Package-private (কোনো modifier না দিলে) মানে একই package-এর class ছাড়া কেউ দেখতে পাবে না। Helper class/method-কে package-private রাখুন — অন্য package এদের উপর depend করতে পারবে না, তাই refactor-এ ভয় নেই।