Object-oriented Software in Ada 95

Case study: Design of a game 129
A controller of the game (games master) asks each player in turn for a move. When a move is received from a
player the board is asked to validate the move. If this is a valid move the counter of the current player is added
to the board. The board is displayed and the new state of the board is evaluated. This process is repeated until
either the board is filled or neither player can make a move. The player making the last move is asked to
announce the result of the game.
The major objects and verbs identified are:
Objects (nouns)
board
game
cell
counter
player
game
Messages (verbs)
add
announce
ask
evaluated
capture
display
validate
The following messages are sent to individual objects:
board
Display a representation of the board.
Add a counter to the board.
Evaluate the current state of the board.
Validate a proposed move.
player
Announce the result of the game.
Ask for the next move.
cell
Add a counter into a cell on the board.
counter
Display a representation of the counter.
Play
Play the game.
It is more appropriate to deal with classes than to deal with objects. For example, Board is the class to which
the object board belongs. Using this approach the messages sent to these classes can be refined into the
following list:
Class Message Responsibility of method
Board Add Add a counter into the board.
Check_Move
Check if a player can drop a counter
into a column.
Contents
Return the contents of a cell.
Display
Display a representation of the board.
Now_playing
Say who is now playing on the board.
Set_Up
Populate the board with the initial
contents.
Status
Evaluate the current state of the
board.
Player Announce Announcing that the player has either
won or drawn the game.
Get_Move
Get the next move from the player.
My_Counter
Return the counter that the player
plays with.
Set
Set a player to play with a particular
counter.
© M A Smith - May not be reproduced without permission