🤝Mediator Pattern

Define an object that encapsulates how a set of objects interact — promote loose coupling by keeping objects from referring to each other explicitly.

📌 Key IdeaInstead of N×(N-1) links between UI widgets or services, each colleague notifies Mediator; mediator orchestrates others. Think chat room, dialog coordinating inputs, enterprise service bus style routing.

🖼

Visual

🤝 Colleagues talk through mediator

      ChatRoom (mediator)
       /    |    \
   UserA  UserB  UserC   // users don't reference each other directly

Hub-and-spoke replaces mesh — easier to change interaction rules in one place.

✅

When to Use

Many objects interact in complex ways — hard to track direct references.
UI forms where enabling one field affects several others.
Air traffic control style coordination — rules centralized.

🚫

When NOT to Use

Simple one-to-one relationships — mediator is ceremony.
Mediator risks becoming a god object — split bounded contexts.

🚨

Anti-Pattern (What NOT to Do)

Peers reference each other directlyBadjava

class UserA {
    UserB b;
    void send(String msg) { b.receiveFromA(msg); }
}
// Adding UserC → edit A, B, ... mesh explosion

N² coupling — change one participant, ripple everywhere.

✅

The Right Way

Room mediates — peers only know mediatorGoodjava

interface Chat {
    void join(User u);
    void say(User u, String msg);
}

record User(String name, Chat room) {
    void say(String m) { room.say(this, m); }
}

Chat implementation forwards messages — rules live in one place.

⚖️

Pros & Cons

👍 Pros

Decouples colleagues — swap interaction policy without touching each class.
Simplifies reuse of individual components.

👎 Cons

Mediator itself can grow huge.
Indirection — harder to trace flow than explicit calls.

🔀

Common Confusions Cleared Up

Mediator vs Observer

Observer broadcast one-to-many notifications (subject doesn't orchestrate detailed dialogue). Mediator actively coordinates multi-step protocols between peers.

Mediator vs Facade

Facade simplifies outward API to subsystem (often one-way convenience). Mediator manages peer-to-peer conversations behind the scenes.

🎤

Interview Q&A

🔮

Likely Follow-up Questions

When does Mediator become an anti-pattern (fat controller)?

🧾

Quick Cheat Sheet

IntentCentral hub for colleague interactions.

vs ObserverOrchestrate vs broadcast.

RiskGod mediator.

🔗