Mediator Design Pattern in Java

// Java code to illustrate Mediator Pattern
// All public class codes should be put in
// different files.

public interface Mediator {

    // The mediator interface
    public void addBuyer(Buyer buyer);
    public void findHighestBidder();
}

public class AuctionMediator implements Mediator {

    // this class implements the interface and holds 
    // all the buyers in a Array list.
    // We can add buyers and find the highest bidder
    private ArrayList buyers;

    public AuctionMediator()
    {
        buyers = new ArrayList<>();
    }

    @Override
    public void addBuyer(Buyer buyer)
    {
        buyers.add(buyer);
        System.out.println(buyer.name + " was added to" +
                "the buyers list.");
    }

    @Override
    public void findHighestBidder()
    {
        int maxBid = 0;
        Buyer winner = null;
        for (Buyer b : buyers) {
            if (b.price > maxBid) {
                maxBid = b.price;
                winner = b;
            }
        }
        System.out.println("The auction winner is " + winner.name + 
        ". He paid " + winner.price + "$ for the item.");
    }
}

public abstract class Buyer {
    
    // this class holds the buyer
    protected Mediator mediator;
    protected String name;
    protected int price;

    public Buyer(Mediator med, String name)
    {
        this.mediator = med;
        this.name = name;
    }

    public abstract void bid(int price);

    public abstract void cancelTheBid();
}

public class AuctionBuyer extends Buyer {

    // implementation of the bidding process
    // There is an option to bid and an option to
    // cancel the bidding 
    public AuctionBuyer(Mediator mediator,
                                String name)
    {
        super(mediator, name);
    }

    @Override
    public void bid(int price)
    {
        this.price = price;
    }

    @Override
    public void cancelTheBid()
    {
        this.price = -1;
    }
}

public class Main {

    /* This program illustrate an auction. The AuctionMediator 
    is responsible for adding the buyers, and after each 
    buyer bid a certain amount for the item, the mediator
    know who won the auction. */
    public static void main(String[] args)
    {

        AuctionMediator med = new AuctionMediator();
        Buyer b1 = new AuctionBuyer(med, "Tal Baum");
        Buyer b2 = new AuctionBuyer(med, "Elad Shamailov");
        Buyer b3 = new AuctionBuyer(med, "John Smith");

        // Create and add buyers
        med.addBuyer(b1);
        med.addBuyer(b2);
        med.addBuyer(b3);

        System.out.println("Welcome to the auction. Tonight " +
                        "we are selling a vacation to Vegas." + 
                        " please Bid your offers.");
        System.out.println("--------------------------------" + 
                                        "---------------");
        System.out.println("Waiting for the buyer's offers...");

        // Making bids
        b1.bid(1800);
        b2.bid(2000);
        b3.bid(780);
        System.out.println("---------------------------------" + 
                                            "--------------");
        med.findHighestBidder();

        b2.cancelTheBid();
        System.out.print(b2.name + " Has canceled his bid!, " + 
                                            "in that case ");
        med.findHighestBidder();
    }
}