Develop Reference

KeyListener in a new thread for game input capture

I am making an old school Snake game in Java with Swing. I've read that in order to capture input in real time I need to run my game loop in a new thread so that It's wait() method won't interfere with the input capture. I've made InputCapture class implementing KeyListener and I've implemented keyPressed() method like that:
public class InputCapture implements KeyListener {
private Direction capturedDirection;
//Methods
#Override
public void keyPressed(KeyEvent e) {
boolean inputConsoleDebug = true;
if (e.getKeyCode() == KeyEvent.VK_LEFT) {
capturedDirection = Direction.left;
if (inputConsoleDebug) System.out.println("LEFT");
} else if (e.getKeyCode() == KeyEvent.VK_RIGHT) {
capturedDirection = Direction.right;
if (inputConsoleDebug) System.out.println("RIGHT");
} else if (e.getKeyCode() == KeyEvent.VK_UP) {
capturedDirection = Direction.up;
if (inputConsoleDebug) System.out.println("UP");
} else if (e.getKeyCode() == KeyEvent.VK_DOWN) {
capturedDirection = Direction.down;
if (inputConsoleDebug) System.out.println("DOWN");
}
}
#Override
public void keyReleased(KeyEvent e) {
}
#Override
public void keyTyped(KeyEvent e) {
}
public Direction getCapturedDirection() {
return capturedDirection;
}
}
Then I've made Game class extending Thread and I've put game loop code into run() method:
public class Game extends Thread {
private Board board;
private Snake snake;
private JFrame frame;
private long waitTime;
private int difficultyStep;
private Direction inputDirection;
private InputCapture inputManager;
//Constructors
Game(Dimension boardSize) {
//Set difficulty
int applesToWin = boardSize.width * boardSize.height - 1;
final int easiestWaitTime = 1000;
final int hardestWaitTime = 100;
difficultyStep = (easiestWaitTime - hardestWaitTime) / applesToWin;
waitTime = easiestWaitTime;
//Set starting point
final int startingPointX = boardSize.width / 2;
final int startingPointy = boardSize.height / 2;
//Set board and snake
board = new Board(boardSize);
snake = new Snake(board, startingPointX, startingPointy);
//Set window Frame
frame = new JFrame(SnakeApplication.getApplicationName());
frame.setContentPane(board);
frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
frame.pack();
frame.setResizable(false);
frame.addWindowListener(new WindowAdapter() {
#Override
public void windowClosing(WindowEvent e) {
super.windowClosing(e);
interrupt();
}
});
//Set input manager
inputManager = new InputCapture();
frame.addKeyListener(inputManager);
inputDirection = null;
}
//Methods
public void run() {
board.spawnApple();
while (!isWon()) {
try {
sleep(waitTime);
} catch (InterruptedException e) {
return;
}
try {
inputDirection = inputManager.getCapturedDirection();
snake.move(inputDirection);
} catch (LosingMove e) {
showGameOverDialog();
return;
}
board.repaint();
}
showWinDialog();
}
JFrame getFrame() {
return frame;
}
private boolean isWon() {
for (int row = 0; row < board.getFields().length; row++) {
for (int col = 0; col < board.getFields()[0].length; col++) {
if (!(board.getFields()[row][col].getContent() instanceof Snake.SnakeNode)) return false;
}
}
return true;
}
private void showGameOverDialog() {
JFrame gameOverFrame = new JFrame();
JOptionPane.showMessageDialog(gameOverFrame, "Game Over!");
}
private void showWinDialog() {
JFrame gameOverFrame = new JFrame();
JOptionPane.showMessageDialog(gameOverFrame, "You Win!");
}
}
In my MainMenu class I've made startNewGame() method that is called when New Game button is clicked. This method creates Game object and starts a new thread by calling start() method.
public class MainMenu {
//Form components references
private JButton exitButton;
private JFrame frame;
private JPanel mainPanel;
private JButton newGameButton;
private JLabel titleLabel;
//Constructors
MainMenu() {
//Set window Frame
frame = new JFrame(SnakeApplication.getApplicationName());
frame.setContentPane(mainPanel);
frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
frame.setResizable(false);
frame.pack();
newGameButton.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
startNewGame();
}
});
exitButton.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
exitGame();
}
});
}
JFrame getFrame() {
return frame;
}
private Dimension showBoardSizeDialog() {
Frame boardSizeFrame = new Frame();
int width = Integer.parseInt(JOptionPane.showInputDialog(boardSizeFrame, "Set board's width:"));
int height = Integer.parseInt(JOptionPane.showInputDialog(boardSizeFrame, "Set board's height:"));
return new Dimension(width, height);
}
private void startNewGame() {
Dimension boardSize = showBoardSizeDialog();
frame.setVisible(false);
Game game = new Game(boardSize);
game.getFrame().setVisible(true);
//Starting game loop in a new thread
game.start();
try {
game.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
frame.setVisible(true);
}
}
But when testing the app it gets stuck in the game loop and doesn't capture input at all. Why? I was trying to debug It, but every time the new thread is started it gets stuck in game loop. The Board itself is painted only when main thread ends its execution. Why? Shouldn't It be repainted many times during game loop if execution is stucked there?
Also, I've made thread interrupt when frame's close button is clicked (red X button) so execution could get back to MainMenu and reappear it, but clicking red close button has no effect.

