Thank you fellow programmers. I found the answers very useful.
(1) Why does the program freeze?
When the program first starts, game.play() gets executed by the main thread, which is the thread that executes main. However, when the "New Game" button is pressed, game.play() gets executed by the event dispatch thread (instead of the main thread), which is the thread responsible for executing event-handling code and updating the user interface. The while loop (in play()) only terminates if selection == 0 evaluates to false. The only way selection == 0 evaluates to false is if didUserMakeSelection becomes true. The only way didUserMakeSelection becomes true is if the user presses one of the numbered buttons. However, the user cannot press any numbered button, nor the "New Game" button, nor exit the program. The "New Game" button doesn't even pop back out, because the event dispatch thread (which would otherwise repaint the screen) is too busy executing the while loop (which is effectively inifinte for the above reasons).
(2) How can the program be rewritten to fix the problem?
Since the problem is caused by the execution of game.play() in the event dispatch thread, the direct answer is to execute game.play() in another thread. This can be accomplished by replacing
if (pressedButton.getText() == "New Game") {
game.play();
}
with
if (pressedButton.getText() == "New Game") {
Thread thread = new Thread() {
public void run() {
game.play();
}
};
thread.start();
}
However, this results in a new (albeit more tolerable) problem: Each time the "New Game" button is pressed, a new thread is created. Since the program is very simple, it's not a big deal; such a thread becomes inactive (i.e. a game finishes) as soon as the user presses a numbered button. However, suppose it took longer to finish a game. Suppose, while a game is in progress, the user decides to start a new one. Each time the user starts a new game (before finishing one), the number of active threads increments. This is undesirable, because each active thread consumes resources.
The new problem can be fixed by:
(1) adding import statements for Executors, ExecutorService, and Future, in Game.java
import java.util.concurrent.Executors;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Future;
(2) adding a single-thread executor as a field under Game
private ExecutorService gameExecutor = Executors.newSingleThreadExecutor();
(3) adding a Future, representing the last task submitted to the single-thread executor, as a field under Game
private Future<?> gameTask;
(4) adding a method under Game
public void startNewGame() {
if (gameTask != null) gameTask.cancel(true);
gameTask = gameExecutor.submit(new Runnable() {
public void run() {
play();
}
});
}
(5) replacing
if (pressedButton.getText() == "New Game") {
Thread thread = new Thread() {
public void run() {
game.play();
}
};
thread.start();
}
with
if (pressedButton.getText() == "New Game") {
game.startNewGame();
}
and finally,
(6) replacing
public void play() {
int selection = 0;
while (selection == 0) {
selection = userInterface.getSelection();
}
System.out.println(selection);
}
with
public void play() {
int selection = 0;
while (selection == 0) {
selection = userInterface.getSelection();
if (Thread.currentThread().isInterrupted()) {
return;
}
}
System.out.println(selection);
}
To determine where to put the if (Thread.currentThread().isInterrupted()) check, look at where the method lags. In this case, it is where the user has to make a selection.
There is another problem. The main thread could still be active. To fix this, you can replace
public static void main(String[] args) {
Game game = new Game();
game.play();
}
with
public static void main(String[] args) {
Game game = new Game();
game.startNewGame();
}
The code below applies the above modifications (in addition to a checkThreads() method):
import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.concurrent.Executors;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Future;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
public class Game {
private GraphicalUserInterface userInterface;
private ExecutorService gameExecutor = Executors.newSingleThreadExecutor();
private Future<?> gameTask;
public Game() {
userInterface = new GraphicalUserInterface(this);
}
public static void main(String[] args) {
checkThreads();
Game game = new Game();
checkThreads();
game.startNewGame();
checkThreads();
}
public static void checkThreads() {
ThreadGroup mainThreadGroup = Thread.currentThread().getThreadGroup();
ThreadGroup systemThreadGroup = mainThreadGroup.getParent();
System.out.println("\n" + Thread.currentThread());
systemThreadGroup.list();
}
public void play() {
int selection = 0;
while (selection == 0) {
selection = userInterface.getSelection();
if (Thread.currentThread().isInterrupted()) {
return;
}
}
System.out.println(selection);
}
public void startNewGame() {
if (gameTask != null) gameTask.cancel(true);
gameTask = gameExecutor.submit(new Runnable() {
public void run() {
play();
}
});
}
}
class GraphicalUserInterface extends JFrame implements ActionListener {
private Game game;
private JButton newGameButton = new JButton("New Game");
private JButton[] numberedButtons = new JButton[3];
private JPanel southPanel = new JPanel();
private int selection;
private boolean isItUsersTurn = false;
private boolean didUserMakeSelection = false;
public GraphicalUserInterface(Game game) {
this.game = game;
newGameButton.addActionListener(this);
for (int i = 0; i < 3; i++) {
numberedButtons[i] = new JButton((new Integer(i+1)).toString());
numberedButtons[i].addActionListener(this);
southPanel.add(numberedButtons[i]);
}
getContentPane().add(newGameButton, BorderLayout.NORTH);
getContentPane().add(southPanel, BorderLayout.SOUTH);
pack();
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setLocationRelativeTo(null);
setVisible(true);
}
public void actionPerformed(ActionEvent event) {
JButton pressedButton = (JButton) event.getSource();
if (pressedButton.getText() == "New Game") {
game.startNewGame();
Game.checkThreads();
}
else if (isItUsersTurn) {
selection = southPanel.getComponentZOrder(pressedButton) + 1;
didUserMakeSelection = true;
}
}
public int getSelection() {
if (!isItUsersTurn) {
isItUsersTurn = true;
}
if (didUserMakeSelection) {
isItUsersTurn = false;
didUserMakeSelection = false;
return selection;
}
else {
return 0;
}
}
}
References
The Java Tutorials: Lesson: Concurrency
The Java Tutorials: Lesson: Concurrency in Swing
The Java Virtual Machine Specification, Java SE 7 Edition
The Java Virtual Machine Specification, Second Edition
Eckel, Bruce. Thinking in Java, 4th Edition. "Concurrency & Swing: Long-running tasks", p. 988.
How do I cancel a running task and replace it with a new one, on the same thread?
