Solution for Programming Exercise 6.10
This page contains a sample solution to one of the exercises from Introduction to Programming Using Java.
Exercise 6.10:
In the Blackjack game from Exercise 6.9, the user can click on the "Hit", "Stand", and "NewGame" buttons even when it doesn't make sense to do so. It would be better if the buttons were disabled at the appropriate times. The "New Game" button should be disabled when there is a game in progress. The "Hit" and "Stand" buttons should be disabled when there is not a game in progress. The instance variable gameInProgress tells whether or not a game is in progress, so you just have to make sure that the buttons are properly enabled and disabled whenever this variable changes value. I strongly advise writing a subroutine that can be called whenever it is necessary to set the value of the gameInProgress variable. Then the subroutine can take responsibility for enabling and disabling the buttons. Recall that if bttn is a variable of type JButton, then bttn.setEnabled(false) disables the button and bttn.setEnabled(true) enables the button.
As a second (and more difficult) improvement, make it possible for the user to place bets on the Blackjack game. When the applet starts, give the user $100. Add a JTextField to the strip of controls along the bottom of the applet. The user can enter the bet in this JTextField. When the game begins, check the amount of the bet. You should do this when the game begins, not when it ends, because several errors can occur: The contents of the JTextField might not be a legal number. The bet that the user places might be more money than the user has, or it might be <= 0. You should detect these errors and show an error message instead of starting the game. The user's bet should be an integral number of dollars.
It would be a good idea to make the JTextField uneditable while the game is in progress. If betInput is the JTextField, you can make it editable and uneditable by the user with the commands betInput.setEditable(true) and betInput.setEditable(false).
In the paintComponent() method, you should include commands to display the amount of money that the user has left.
There is one other thing to think about: Ideally, the applet should not start a new game when it is first created. The user should have a chance to set a bet amount before the game starts. So, in the constructor for the drawing surface class, you should not call doNewGame(). You might want to display a message such as "Welcome to Blackjack" before the first game starts.
Here is an applet version of my program:
In the original applet, the button variables are declared in the constructor of the main class, where the buttons are created. There are no instance variables that refer to the buttons, so it is not possible to do anything with the buttons outside the constructor. For this exercise, references to the buttons must be stored in instance variables. The program uses instance variables hitButton, standButton, and newGameButton to refer to the buttons. These variables are required in order to call the buttons' setEnabled() methods. I also introduced a JTextField, betInput, as an instance variable and modified the constructor so that it creates the text field and adds it to the panel that occupies the SOUTH position in the main panel.
The buttons must be enabled and disabled whenever the value of the variable gameInProgress changes. At the same time, the text field should be made editable or non-editable. As recommended in the exercise, I wrote a method for changing the value of gameInProgress. This method also sets the buttons and text field to reflect the state of the program, which allows the state of the buttons and text field to be controlled in one location:
/**
* This method is called whenever the value of the gameInProgress
* property has to be changed. In addition to setting the value
* of the gameInProgress variable, it also enables and disables
* the buttons and text input box to reflect the state of the
* game.
* @param inProgress The new value of gameInProgress.
*/
private void setGameInProgress( boolean inProgress ) {
gameInProgress = inProgress;
if (gameInProgress) {
hitButton.setEnabled(true);
standButton.setEnabled(true);
newGameButton.setEnabled(false);
betInput.setEditable(false);
}
else {
hitButton.setEnabled(false);
standButton.setEnabled(false);
newGameButton.setEnabled(true);
betInput.setEditable(true);
}
}
Once this routine is available, then any line in the old program that said "gameInProgress = false;" should be changed to "setGameInProgress(false);". And any line that said "gameInProgress = true;" should be changed to "setGameInProgress(true);". In this way, we can be sure that the buttons are always properly enabled and disabled. Note that I added lines to the BlackjackGUI2 constructor to disable hitButton and standButton when they are created. This ensures that they have the correct state at the start of the program, before the first game has been started.
You should understand why I used a subroutine to set the value of gameInProgress. Every time gameInProgress changes, each of the buttons has to be enabled or disabled and the text field has to be made editable or uneditable. That's four extra lines of code each time the program says gameInProgress = true or gameInProgress = false. We can avoid some extra typing by calling the subroutine. Furthermore, if we always call the subroutine to set the value of gameInProgress, we can be sure that the states of the buttons and text field will always be set correctly to match the value of gameInProgress.