The program freezes because of the call to game.join() in startNewGame. join keeps the thread it was called from from continuing execution until the thread it was called on dies. In your situation, join defeats the purpose of using another thread, so you should just remove that.
There are other issues, though. You probably shouldn't use a thread. You should probably use a Swing Timer. Swing isn't thread-safe, and I can already see a few places where your code isn't thread-safe either. (For example, you need to declare capturedDirection as volatile.) Writing correct multi-threaded code with Swing is a bit complicated and it would be much simpler to just use a timer.
Otherwise, if you don't use a timer, you need to use e.g. synchronization between the game thread (which writes to shared game state) and the Swing thread which does painting (and presumably reads from shared game state). If you don't, you may run in to problems that are hard to diagnose.
Also see The Use of Multiple JFrames: Good or Bad Practice?

You should make your Game class extending Runnable instead of Thread.
Then to have the game in a different thread:
Game theGame = ... // initialization code here
new Thread(theGame).start();

paintComponent() gets called only when Minimizing-Maximizing Screen

I am trying to develop a game in which I need to draw a grid. For that I am using the paintComponent(Graphics g) method which is being called by repaint() method.
The problem is that the repaint method is inside the infinite While loop and it never calls the paintComponent() method unless I minimize and maximize the screen. After that it works fine and calls the paintComponent() perfectly in the while loop.
So in short, I need to trigger it by Minimizing-Maximizing the screen.
Can anybody help me out?
In the code you can see 3 classes namely, Frame.java, MenuHandler.java & Screen.java. Code starts from the main method in Frame class and it adds the Screen class to itself as Screen extends JPanel. However, the control goes to the Screen class only when user selects "Create Map" on the menu. Then MenuHandler class passes the control to the Screen class where the run method is called in the createMap method and I invoking the repaint method inside this run method.
package so;
public class Screen extends JPanel implements Runnable {
Frame frame;
public Screen(Frame frame) {
this.frame = frame;
}
public void createMap() {
thread.start();
}
public void paintComponent(Graphics g) {
g.setColor(Color.BLUE);
}
#Override
public void run() {
System.out.println("Success******");
long lastFrame = System.currentTimeMillis();
int frames = 0;
running = true;
scene = 0;
// the map grid would be refreshed every 2 ms so that we don't get the
// flickering effect
while (running) {
frames++;
if (System.currentTimeMillis() - 1000 >= lastFrame) {
fps = frames;
frames = 0;
lastFrame = System.currentTimeMillis();
}
// to draw stuff all the time on the screen : goes around 2 millions
// frames per second. Which is of no use.
repaint();
try {
Thread.sleep(2);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.exit(0);
}
}
Timer code in the run method:
public void run() {
System.out.println("Success");
long lastFrame = System.currentTimeMillis();
int frames = 0;
running = true;
scene = 0;
// the map grid would be refreshed every 2 ms so that we don't get the
// flickering effect
while (running) {
frames++;
if (System.currentTimeMillis() - 1000 >= lastFrame) {
fps = frames;
frames = 0;
lastFrame = System.currentTimeMillis();
}
System.out.println("before repaint");
// to draw stuff all the time on the screen : goes around 2 millions
// frames per second. Which is of no use.
ActionListener taskPerformer = new ActionListener() {
public void actionPerformed(ActionEvent evt) {
repaint();
}
};
new Timer(200, taskPerformer).start();
System.out.println("after repaint");
try {
Thread.sleep(2);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.exit(0);
}

I figured out the issue. it was just one line that I had to add to trigger the paintComponent method instead of doing it by minimizing-maximizing the window.
Frame was my top level container and I was adding Screen component (that extends JPanel and has the implementation of paintComponent) to the frame. So while adding, earlier I was doing
frame.add(screen);
I changed this to:
frame.getContentPane().add(screen);
frame.getContentPane().validate();
Calling the validate method after adding it did it for me. I don't know if it makes sense but yes that was the only line that worked for me.
Hope it helps.

