public interface Jsonizer {
    String toJson(); // or whatever you want it to return
}

public class ActorsManager implements Jsonizer {

    private static final int NOT_FOUND = -1;

    private List<Actor> actors = new ArrayList<>();

    public void addActor(Actor actor) {
        if (!actors.contains(actor)) {
            actors.add(actor);
        }
    }

    public Actor getActor(Player player) {
        int index = getActorIndex(player);
        return index == NOT_FOUND ? null : actors.get(i);
    }

    public void removeActor(Player player) {
        int index = getActorIndex(player);
        if (index != NOT_FOUND) {
            actors.remove(index);
        }
    }

    private int getActorIndex(Player player) {
        int index = NOT_FOUND;
        int size = actors.size();
        for (int i = 0; i < size; i ++) {
            if (actors.get(i).getPlayer() == player) {
                index = i;
                break;
            }
        }
        return index;
    }

    @Override
    public String toJson() {
        StringBuilder json = new StringBuilder("[");
        int size = actors.size();
        for (int i = 0; i < size - 1; i ++) {
            json.append(actors.get(i).toJson()).append(", ");
        }
        if (size > 0) {
            json.append(actors.get(size - 1).toJson());
        }
        return json.append(']').toString();
    }
}

public class Actor implements Jsonizer {

    private Player player;

    public Actor(Player player) {
        this.player = player;
    }

    public Player getPlayer() {
        return player;
    }

    @Override
    public String toJson() {
        return "{}"; // whatever
    }

    public static Actor fromJson(Player player, String json) {
        return null; // whatever
    }
}