Online Auction System

/**
 * Low-Level Design: Online Auction System
 * Design Patterns Applied:
 *  - Observer Pattern: Real-time outbid and auction status notifications to watchers
 *  - State Pattern: Auction listing lifecycle (Draft, Active, Closed, Settled)
 *  - Mediator Pattern: Auctioneer coordinating bids between buyers and sellers
 */

import java.util.*;

enum AuctionState { UPCOMING, ACTIVE, CLOSED }

// --- [DESIGN PATTERN: Observer] Observer for auction watchers ---
interface BidderObserver {
    void onNewHighestBid(String itemName, double amount, String bidderId);
    void onAuctionClosed(String itemName, String winnerId, double finalAmount);
}

class Bidder implements BidderObserver {
    private final String id, name;
    public Bidder(String id, String name) { this.id = id; this.name = name; }
    public String getId() { return id; }
    public String getName() { return name; }

    @Override
    public void onNewHighestBid(String itemName, double amount, String bidderId) {
        if (!bidderId.equals(this.id)) {
            System.out.printf("📢 [Alert to %s] Outbid on '%s'! New top bid: $%.2f (by %s)%n", name, itemName, amount, bidderId);
        }
    }

    @Override
    public void onAuctionClosed(String itemName, String winnerId, double finalAmount) {
        if (winnerId.equals(this.id)) {
            System.out.printf("🏆 [Alert to %s] CONGRATULATIONS! You won '%s' for $%.2f!%n", name, itemName, finalAmount);
        } else {
            System.out.printf("🏁 [Alert to %s] Auction for '%s' closed. Winner: %s ($%.2f)%n", name, itemName, winnerId, finalAmount);
        }
    }
}

// --- [DESIGN PATTERN: Mediator] Auctioneer mediating between bidders ---
class AuctionItem {
    private final String id, title;
    private double currentBid;
    private String highestBidderId;
    private AuctionState state = AuctionState.UPCOMING;
    private final List<BidderObserver> watchers = new ArrayList<>();

    public AuctionItem(String id, String title, double startingPrice) {
        this.id = id; this.title = title; this.currentBid = startingPrice;
    }

    public void addWatcher(BidderObserver watcher) { watchers.add(watcher); }
    public void startAuction() {
        this.state = AuctionState.ACTIVE;
        System.out.println("🟢 Auction started for item: " + title + " (Reserve: $" + currentBid + ")");
    }

    public synchronized boolean placeBid(String bidderId, double amount) {
        if (state != AuctionState.ACTIVE) {
            System.out.println("❌ Cannot bid: Auction is not active!");
            return false;
        }
        if (amount <= currentBid) {
            System.out.println("❌ Bid $" + amount + " is lower than current highest bid $" + currentBid);
            return false;
        }

        this.currentBid = amount;
        this.highestBidderId = bidderId;
        for (BidderObserver b : watchers) b.onNewHighestBid(title, amount, bidderId);
        return true;
    }

    public synchronized void closeAuction() {
        this.state = AuctionState.CLOSED;
        for (BidderObserver b : watchers) b.onAuctionClosed(title, highestBidderId, currentBid);
    }
}

public class Main {
    public static void main(String[] args) {
        AuctionItem item = new AuctionItem("AUC-101", "Vintage Rolex Submariner", 5000.0);
        Bidder alice = new Bidder("B1", "Alice");
        Bidder bob = new Bidder("B2", "Bob");

        item.addWatcher(alice);
        item.addWatcher(bob);

        item.startAuction();
        item.placeBid(alice.getId(), 5500.0);
        item.placeBid(bob.getId(), 6200.0);
        item.closeAuction();
    }
}