Why my GUI won't update even tho repaint() is being called?

I am having some difficulties using swing workers, timers, and I am actually a little confused.
As far as my understanding goes, I have to put on a timer to set-up recurring tasks that have to be called by the EDT.
I'm trying to make a program that shows graphically a sorting alghoritm (like this : https://www.youtube.com/watch?v=kPRA0W1kECg )
I just don't understand why the GUI won't refresh. I am quite sure the repaint method is being called since I put a sysout showing me the ordered values and it seems to work , but the GUI just... doesn't change.
Here's my code:
public class MainWindow {
private JFrame frame;
JPanel panel;
public final static int JFRAME_WIDTH = 800;
public final static int JFRAME_HEIGHT = 600;
public final static int NELEM = 40;
ArrayList<Double> numbers;
ArrayList<myRectangle> drawables = new ArrayList<myRectangle>();
Lock lock = new ReentrantLock();
Condition waitme = lock.newCondition();
public static void main(String[] args) {
EventQueue.invokeLater(new Runnable() {
public void run() {
try {
MainWindow window = new MainWindow();
window.frame.setVisible(true);
} catch (Exception e) {
e.printStackTrace();
}
}
});
}
public MainWindow() {
initialize();
}
private void initialize() {
frame = new JFrame();
frame.setBounds(100, 100, JFRAME_WIDTH + 20, JFRAME_HEIGHT + 40);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
panel = new myPanel();
frame.getContentPane().add(panel, BorderLayout.CENTER);
Timer timer = new Timer(500, new ActionListener() {
#Override
public void actionPerformed(ActionEvent arg0) {
lock.lock();
try{
//Updating the gui
panel.repaint();
panel.revalidate();
//Giving the OK to the sorting alghoritm to proceed.
waitme.signal();
}catch(Exception e){
e.printStackTrace();
}finally{
lock.unlock();
}
}
});
timer.start();
SwingWorker<Integer, String> sw = new SwingWorker<Integer, String>(){
#Override
protected Integer doInBackground() throws Exception {
mapAndCreate();
bubbleSort();
return null;
}
};
sw.execute();
}
private void bubbleSort() throws InterruptedException{
for(int i=0; i < NELEM; i++){
for(int j=1; j < (NELEM-i); j++){
if(drawables.get(j-1).wid > drawables.get(j).wid){
//swap the elements!
myRectangle temp = drawables.get(j-1);
drawables.set(j-1, drawables.get(j));
drawables.set(j, temp);
lock.lock();
try{
//Wait for the GUI to update.
waitme.await();
}catch(Exception e){
e.printStackTrace();
}finally{
lock.unlock();
}
}
}
}
}
/***
* Function that maps values from 0 to 1 into the rectangle width.
*/
private void mapAndCreate() {
double max = 0;
numbers = new ArrayList<Double>(NELEM);
//Finding maximum.
for(int i = 0; i < NELEM; i++){
Double currElem = Math.random();
if(currElem > max) max = currElem;
numbers.add(currElem);
}
//Mapping process
int offset = 0;
for(int j = 0; j < NELEM; j++){
Integer mapped = (int) (( JFRAME_WIDTH * numbers.get(j) ) / max);
myRectangle rect = new myRectangle(offset , mapped);
drawables.add(rect);
offset += JFRAME_HEIGHT / NELEM;
}
}
private class myRectangle{
int myy , wid , colorR,colorG,colorB;
public myRectangle(int y , int wid){
this.myy = y;
this.wid = wid;
Random r = new Random();
colorR = r.nextInt(255);
colorG = r.nextInt(255);
colorB = r.nextInt(255);
}
}
private class myPanel extends JPanel{
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
for(myRectangle rectan : drawables){
Graphics2D graphics2D = (Graphics2D)g;
System.out.println(rectan.wid);
Rectangle2D.Double rect = new Rectangle2D.Double(0,rectan.myy,rectan.wid,JFRAME_HEIGHT / NELEM);
graphics2D.setColor(new Color(rectan.colorR,rectan.colorG,rectan.colorB));
graphics2D.fill(rect);
}
System.out.println("====================================================================================================");
}
}
}

