Java Decorator Pattern Lets Developers Add Features Without Modifying Existing Code

The Decorator Design Pattern in Java allows new behavior to be added to existing objects dynamically by wrapping them in another object, without altering their source code. It follows the Open/Closed Principle, meaning software components remain closed for modification but open for extension. The pattern uses four key components — a common interface, a base implementation, an abstract decorator, and concrete decorators — to layer functionality at runtime. A practical example is a coffee ordering system where add-ons like milk or sugar are applied dynamically rather than through dozens of separate subclasses. Java's own java.io package uses this pattern, with classes like BufferedInputStream decorating a base InputStream to provide additional capabilities.