Behavioral 8 — Mediator

Users talk only to the room, never directly to each other. N users need N links, not N×N.

public interface ChatMediator {
  void join(User u);
  void leave(User u);
  void send(String from, String msg);
}

public final class ChatRoom implements ChatMediator {
  private final Map<String, User> members = new ConcurrentHashMap<>();

  public void join(User u)  { members.put(u.name(), u); send("system", u.name() + " joined"); }
  public void leave(User u) { members.remove(u.name()); }

  public void send(String from, String msg) {
    for (User u : members.values()) {
      if (!u.name().equals(from)) u.receive(from, msg);   // the rule lives here, in one place
    }
  }
}

public final class User {
  private final String name;
  private final ChatMediator room;
  public User(String name, ChatMediator room) { this.name = name; this.room = room; }
  public String name() { return name; }
  public void say(String msg) { room.send(name, msg); }       // talks to the mediator only
  public void receive(String from, String msg) { System.out.println(name + " <- " + from + ": " + msg); }
}