Most OSs (or rather the UI frameworks which they use) don't support concurrent access. Simply put, you can't render two strings of text at the same time.
That's why Swing runs all rendering operations in the UI thread. Calling rendering functions (like paint()) outside of the UI thread can cause all kinds of problems. So when you do it, Swing will just remember "I should repaint" and return (instead of doing any actual work). That way, Swing protects you but most people would prefer to get an error with a useful message.
A timer always also means that there is a thread somewhere which executes when the timer runs out. This is not the UI thread of Swing. So any paing operations there must be wrapped with EventQueue.invokeLater() or similar.
Another common bug is to hog the UI thread (so no rendering happens because you do complex calculations there). That's what the SwingWorker is for. Again, in most methods of the SwingWorker, calling methods which would render something is forbidden (-> use invokeLater()).
So my guess is that the UI thread waits for the lock and the lock simply isn't unlocked early or often enough. See this demo how to do a simple animation in Swing.
public class TimerBasedAnimation extends JPanel implements ActionListener {
public void paint(Graphics g) {
// setup
// do some first-run init stuff
// calculate the next frame
// render frame
}
public void actionPerformed(ActionEvent e) {
repaint();
}
public static void main(String[] args) {
JFrame frame = new JFrame("TimerBasedAnimation");
frame.add(new TimerBasedAnimation());
...
}
}
As you can see in the code doesn't lock. Instead, you just send "render now" events from actionPerformed to Swing. Some time later, Swing will call paint(). There is no telling (and no way to make sure or force Swing) when this will happen.
So good animation code will take the current time, calculate the animation state at that time and then render it. So it doesn't blindly step through N phases in M seconds. Instead, it adjusts for every frame to create the illusion that the animation is smooth when it really isn't.
Related:
Java: Safe Animations with Swing
How to Use Swing Timers

Java repaint() method doesn't always work

There is a problem with the repaint() method in Java. I made a new thread that constantly repaints the screen. When I release the spacebar I want my player to fall smoothly by setting its position and then waiting for 50 milliseconds and looping that 20 times. Instead, it waits the whole amount of time in the loop, then repaints. I am wondering why it doesn't constantly repaint the changes in the players co-ordinates. Thank you.
(Edit) Thanks everyone for the help. This is my first time using stack overflow, and I am only 13 and still learning java, so I probably will go back to the tutorials again.
My 'a' class (main):
public class a {
public static void main(String[] args) {
JFrame frame = new JFrame("StickFigure Game");
frame.setSize(740, 580);
frame.setDefaultCloseOperation(frame.EXIT_ON_CLOSE);
frame.setLocationRelativeTo(null);
frame.setVisible(true);
b board = new b();
frame.add(board);
frame.addKeyListener(board);
}
}
My 'b' class (JPanel/drawing):
public class b extends JPanel implements KeyListener {
c player = new c();
public class MyRunnable implements Runnable {
public void run() {
while (true)
repaint();
}
}
MyRunnable run = new MyRunnable();
public void paint(Graphics g) {
super.paint(g);
Graphics2D g2d = (Graphics2D) g;
g2d.drawImage(player.getImage(), player.getX(), player.getY(), 80, 140,
null);
}
public b() {
Thread thread = new Thread(new MyRunnable());
thread.start();
}
public static void slow(int n) {
long t0, t1;
t0 = System.currentTimeMillis();
do {
t1 = System.currentTimeMillis();
} while (t1 - t0 < n);
}
public void keyPressed(KeyEvent e) {
if (e.getKeyCode() == KeyEvent.VK_D) {
player.setPos(player.getX() + 6, player.getY());
}
if (e.getKeyCode() == KeyEvent.VK_A) {
player.setPos(player.getX() - 6, player.getY());
}
if (e.getKeyCode() == KeyEvent.VK_SPACE) {
player.setPos(player.getX(), player.getY() - 60);
}
}
public void keyReleased(KeyEvent e) {
if (e.getKeyCode() == KeyEvent.VK_SPACE) {
for (int i = 0; i < 20; i++) {
slow(50);
player.setPos(player.getX(), player.getY() + 2);
}
}
}
public void keyTyped(KeyEvent e) {
}
}
my 'c' class (player):
public class c {
private ImageIcon i = new ImageIcon("guy.png");
private Image img = i.getImage();
private int x = 0;
private int y = 100;
public void wait(int what) {
try {
Thread.sleep(what);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
public c() {
}
public Image getImage() {
return img;
}
public int getX() {
return x;
}
public int getY() {
return y;
}
public void setPos(int mx, int my) {
x = mx;
y = my;
}
}

I haven't gone through all the code here but here are some pointers:
Swing has its own concurrency mechanisms which allow you to handle UI updates. You can use a Swing Timer rather than a raw Thread. Related is the use of Thread.sleep - don't do this, it only blocks the EDT and prevents UI updates.
The Swing paint chain mechanism requires you to override paintComponent rather than paint.
Always use Key Bindings rather than KeyListeners in Swing. KeyListeners require component focus to work to interact with the KeyEvents. Key Bindings do not have this limitation.