The changes that I've discussed so far are enough to complete the first part of the exercise, enabling and disabling the buttons. We still have to implement betting.
The JTextField for the user's input is created with the command "betInput = new JTextField("10",5);". The first parameter in the constructor specifies the initial content of the text input box. This is meant as a reasonable value for the bet, but the user can change it if he wants to. The second parameter is important. It specifies the number of characters that the text field is designed to hold. The preferred size of the JTextField is computed based on this number of characters. If the text field were being used in a context where it would be stretched to fit the available size, such as in a GridLayout, the preferred size would not be so important. However, in this applet, the JTextField is used with a FlowLayout, and it will appear at exactly its preferred size. If you leave out the second parameter in the constructor, the JTextField will be sized to fit its contents, "10", and it will look too small.
The CardPanel class contains two new instance variables for managing the user's bets. One variable, usersMoney, records the amount of money that the user has. The other, betAmount, records the amount of the user's bet on the current game. The value of usersMoney is initialized to 100 in the constructor. At the end of a game, if the user wins, the betAmount is added to the user's money, and if the user loses, the betAmount is subtracted from the user's money. We have to decide what happens if the user runs out of money. One possibility would be to shut the game down, but that seems drastic since it's only play money anyway. So, if the value of usersMoney drops to zero, I give the user another $100 at the start of the next game.
At the beginning of a game, the program has to look at the number in the text field to determine how much money the user wants to bet on the game. Several things can go wrong at this time. Since it's a little complicated, I wrote a method to check the contents of betInput:
/**
* This is called when the user wants to start a new game. It tries to
* read the amount of the user's bet from the betInput text field. If an error
* occurs, the message in the panel is changed to inform the user of the error.
* @return true if the bet is read without error, or false if an error occurs
*/
private boolean checkBet() {
int amount;
try {
amount = Integer.parseInt( betInput.getText() );
}
catch (NumberFormatException e) {
message = "The bet amount must be a legal positive integer.";
repaint();
return false;
}
if (amount <= 0) {
message = "The bet amount must be a positive integer.";
repaint();
return false;
}
if (amount > usersMoney) {
message = "You can't bet more money than you have!";
repaint();
return false;
}
betAmount = amount;
return true;
}
The doNewGame() method starts by checking if usersMoney is 0; if so, it gives the user another $100 by setting usersMoney to 100. Then it checks the user's bet by calling the checkBet() method. If the return value is false, meaning that the value in the text field was not a valid bet, then the doNewGame() method returns without starting a new game. Otherwise, it starts the game as it did in the old program.
When the game ends for any reason, the user's money has to be adjusted. There are many points in the source code where the game ends. In each of those places, I inserted a line "usersMoney = usersMoney + betAmount" if the user won or "usersMoney = usersMoney - betAmount" if the user lost. It might have been a good idea to write another subroutine to handle this task.
I also added some code to the paintComponent() method to display the user's current amount of money. To accommodate this, this version of the panel has to be about 30 pixels taller than the previous version.
One of the tricky parts of this assignment is to arrange things so that a game does not start as soon as the applet is created. It's no problem to take the "doNewGame();" statement out of the constructor in the CardPanel class. Some initialization has to be done there instead:
usersMoney = 100; message = "Welcome to Blackjack! You start with $100.";
However, when I did this, I ran into a NullPointerException in the paintComponent() method because the paintComponent() method assumed that two Hand objects, dealerHand and playerHand, exist. These objects are created in the doNewGame() method, so removing the call to doNewGame() form the constructor meant that the hands were null when paintComponent() was first called. This just required a simple modification in the paintComponent() method to deal with this possibility. I decided simply to return from paintComponent() after drawing the messages, if the hands don't exist. (An alternative would be to have some fancy introductory screen.)
(I should also confess that I had problems with null pointer exceptions when I tried to run the program without removing the call to doNewGame() from the constructor. In this case, the problem was more subtle: The CardPanel constructor was being called before the button and text field objects had been created, so doNewGame() was trying to enable/disable buttons that didn't exist yet.)
