Java Reference
In-Depth Information
initBoard() ;
return true ;
return false ;
Note that the JOptionPane.showMessageDialog(window, s) method displays the
string s in a pop-up window (a.k.a., a dialog box). The parameter window is the parent
window, which in this case is the JApplet object. The dialog box that is created is modal,
that is, no other window can be accessed until the window is closed. Fortunately, the dialog
window will have an OK button that can be used to close the window. Note that it does
not matter whether the isGameOver method first checks to see if X wins or whether it first
checks to see if Y wins. The reason is that the program logic prevents both players from
winning at the same time.
Next, let us examine the computerMove method. It first checks the middle square. If it is
empty, then it always places the letter O there. Otherwise, the method goes through all the
squares and calls the computeScore method on each square. The computeScore method
evaluates the score for placing a symbol in the square. The square with the greatest score
is then chosen and the character o is placed there.
public void computerMove () {
if (! squares . get(4) .hasValue()) { // if middle is empty
squares . get(4) . placeCharacter( 'o' );
return ;
Square bestRectangle = squares . get(0) ;
int best = computeScore(bestRectangle) ;
for (Square r : squares) {
if (computeScore(r) > best)
best = computeScore(r) ;
bestRectangle = r ;
} bestRectangle . placeCharacter( 'o' );
The real magic or the AI (stands for artificial intelligence ) of the program happens in
the computeScore method. The method is smart enough to access the benefits of choosing
between different squares of the board. Below is our implementation.
public int computeScore(Square r)
if (r.hasValue())
return 0;
if (winsWithNextMove( r ,
'o' )) {
return 4;
if (winsWithNextMove( r ,
'x' )) {
return 3;
return 2;
The method first checks to see if the square is occupied. An occupied square is a terrible
choice and therefore a score of 0 is returned. Next, the method checks to see if the computer
can win if it places its sign in the square. If this is the case, then this is an automatic win
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