Inheritance and Polymorphism

Inheritance and Polymorphism

Inheritance lets a class reuse behaviour from a parent class. Polymorphism lets you write code that works with multiple types through a shared interface.

public class Animal {
    protected String name;

    public Animal(String name) { this.name = name; }

    public String sound() { return "..."; }
}

public class Dog extends Animal {
    public Dog(String name) { super(name); }

    @Override
    public String sound() { return "Woof"; }
}

public class Cat extends Animal {
    public Cat(String name) { super(name); }

    @Override
    public String sound() { return "Meow"; }
}

Runtime Polymorphism

List<Animal> animals = List.of(new Dog("Rex"), new Cat("Mimi"));
for (Animal a : animals) {
    System.out.println(a.name + " says " + a.sound());
}
// Rex says Woof
// Mimi says Meow

The correct sound() method is selected at runtime based on the actual object type — this is called dynamic dispatch.

final, abstract, and sealed

• final class — cannot be extended
• abstract class — cannot be instantiated; subclasses must implement abstract methods
• sealed class (Java 17+) — only permitted subclasses can extend it

Interview Tip

Distinguish method overriding (runtime polymorphism, @Override) from method overloading (compile-time, same name different parameters). Interviewers ask about this constantly.