MyModelFactory.java my ver.

package uk.ac.bris.cs.scotlandyard.model;

import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableSet;

import javax.annotation.Nonnull;
import java.util.HashSet;
import java.util.Set;

import uk.ac.bris.cs.scotlandyard.model.ScotlandYard.Factory;

public final class MyModelFactory implements Factory<Model> {

    @Nonnull
    @Override
    public Model build(GameSetup setup, Player mrX, ImmutableList<Player> detectives) {
        Board.GameState initialState = new MyGameStateFactory().build(setup, mrX, detectives);
        Set<Model.Observer> observerRegistry = new HashSet<>();

        return new Model() {

            private Board.GameState currentState = initialState;

            @Nonnull
            @Override
            public Board getCurrentBoard() {
                return currentState;
            }

            @Override
            public void registerObserver(@Nonnull Observer observer) {
                if (observer == null) throw new NullPointerException("Observer cannot be null");
                if (!observerRegistry.add(observer)) {
                    throw new IllegalArgumentException("Observer is already registered");
                }
            }

            @Override
            public void unregisterObserver(@Nonnull Observer observer) {
                if (observer == null) throw new NullPointerException("Observer cannot be null");
                if (!observerRegistry.remove(observer)) {
                    throw new IllegalArgumentException("Observer is not registered");
                }
            }

            @Nonnull
            @Override
            public ImmutableSet<Observer> getObservers() {
                return ImmutableSet.copyOf(observerRegistry);
            }

            @Override
            public void chooseMove(@Nonnull Move move) {
                if (!currentState.getAvailableMoves().contains(move)) {
                    throw new IllegalArgumentException("Invalid move: " + move);
                }

                // Advance the game state
                currentState = currentState.advance(move);

                // Determine the correct event type
                Model.Observer.Event event = currentState.getWinner().isEmpty()
                        ? Model.Observer.Event.MOVE_MADE
                        : Model.Observer.Event.GAME_OVER;

                // Notify observers
                for (Observer observer : observerRegistry) {
                    observer.onModelChanged(getCurrentBoard(), event);
                }
            }
        };
    }
}