Understanding the Decorator Pattern in Java: Core Roles and Practical Implementation

The Decorator pattern is a structural design pattern that allows behavior to be added to individual objects dynamically without affecting other objects of the same class.

It consists of four essential roles: the Component interface, ConcreteComponent classes that implement the core functionality, an abstract Decorator class that also implements the Component interface and holds a reference to a Component, and ConcreteDecorator classes that extend the Decorator to add extra behavior.

A UML class diagram (not shown here) visualises these relationships, where the Decorator and ConcreteDecorator can be stacked to achieve multiple layers of augmentation.

public interface Car { void run(); }

public class BenzCar implements Car { @Override public void run() { System.out.println("奔驰开车了！"); } } public class BmwCar implements Car { @Override public void run() { System.out.println("宝马开车了！"); } } public class TeslaCar implements Car { @Override public void run() { System.out.println("特斯拉开车了！"); } }

public class CarDecorator implements Car { protected Car decoratedCar; public CarDecorator(Car decoratedCar) { this.decoratedCar = decoratedCar; } @Override public void run() { decoratedCar.run(); } }

public class AutoCarDecorator extends CarDecorator { public AutoCarDecorator(Car decoratedCar) { super(decoratedCar); } @Override public void run() { decoratedCar.run(); autoRun(); } private void autoRun() { System.out.println("开启自动驾驶"); } } public class FlyCarDecorator extends CarDecorator { public FlyCarDecorator(Car decoratedCar) { super(decoratedCar); } @Override public void run() { decoratedCar.run(); fly(); } private void fly() { System.out.println("开启飞行汽车模式"); } }

public class Client { public static void main(String[] args) { Car benzCar = new BenzCar(); Car bmwCar = new BmwCar(); Car teslaCar = new TeslaCar(); CarDecorator autoBenzCar = new AutoCarDecorator(benzCar); CarDecorator flyAutoBmwCar = new FlyCarDecorator(new AutoCarDecorator(bmwCar)); benzCar.run(); bmwCar.run(); teslaCar.run(); autoBenzCar.run(); flyAutoBmwCar.run(); } }

Beyond the illustrative car example, the JDK itself uses the Decorator pattern: various concrete InputStream subclasses (e.g., FileInputStream , ByteArrayInputStream ) inherit from the abstract InputStream , while FilterInputStream acts as a decorator that holds an InputStream instance and adds functionality such as buffering ( BufferedInputStream ) or data‑type reading ( DataInputStream ).

This demonstrates how the Decorator pattern provides a flexible mechanism for extending behavior without modifying existing code, making it a fundamental technique in object‑oriented design.