Here is the new version of the blackjack source code, with changes from Exercise 6.9 shown in red:
import java.awt.*; import java.awt.event.*; import javax.swing.*; /** * In this program, the user plays a game of Blackjack. The * computer acts as the dealer. The user plays by clicking * "Hit!" and "Stand!" buttons. The user can place bets. * At the beginning of the game, the user is given $100. * * This class defines a panel, but it also contains a main() * routine that makes it possible to run the program as a * stand-alone application. * * If this panel is used in an applet the size should be about 466 pixels * wide and 365 pixels high. That width is just big enough to show * 2 rows of 5 cards. The height is probably a little bigger * than necessary, to allow for variations in the size of buttons * from one platform to another. * * This program depends on the following classes: Card, Hand, * BlackjackHand, Deck. */ public class BlackjackGUI2 extends JPanel { /** * The main routine simply opens a window that shows a BlackjackGUI2. */ public static void main(String[] args) { JFrame window = new JFrame("Blackjack"); BlackjackGUI2 content = new BlackjackGUI2(); window.setContentPane(content); window.pack(); // Set size of window to preferred size of its contents. window.setResizable(false); // User can't change the window's size. Dimension screensize = Toolkit.getDefaultToolkit().getScreenSize(); window.setLocation( (screensize.width - window.getWidth())/2, (screensize.height - window.getHeight())/2 ); window.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE ); window.setVisible(true); } private JButton hitButton; // The three buttons that control the game. private JButton standButton; private JButton newGameButton; private JTextField betInput; // Where the user inputs the amount of his bet. /** * The constructor lays out the panel. A CardPanel occupies the CENTER * position of the panel (where CardPanel is a subclass of JPanel that is * defined below). On the bottom is a panel that holds three buttons. * The CardPanel listens for ActionEvents from the buttons and does all * the real work of the program. */ public BlackjackGUI2() { setBackground( new Color(130,50,40) ); setLayout( new BorderLayout(3,3) ); CardPanel board = new CardPanel(); add(board, BorderLayout.CENTER); JPanel buttonPanel = new JPanel(); buttonPanel.setBackground( new Color(220,200,180) ); add(buttonPanel, BorderLayout.SOUTH); // NOTE: Declarations of hitButton, standButton, newGameButton were moved // out of the constructor. Previously, they were local variables. hitButton = new JButton( "Hit!" ); hitButton.setEnabled(false); hitButton.addActionListener(board); buttonPanel.add(hitButton); standButton = new JButton( "Stand!" ); standButton.setEnabled(false); standButton.addActionListener(board); buttonPanel.add(standButton); newGameButton = new JButton( "New Game" ); newGameButton.addActionListener(board); buttonPanel.add(newGameButton); buttonPanel.add(new JLabel(" Bet:", JLabel.RIGHT)); betInput = new JTextField("10", 5); betInput.setMargin( new Insets(3,3,3,3) ); buttonPanel.add(betInput); setBorder(BorderFactory.createLineBorder( new Color(130,50,40), 3) ); } // end constructor /** * A nested class that displays the game and does all the work * of keeping track of the state and responding to user events. */ private class CardPanel extends JPanel implements ActionListener { Deck deck; // A deck of cards to be used in the game. BlackjackHand dealerHand; // Hand containing the dealer's cards. BlackjackHand playerHand; // Hand containing the user's cards. String message; // A message drawn on the canvas, which changes // to reflect the state of the game. boolean gameInProgress; // Set to true when a game begins and to false // when the game ends. Font bigFont; // Font that will be used to display the message. Font smallFont; // Font that will be used to draw the cards. int usersMoney; // The amount of money that the user currently has. int betAmount; // The bet amount, read from betInput when game starts. /** * The constructor creates the fonts and starts the first game. * It also sets a preferred size of 460-by-330 for the panel. * The paintComponent() method assumes that this is in fact the * size of the