Question

I’m working on a tic tac toe game and am wanting to implement an undo method. What I feel is the best way to do this is just setup another (multiple?) stacks, and make a copy of the ‘moves’ that have just taken place. Then, if undo is called, just pop the last move and repopulate the game board.

So yeah, I have the idea, but can’t figure out how to implement it.

Some of what I have:

To set:

public void set(Position p, int v, int n) throws IOException { 
    if (board[p.x][p.y][p.z]!= 0) throw new IOException("Position taken");

    //Restrict 222 until all other's have been used
    if (n != 26) {
        if (p.x == 1 && p.y == 1 && p.z ==1) {
            throw new IOException("[2,2,2] cannot be played until all other positions have been taken");
        }
    }

    //Enforce x=1 for first 9, x=3 for next 9
    if (n < 9 ) {
        if (p.x != 0) throw new IOException("Please play on x=1 for the first 9 moves");
    }

    if (n >= 9 && n < 18) {
        if (p.x != 2) throw new IOException("Please play on x=3 for the first 9 moves");
    }

    board[p.x][p.y][p.z] = v;
}

Then there is a board method to build the board, a display method, and of course one to check for 3 in a row.

Thanks for any advice

Answer 1

There is design pattern to do undo and redo. The command design pattern. It involes

public interface ICommand{
   void execute();
   void undo();
   void redo();
}

implement the above interface to perform your move, execute will encapsulate your action.

class MoveCommand implements ICommand{//parameter to store current board state
  public MoveCommand(){
    // new board state is validated
  }
  public void execute(){
    // change the board state
  }
 public void undo(){ // restore
 }
 public void redo(){ // apply again if possible
 }
}

now create a new class that will be CommandDispatcher

class CommandDispatcher{
 private List<ICommand> commands = new ArrayList<ICommand>();
 public CommandDispatcher(){
 }
 private ICommand currentCommand = null;
 public void setCommand(ICommand cmd){
   currentCommand  = cmd;
   cmd.execute();
   commands.add(cmd);
 }
 public void undoAll(){
    for(ICommand cmd : commands){cmd.undo();}
}
 public void undo(){
 commands.remove(commands.size()-1);
 currentCommand = commands.get(commands.size()-1)
}
public void redo(){
if(null!=currentCommand) currentCommand.redo();
}

}

This way you can preserve the state of your application and prevent yourself from getting nullpointer exceptions.
The method redo() will call execute() method. I just added it for clarity.