"There is a problem with the repaint() method in java." Did you consider that perhaps the problem is with your code instead? You are blocking the event thread and giving the system no time to do the intermediate repaints. In particular, this method:
public static void slow (int n){
long t0,t1;
t0=System.currentTimeMillis();
do{
t1=System.currentTimeMillis();
}
while (t1-t0<n);
}
and this loop:
for(int i = 0;i<20;i++){
slow(50);
player.setPos(player.getX(), player.getY()+2);
}
do not relinquish control to the system so that repaints can actually happen. Rewrite those using Swing timers. Look at this tutorial for an introduction on how to use these.
Also, your thread that constantly calls repaint() in a tight loop:
public void run(){
while(true) repaint();
}
is a terrible idea. You don't need to call repaint() at full CPU speed. Once every 30 milliseconds or so is fine for animation. Again, consider using Swing utilities to do this rather than writing your own looping thread.

The repaint is only a "request" to paint as soon as possible. so when you call it it causes a call to the paint method as soon as possible.
from here
So basically you just flooding the scheduled calls of paint or update with while(true) repaint();.
Oracle's stance on painting in AWT and Swing
One way you could do it, or should I say how I would do it, is to make your c class implement KeyListener, so that when a key is pressed (and only when it is pressed) you update it's location.
So move your KeyListener methods to class c, in your class b constructor you can add the call this.addKeyListener(player) or make a method void addPlayer(c player) that adds it.

How do I make this java for loop pause for 1/2 a second between each iteration?

private class MultipleGensListener implements ActionListener
{
public void actionPerformed(ActionEvent e)
{
for(int i = 0; i < 25; i++)
{
game.runSimulationOneGen();
changeGrid();
}
}
}
//this is the loop. The changeGrid method displays a game grid on a GUI but
// only the 25th iteration is visible on screen. I would like each one to be
// visible for about a half a second before the loop continues.
// I have seen some questions answered on here that are very close to what I'm asking,
// but I just don't really understand how to apply it to my program..
// thanks for any help.

If the code performed by the simulation is quick and does not consume too much CPU and time, then consider using a Swing Timer to do your looping and delay. Otherwise, you'll need to use a background thread such as can be done with a SwingWorker object.
For e.g. if using both Swing Timer and SwingWorker:
private class MultipleGensListener implements ActionListener {
protected static final int MAX_INDEX = 25;
public void actionPerformed(ActionEvent e) {
int timerDelay = 500; // ms delay
new Timer(timerDelay, new ActionListener() {
int index = 0;
public void actionPerformed(ActionEvent e) {
if (index < MAX_INDEX) { // loop only MAX_INDEX times
index++;
// create the SwingWorker and execute it
new SwingWorker<Void, Void>() {
#Override
protected Void doInBackground() throws Exception {
game.runSimulationOneGen(); // this is done in background thread.
return null;
}
#Override
protected void done() {
changeGrid(); // this is called on EDT after background thread done.
}
}.execute(); // execute the SwingWorker
} else {
((Timer) e.getSource()).stop(); // stop the timer
}
}
}).start(); // start the Swing timer
}
}

NEVER BLOCK THE GUI EVENT THREAD
you can use a timer for that and have it only fire 25 times
final Timer t = new Timer(500,null);
t.addActionListener(new ActionListener(){
int i=0;
public void actionPerformed(ActionEvent e){
game.runSimulationOneGen();//run 1 iteration per tick
changeGrid();
if(i>25){t.stop();}
i++;
}
});
t.setRepeats(true);
t.start();
btw the reason only the last iteration is shown is that gui updates (redraws) are done in a separate event, but to let another event trigger you need to return from the listener method which you didn't
the Timer I showed is a more elaborate iteration which lets other events run in between iterations allowing the gui to show the changes

check my post that shows both methods java.swing.Timer#setDelay(int)
and
correct usage of Thread.sleep(int)
java wait cursor display problem