panel (although it can be a little taller with * no bad effect). */ CardPanel() { setPreferredSize( new Dimension(460,330) ); setBackground( new Color(0,120,0) ); smallFont = new Font("SansSerif", Font.PLAIN, 12); bigFont = new Font("Serif", Font.BOLD, 16); usersMoney = 100; message = "Welcome to Blackjack! Make your bet and hit \"New Game\"."; } /** * This method is called whenever the value of the gameInProgress * property has to be changed. In addition to setting the value * of the gameInProgress variable, it also enables and disables * the buttons and text input box to reflect the state of the * game. * @param inProgress The new value of gameInProgress. */ private void setGameInProgress( boolean inProgress ) { gameInProgress = inProgress; if (gameInProgress) { hitButton.setEnabled(true); standButton.setEnabled(true); newGameButton.setEnabled(false); betInput.setEditable(false); } else { hitButton.setEnabled(false); standButton.setEnabled(false); newGameButton.setEnabled(true); betInput.setEditable(true); } } /** * This is called when the user wants to start a new game. It tries to * read the amount of the user's bet from the betInput text field. If an error * occurs, the message in the panel is changed to inform the user of the error. * @return true if the bet is read without error, or false if an error occurs */ private boolean checkBet() { int amount; try { amount = Integer.parseInt( betInput.getText() ); } catch (NumberFormatException e) { message = "The bet amount must be a legal positive integer."; repaint(); return false; } if (amount <= 0) { message = "The bet amount must be a positive integer."; repaint(); return false; } if (amount > usersMoney) { message = "You can't bet more money than you have!"; repaint(); return false; } betAmount = amount; return true; } /** * Respond when the user clicks on a button by calling the appropriate * method. Note that the buttons are created and listening is set * up in the constructor of the BlackjackPanel class. */ public void actionPerformed(ActionEvent evt) { String command = evt.getActionCommand(); if (command.equals("Hit!")) doHit(); else if (command.equals("Stand!")) doStand(); else if (command.equals("New Game")) doNewGame(); } /** * This method is called when the user clicks the "Hit!" button. First * check that a game is actually in progress. If not, give an error * message and exit. Otherwise, give the user a card. The game can end * at this point if the user goes over 21 or if the user has taken 5 cards * without going over 21. */ void doHit() { if (gameInProgress == false) { // Should not be possible! message = "Click \"New Game\" to start a new game."; repaint(); return; } playerHand.addCard( deck.dealCard() ); if ( playerHand.getBlackjackValue() > 21 ) { usersMoney = usersMoney - betAmount; message = "You've busted! Sorry, you lose."; setGameInProgress(false); } else if (playerHand.getCardCount() == 5) { usersMoney = usersMoney + betAmount; message = "You win by taking 5 cards without going over 21."; setGameInProgress(false); } else { message = "You have " + playerHand.getBlackjackValue() + ". Hit or Stand?"; } repaint(); } /** * This method is called when the user clicks the "Stand!" button. * Check whether a game is actually in progress. If it is, the game * ends. The dealer takes cards until either the dealer has 5 cards * or more than 16 points. Then the winner of the game is determined. */ void doStand() { if (gameInProgress == false) { // Should not be possible! message = "Click \"New Game\" to start a new game."; repaint(); return; } setGameInProgress(false); while (dealerHand.getBlackjackValue() <= 16 && dealerHand.getCardCount() < 5) dealerHand.addCard( deck.dealCard() ); if (dealerHand.getBlackjackValue() > 21) { usersMoney = usersMoney + betAmount; message = "You win! Dealer has busted with " + dealerHand.getBlackjackValue() + "."; } else if (dealerHand.getCardCount() == 5) { usersMoney = usersMoney - betAmount; message = "Sorry, you lose. Dealer took 5 cards without going over 21."; } else if (dealerHand.getBlackjackValue() > playerHand.getBlackjackValue()) { usersMoney = usersMoney - betAmount; message = "Sorry, you lose, " + dealerHand.getBlackjackValue() + " to " + playerHand.getBlackjackValue() + "."; } else