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

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

In Java, polymorphism is implemented through inheritance and method overriding. For example, a base class declares a method, and subclasses implement it in their own way.

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"
    }
}

Here, a variable of type Animal can refer to objects of different subclasses, and the call to the sound() method will be executed depending on the actual type of the object — this is polymorphism.