Pubali Bank Ltd. Senior Officer (SD) Written Test: 2018প্রশ্ন ও সমাধান

Inheritance lets a new (derived/child) class acquire the fields and methods of an existing (base/parent) class, enabling code reuse and an "is-a" hierarchy. Polymorphism means one interface can take many forms — the same method call behaves differently depending on the actual object, achieved via overriding (runtime/dynamic polymorphism) and overloading (compile-time/static polymorphism).

classDiagram
  Animal <|-- Dog
  Animal <|-- Cat
  class Animal{
    +String name
    +makeSound()
  }
  class Dog{
    +makeSound()
  }
  class Cat{
    +makeSound()
  }

class Animal {
    String name;
    Animal(String n) { name = n; }
    void makeSound() { System.out.println(name + " makes a sound"); }
}
class Dog extends Animal {                          // Inheritance
    Dog(String n) { super(n); }
    @Override
    void makeSound() { System.out.println(name + " barks"); }   // Overriding -> Polymorphism
}
class Cat extends Animal {
    Cat(String n) { super(n); }
    @Override
    void makeSound() { System.out.println(name + " meows"); }
}
public class Main {
    public static void main(String[] args) {
        Animal[] pets = { new Dog("Rex"), new Cat("Tom") };
        for (Animal a : pets) a.makeSound();          // same call, different behaviour
    }
}

Output:

Rex barks
Tom meows

Dog and Cat inherit the name field from Animal; calling makeSound() through an Animal reference resolves at runtime to the actual object's own version — that is polymorphism.

Exam takeaway: Inheritance = reuse via "is-a"; Polymorphism = one interface, many implementations, resolved by overriding (runtime) or overloading (compile-time).