if (dealerHand.getBlackjackValue() == playerHand.getBlackjackValue()) { usersMoney = usersMoney - betAmount; message = "Sorry, you lose. Dealer wins on a tie."; } else { usersMoney = usersMoney + betAmount; message = "You win, " + playerHand.getBlackjackValue() + " to " + dealerHand.getBlackjackValue() + "!"; } repaint(); } /** * Called by the constructor, and called by actionPerformed() if the * user clicks the "New Game" button. Start a new game. Deal two cards * to each player. The game might end right then if one of the players * had blackjack. Otherwise, gameInProgress is set to true and the game * begins. */ void doNewGame() { if (gameInProgress) { // If the current game is not over, it is an error to try // to start a new game. This shouldn't be possible because // the new game button is disabled while a game is in progress, // but it doesn't hurt anything to check anyway. message = "You still have to finish this game!"; repaint(); return; } if (usersMoney == 0) { // The user has run out of money; give the user another $100. usersMoney = 100; } if ( ! checkBet() ) { // The user's bet was not legal, so we can't start a game. // The checkBet method has already given an error message. return; } deck = new Deck(); // Create the deck and hands to use for this game. dealerHand = new BlackjackHand(); playerHand = new BlackjackHand(); deck.shuffle(); dealerHand.addCard( deck.dealCard() ); // Deal two cards to each player. dealerHand.addCard( deck.dealCard() ); playerHand.addCard( deck.dealCard() ); playerHand.addCard( deck.dealCard() ); if (dealerHand.getBlackjackValue() == 21) { message = "Sorry, you lose. Dealer has Blackjack."; usersMoney = usersMoney - betAmount; setGameInProgress(false); } else if (playerHand.getBlackjackValue() == 21) { message = "You win! You have Blackjack."; usersMoney = usersMoney + betAmount; setGameInProgress(false); } else { message = "You have " + playerHand.getBlackjackValue() + ". Hit or stand?"; setGameInProgress(true); } repaint(); } // end newGame(); /** * The paint method shows the message at the bottom of the * canvas, and it draws all of the dealt cards spread out * across the canvas. */ public void paintComponent(Graphics g) { super.paintComponent(g); // fill with background color. g.setFont(bigFont); g.setColor(Color.GREEN); g.drawString(message, 10, getHeight() - 10); // Draw a message telling how much money the user has. g.setColor(Color.YELLOW); if (usersMoney > 0) g.drawString("You have $" + usersMoney, 10, getHeight() - 35); else g.drawString("YOU ARE BROKE! (I will give you another $100.)", 10, getHeight() - 32 ); if (dealerHand == null) return; // the first game has not yet started. // Draw labels for the two sets of cards. g.setColor(Color.GREEN); g.drawString("Dealer's Cards:", 10, 23); g.drawString("Your Cards:", 10, 153); // Draw dealer's cards. Draw first card face down if // the game is still in progress, It will be revealed // when the game ends. g.setFont(smallFont); if (gameInProgress) drawCard(g, null, 10, 30); else drawCard(g, dealerHand.getCard(0), 10, 30); for (int i = 1; i < dealerHand.getCardCount(); i++) drawCard(g, dealerHand.getCard(i), 10 + i * 90, 30); // Draw the user's cards. for (int i = 0; i < playerHand.getCardCount(); i++) drawCard(g, playerHand.getCard(i), 10 + i * 90, 160); } // end paintComponent(); /** * Draws a card as a 80 by 100 rectangle with upper left corner at (x,y). * The card is drawn in the graphics context g. If card is null, then * a face-down card is drawn. (The cards are rather primitive!) */ void drawCard(Graphics g, Card card, int x, int y) { if (card == null) { // Draw a face-down card g.setColor(Color.blue); g.fillRect(x,y,80,100); g.setColor(Color.white); g.drawRect(x+3,y+3,73,93); g.drawRect(x+4,y+4,71,91); } else { g.setColor(Color.white); g.fillRect(x,y,80,100); g.setColor(Color.gray); g.drawRect(x,y,79,99); g.drawRect(x+1,y+1,77,97); if (card.getSuit() == Card.DIAMONDS || card.getSuit() == Card.HEARTS) g.setColor(Color.red); else g.setColor(Color.black); g.drawString(card.getValueAsString(), x + 10, y + 30); g.drawString("of", x+ 10, y + 50); g.drawString(card.getSuitAsString(), x + 10, y + 70); } } // end drawCard() } // end nested class CardPanel } // end class BlackjackGUI2