State design pattern

Common Errors

Assigning state in constructor of state

Don't assign state from within the constructor of a state implementation.

For example:

public class GameState {
    protected final GameManager context;

    public GameState(GameManager context) {
        this.context = context;
    }
}

public class PlayingGameState extends GameState {
    public PlayingGameState(GameManager context) {
        super(context);
        if (noPlayersAreOnline()) {
            context.setState(new WaitingInHubState(context));
        }
    }

    private boolean noPlayersAreOnline() {
        // some implementation which returns false for this example case
    }
}

// some method
context.setState(new PlayingGameState(context));
System.out.printf("The current state type is %s", context.getStateType())

I expect the printout to be "The current state type is WaitingInHubState", but it winds up being "The current state type is PlayingGameState", and that's because there is a timing issue between assigning the state.

• Assign state to PlayingGameState
  • call PlayingGameState constructor
    • assign state to WaitingInHubState
      • call WaitingInHubState constructor
    • context.state is assigned to WaitingInHubState
  • context.state is assigned to PlayingGameState

Thus, never assign the state of the context from within the constructor of a state implementation.