#Decorator

#Overview

Attach additional responsibilities to an object dynamically by wrapping it.

#When to use

• You need flexible, runtime composition of behavior.
• Inheritance would lead to too many subclasses.

#Java example

interface Notifier {
    void send(String message);
}

class EmailNotifier implements Notifier {
    public void send(String message) { /* email */ }
}

class SmsDecorator implements Notifier {
    private final Notifier wrapped;
    SmsDecorator(Notifier wrapped) { this.wrapped = wrapped; }
    public void send(String message) {
        wrapped.send(message);
        /* sms */
    }
}

#TypeScript example

interface Notifier {
  send(message: string): void;
}

class EmailNotifier implements Notifier {
  send(message: string): void {}
}

class SmsDecorator implements Notifier {
  constructor(private wrapped: Notifier) {}
  send(message: string): void {
    this.wrapped.send(message);
  }
}

#Pros and cons

Pros:

• Behavior can be added without subclassing.
• Multiple decorators can be combined.

Cons:

• Many small objects can be hard to debug.
• Order of decorators matters.

#Common pitfalls

• Hiding side effects in deep decorator stacks.
• Mutating shared state across decorators.

#Related patterns

• Composite
• Strategy