Java Programming

Well I want to lock rooms in my game, and only when a command such as 'use key' is executed, the room unlocks. How would I implement this in my game? Taking into account that if a room is locked I would not be able to enter it etc....

Here's my fields, consturctors and some methods for the class Game:
What coding would I need to implement just in the game class to get the locked rooms working?

{private Parser parser;
private Room currentRoom;
private ArrayList inventory;

private void createRooms()
{
Room Garden, Outside;

// create the rooms
Garden = new Room("you are in the garden");
Garden.addItem(new Item("cheese", 4));
Outside = new Room("you are now outside");
Outside.addItem(new Item("milk", 4));

Home.setExits("east", Dungen);
Bridge.setExits("west", Basement);

private boolean processCommand(Command command) 
    {
        if(command.isUnknown())
        {
            System.out.println("I don't know what you mean...");
            return false;
        }

        CommandWord action = command.getCommandWord();
        if (action == CommandWord.HELP)
            printHelp();

private void goRoom(Command command) 
    {
        if(!command.hasSecondWord())
        {
            // if there is no second word, we don't know where to go...
            System.out.println("Go where?");
            return;
        }

        String direction = command.getSecondWord();

        // Try to leave current room.
        Room nextRoom = currentRoom.nextRoom(direction);

        if (nextRoom == null)
            System.out.println("There is no door!");
else 
        {
            currentRoom = nextRoom;
            System.out.println(currentRoom.longDescription());
        }
    }

If anyone else wants me to post some more code from the other classes then please reply doing so. Thanks!