Explain the concept of polymorphism and how it is implemented in object-oriented programming languages.

Polymorphism is the ability of objects of different classes to respond to the same messages (method calls) in different ways. In object-oriented programming languages, this allows for using a single interface to work with different types of objects.

In Java, polymorphism is implemented through:

• Inheritance: subclasses inherit methods from the superclass.
• Method overriding: a subclass can change the behavior of a superclass method.
• References to base type: a variable of a base class can refer to an object of a subclass.

Example:

class Animal {
    void sound() {
        System.out.println("Some sound");
    }
}

class Dog extends Animal {
    @Override
    void sound() {
        System.out.println("Bark");
    }
}

class Cat extends Animal {
    @Override
    void sound() {
        System.out.println("Meow");
    }
}

public class Main {
    public static void main(String[] args) {
        Animal a1 = new Dog();
        Animal a2 = new Cat();
        a1.sound(); // Outputs "Bark"
        a2.sound(); // Outputs "Meow"
    }
}

Thus, polymorphism allows writing more flexible and extensible code, where behavior is determined at runtime depending on the actual object type.