Structural 6 — Bridge

Message urgency × delivery channel: M + N classes instead of M × N.

// Implementation hierarchy: HOW it is delivered
public interface Channel { void deliver(String to, String body); }
public final class EmailChannel implements Channel { public void deliver(String to, String b) { /* SMTP */ } }
public final class SmsChannel   implements Channel { public void deliver(String to, String b) { /* SMS */ } }

// Abstraction hierarchy: WHAT kind of message, holding a bridge to a Channel
public abstract class Notification {
  protected final Channel channel;                 // the bridge
  protected Notification(Channel c) { this.channel = c; }
  public abstract void send(String to, String msg);
}

public final class NormalNotification extends Notification {
  public NormalNotification(Channel c) { super(c); }
  public void send(String to, String msg) { channel.deliver(to, msg); }
}

public final class UrgentNotification extends Notification {
  public UrgentNotification(Channel c) { super(c); }
  public void send(String to, String msg) {
    for (int i = 0; i < 3; i++) channel.deliver(to, "[URGENT] " + msg);  // escalation policy
  }
}

// 2 kinds × 2 channels = 4 combos from 4 classes; adding Push adds ONE class.
// new UrgentNotification(new SmsChannel()).send("+91...", "server down");