Explain the Decorator design pattern and provide an example of its implementation in a software system.

The Decorator design pattern is a structural design pattern that allows adding new functionality to an existing object dynamically without altering its structure. It provides a flexible alternative to subclassing for extending the functionality of an object at runtime.

In this pattern, a decorator class wraps the original object and adds new behaviors or responsibilities to it by implementing the same interface as the original object. The decorator class maintains a reference to the original object and delegates the calls to it, while also adding its own functionality before or after the delegated calls.

An example of the Decorator design pattern can be seen in a software system that deals with different types of beverages, such as coffee or tea. The system may have a base interface called "Beverage" that defines common methods like "getDescription()" and "getCost()".

We can have a concrete implementation of the "Beverage" interface called "Coffee" that represents a basic coffee. Now, let's say we want to add additional functionalities to the coffee, such as adding milk, sugar, or whipped cream. Instead of creating separate subclasses for each combination of coffee with different additives, we can use the Decorator pattern.

We can create decorator classes like "MilkDecorator", "SugarDecorator", and "WhippedCreamDecorator", which implement the "Beverage" interface. Each decorator class will have a reference to the original "Beverage" object and override its methods to add the desired functionality.

For example, the "MilkDecorator" class can have a constructor that takes a "Beverage" object as a parameter and stores it in a member variable. It can then override the "getDescription()" method to append "with milk" to the original description and override the "getCost()" method to add an additional cost for the milk.

Similarly, the "SugarDecorator" class can add sugar to the beverage, and the "WhippedCreamDecorator" class can add whipped cream. Each decorator can be used independently or combined to create different combinations of beverages.

By using the Decorator pattern, we can dynamically add or remove functionalities to the original object at runtime without modifying its structure. This promotes code reusability, flexibility, and maintainability in the software system.