Inheritance — extends, super & Hierarchies

7. Practice Problems

1. Build Animal (with speak()) and Dog extends Animal that overrides speak(). Create one Dog and call speak().
   Animal class বানান (যার speak() আছে), তারপর Dog extends Animal যেটি speak() override করে। একটি Dog বানিয়ে speak() কল করুন।
   ✨ Show Answer

   Main.java

   class Animal {
       void speak() { System.out.println("Some generic sound"); }
   }
   class Dog extends Animal {
       void speak() { System.out.println("Ghew ghew!"); }
   }
   class Main {
       public static void main(String[] args) {
           new Dog().speak();
       }
   }

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2. Write a class Employee with fields name, salary. Then Manager extends Employee with an extra bonus. Use super(...).
   Employee (name, salary) থেকে Manager উত্তরাধিকার নেবে, Manager-এ অতিরিক্ত bonus। super(...) ব্যবহার করুন।
   ✨ Show Answer

   Main.java

   class Employee {
       String name; double salary;
       Employee(String n, double s) { name = n; salary = s; }
       double pay() { return salary; }
   }
   class Manager extends Employee {
       double bonus;
       Manager(String n, double s, double b) {
           super(n, s);
           bonus = b;
       }
       double pay() { return super.pay() + bonus; }
   }
   class Main {
       public static void main(String[] args) {
           Manager m = new Manager("Habib", 60000, 15000);
           System.out.println(m.name + " earns " + m.pay());
       }
   }

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3. Why does Java forbid multiple class inheritance? Explain in 2–3 sentences with the "diamond problem".
   Java multiple class inheritance কেন নিষিদ্ধ — "diamond problem" দিয়ে দুই-তিন বাক্যে ব্যাখ্যা।
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   Answer: If class D extended both B and C, and both B and C extended A and overrode the same method, the compiler would not know which overridden version D inherits — this ambiguity is called the diamond problem. Java sidesteps it by allowing only one direct superclass (interfaces with default methods are allowed in multiples, but their rules resolve conflicts explicitly).

   D যদি B ও C দুটো থেকেই উত্তরাধিকার নিত (এবং দুটোর মূল parent এক), একই method-এর কোন version পাবে তা ambiguous হতো — এটাই diamond problem। Java তাই single class inheritance-এ সীমাবদ্ধ।

4. Design Shape with a method area() returning 0. Then Square extends Shape overriding area(). Print area of a 4×4 square.
   Shape (area() → 0) থেকে Square উত্তরাধিকার নেবে ও area() override করবে। 4×4 Square-এর area print করুন।
   ✨ Show Answer

   Main.java

   class Shape {
       double area() { return 0; }
   }
   class Square extends Shape {
       double side;
       Square(double s) { side = s; }
       double area() { return side * side; }
   }
   class Main {
       public static void main(String[] args) {
           Shape sh = new Square(4);
           System.out.println("Area = " + sh.area());
       }
   }

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5. Give one real-world example where composition is clearly better than inheritance, and justify briefly.
   বাস্তব একটি উদাহরণ দিন যেখানে inheritance-এর চেয়ে composition স্পষ্টতই ভালো — সংক্ষেপে যুক্তি দিন।
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   Answer: A Car should have an Engine, not extend one. A car is not a kind of engine; it contains an engine alongside wheels, seats, and a gearbox. Composition here lets us swap engine types (petrol, electric, hybrid) at runtime without reshaping the class hierarchy — something single inheritance would make painful.

   Car-এর ভেতরে Engine থাকা উচিত, Car কখনো engine-এর ধরন নয়। Composition-এ petrol/electric/hybrid engine runtime-এ বদলানো যায়, inheritance-এ সেটা কঠিন।