🎁Decorator Pattern

Attach additional responsibilities to an object dynamically — transparent wrappers.

📌 Key IdeaImplement the same interface as the wrapped object; delegate core calls inward; add behaviour before/after. Avoids subclass combinatorics (MilkSugarVanillaLatte). JDK poster child: `InputStream` decorators.

🖼

Visual

🎁 Stack behaviours

  Coffee c = new SugarDecorator(
               new MilkDecorator(new SimpleCoffee()));
  // java.io: new BufferedInputStream(
  //              new FileInputStream(path))

Each decorator holds a reference to Component — nesting = stacking features.

✅

When to Use

You need optional cross-cutting layers: compression, encryption, logging, timing.
Subclass explosion if you modeled every combination explicitly.
Behaviour must be added at runtime (configuration, feature flags).

🚫

When NOT to Use

Wrapping fixes wrong interface — that is Adapter.
Object identity matters (==) — decorators change wrapper identity unless handled.

🚨

Anti-Pattern (What NOT to Do)

Subclass explosionBadjava

class MilkSugarCoffee extends Coffee {}
class SugarMilkCoffee extends Coffee {} // combinatorial hell

M×N flavours → M×N classes.

✅

The Right Way

Composable decoratorsGoodjava

interface Coffee { double cost(); }
class MilkDecorator implements Coffee {
    private final Coffee inner;
    public double cost() { return inner.cost() + 0.5; }
}

Order of wrapping can matter for some behaviours.

⚖️

Pros & Cons

👍 Pros

Follows Open/Closed — extend behaviour without editing originals.
Composable — mix decorators in different orders (when safe).

👎 Cons

Small objects everywhere — debugging stack of wrappers.
Order-dependent behaviour can surprise readers.

🔀

Common Confusions Cleared Up

Decorator vs Subclassing

Subclassing is static — compile-time specialisation. Decorator composes behaviour flexibly at runtime by stacking wrappers.

Decorator vs Proxy

Proxy usually controls access (lazy load, security, remote). Decorator adds responsibilities. Same structure — different intent.

🎤

Interview Q&A

🔮

Likely Follow-up Questions

When does decorator order matter (e.g. caching before vs after validation)?

🧾

Quick Cheat Sheet

IntentAdd duties dynamically; same interface.

JDKCollections synchronizedX, IO streams.

vs ProxyEnhance vs control access.

🔗