Null references aren't "shunned" any more so than exceptions, at least by anyone I've ever known or read. I think you're misunderstanding the conventional wisdom.
What's bad is an attempt to access a null reference (or dereference a null pointer, etc.). This is bad because it always indicates a bug; you would never do something like this on purpose, and if you are doing it on purpose, then that's even worse, because it's making it impossible to distinguish expected behaviour from buggy behaviour.
There are certain fringe groups out there who just really hate the concept of nullity for some reason, but as Ed points out, if you don't have null or nil then you'll just have to replace it with something else, which might lead to something worse than a crash (such as data corruption).
Many frameworks, in fact, embrace both concepts; for example, in .NET, a frequent pattern you'll see is a pair of methods, one prefixed by the word Try (such as TryGetValue). In the Try case, the reference is set to its default value (usually null), and in the other case, an exception is thrown. There's nothing wrong with either approach; both are used frequently in environments that support them.
It really all depends on the semantics. If null is a valid return value, as in the general case of searching a collection, then return null. On the other hand, if it is not a valid return value - for example, looking up a record using a primary key that came from your own database - then returning null would be a bad idea because the caller won't be expecting it and probably won't check for it.
It's really very simple to figure out which semantic to use: Does it make any sense for the result of a function to be undefined? If so, you can return a null reference. If not, throw an exception.
I think you may be getting a little bogged down in the English meaning of state, when compared to the State Pattern (or Finite-State Machine, which is really a diagrammatic representation of a State Pattern). Both are appropriate here, but they shouldn't be confused.
The State Pattern is something which should, given various common stimuli, operate on the game state differently at different times in the game. So, as you've rightly concluded, the lobby stage (which I'm assuming to be while people are joining a game) is a State in that context.
The game state is a list of players, pieces, cards, dice, whatever else, and all the information required to simulate them. You might want to call that the context.
Each State should receive a UserClicked message from the application which receives the context and coordinates or area clicked and should operate on those accordingly. The information that you are trying to give ownership of to the State object does not belong there, it belongs in the game itself, alongside the State.
The state may also have a UserPressedKey method, a TimerTicked method or any other kind of stimulus on which it should act. But each of these should act on the game state rather than being the game state.
One important method of the State will be ScreenRefresh, which will draw the context for the user.
Here is a very rough example:
abstract class ApplicationState {
void Begin(GameContext context);
void UserClicked(GameContext context, int x, int y);
void UserPressedKey(GameContext context, char key);
void TimerTicked(GameContext context);
void ScreenRefresh(GameContext context);
protected void OnStateChanged(ApplicationState newState) {
// inform interested parties that state has changed,
// using the observer pattern
}
}
class LobbyState : ApplicationState
{
void Begin(GameContext context) {
context.Players = new User[context.NoOfPlayers];
}
void UserClicked(GameContext context, int x, int y) {
// Find a displayed player box which contains click coords
int index = -1;
for (i=0; i < context.Players.Length; i++) {
if (PlayerBox[i].Contains(x, y)) {
index = i;
}
}
if (index > -1) {
if (context.Player[index] == null) {
context.Player[index] = context.ActivePlayer;
} else if (context.Player[index] == context.ActivePlayer()) {
context.Player[index] = null;
} else {
ErrorSound.Play();
}
} else if (CloseIcon.Contains(x, y)) {
OnStateChanged(new ExittingState());
} else {
ErrorSound.Play();
}
}
void UserPressedKey(GameContext context, char key) {
if (key = 'J' or key = 'j') {
if (! context.AddPlayerRandomly(context.ActiveUser)) {
ErrorSound.Play();
}
if (context.GameIsNowFull()) {
OnStateChanged(new InitializingState());
}
} else if key = Esc {
if (!context.RemovePlayer(context.ActiveUser)) {
ErrorSound.Play();
}
} else {
ErrorSound.Play();
}
}
void TimerTicked(GameContext context) {
// there is no use for a game timer while
// we're trying to fill the game, but you
// want one to tick anyway, because the
// application doesn't know which state
// it is in.
}
void ScreenRefresh(GameContext context) {
DrawPlayerBoxes(context.Screen, context.Players);
DrawExitIcon(context.Screen);
}
}
class InitializingState : ApplicationState {
private double waitSpinnerAngle = 0;
void Begin(GameContext context) {
context.RandomizePlayOrder();
context.ShuffleCards();
// all other game initialization rules here
OnContextChanged(new PlayerUpState(context.Players[0]));
}
void UserClicked(GameContext context, int x, int y) {
// Not responding to user input for a moment
ErrorSound.Play();
}
void UserPressedKey(GameContext context, char key) {
// Not responding to user input for a moment
ErrorSound.Play();
}
void TimerTicked(GameContext context) {
waitSpinnerAngle += 0.05;
if (waitSpinnerAngle > 1) waitSpinnerAngle = 0;
}
void ScreenRefresh(GameContext context) {
DrawWaitSpinner(waitSpinnerAngle);
}
}
See where I'm going with this? The Application itself is then as simple as setting the initial State, hooking a listener into the OnStateChanged event and calling Begin. When the listener hears that the event is called, unhook your old State, hook in the new one, and call Begin again.
Everything else is triggered by events from the mouse, keyboard or timer, passed directly to the current State without knowledge of which State the game is currently in. If you are running this game across a network then you will also need an event for changes of state received from other players.
Everything in your State and Context is now very unit-testable and separation of concerns are observed. Although, you may want to refactor that LobbyState#UserClicked method a bit, among other things.
Remember, this is just a free example and you get what you paid for it. Don't try to apply it directly to your game. Just use it to understand how the State Pattern should work in the context of a game.
Best Answer
There's a detailed discussion of this on Ward's Wiki. Generally, the use of exceptions for control flow is an anti-pattern, with many notable situation - and language-specific (see for example Python) cough exceptions cough.
As a quick summary for why, generally, it's an anti-pattern:
Read the discussion at Ward's wiki for much more in-depth information.
See also a duplicate of this question, here