Develop Reference

javaFX - creating smooth animation without creating a new Thread

I have a car in my javaFX project where the position of the car(Node) should change(the car should jump smoothly) when SPACE is pressed . so I have used an event handler which invokes a method named :moveUp()
scene.setOnKeyPressed(new EventHandler<KeyEvent>() {
#Override
public void handle(KeyEvent event) {
switch (event.getCode())
{
case SPACE:
moveUp();
break;
}
}
});
This creates a new Thread where the speed of the car is changed 10 times with an interval of 75 milliseconds.
private void moveUp() {
new Thread(new Runnable() {
#Override
public void run() {
carSpeed = 10;
for(;carSpeed>=0;carSpeed--)
{
try {
Thread.currentThread().sleep(75);
} catch (InterruptedException e) {
e.printStackTrace();
}
carPosition_X+=carSpeed;
carPosition_Y-=carSpeed;
car.relocate(carPosition_X,carPosition_Y);
}
for(carSpeed=0;carSpeed<=10;carSpeed++)
{
try {
Thread.currentThread().sleep(75);
} catch (InterruptedException e) {
e.printStackTrace();
}
carPosition_X+=carSpeed;
carPosition_Y+=carSpeed;
car.relocate(carPosition_X,carPosition_Y);
}
}
}).start();
}
This code is doing like this (pressing SPACE once and jumping the car):
If I don't use a different thread the GUI thread will be freeze and if I don't use Thread.sleep() the car will jump abruptly(not smoothly). This code is doing well . But I have learnt that thread.start() doesn't guarantee immediate execution of the thread. How can I guarantee immediate execution ?

I would suggest to stay in the FX Application Thread and to use the class AnimationTimer. Here is a short demo for a smooth jump:
private void moveUp() {
new AnimationTimer() {
long startTime = -1;
double initCarPosition_Y;
#Override
public void handle(long now) {
if(startTime == -1){
startTime = now;
initCarPosition_Y = carPosition_Y;
carSpeedX = 3d;
carSpeedY = -15d;
}
double time = (now - startTime) / 1E9d;
carPosition_X += carSpeedX * time;
carPosition_Y += carSpeedY * time;
if(carSpeed > 0 && initCarPosition_Y <= carPosition_Y){
carPosition_Y = initCarPosition_Y;
stop();
}
carSpeedY += 0.8d * time; //gravity
car.relocate(carPosition_X, carPosition_Y);
}
}.start();
}
This approach gives you full and direct control over what happens in every single frame. However, javaFX also provides high level animation classes including predefined interpolators and transitions. Suitable for alternative approaches could be the following classes:
PathTransition: Allows you to define points and curves which a given node is animated along.
TimeLine: Allows you to define arbitrary animation key frames based on properties like the position of a node.
Note that generally working with these high level classes could become challenging when you want to animate an user controlled actor like your car. These classes all take an exact duration for the animation to last. For example when you want to translate a node as long as a specific key is pressed, you don't know the duration of the animation in beforehand.

Java Thread.sleep() causing JFrame to flicker when resized

So I have a game that I'm trying to make and in the game loop, I call Thread.sleep(). Else where, I have code that maintains the aspect ratio of the window when resizing. This works great, except that I get weird flickering when I'm resizing. I've narrowed the problem down to Thread.sleep(), when I take this line out, my program works just as expected, but this causes the CPU to spike so high that on my Macbook, the Activity Monitor app says my game is using 170+%! Now this is problematic and exactly why I put the sleep line in there anyway. I've heard that sleeping on the event dispatch thread will cause this effect, but I am running this loop in a new thread, so I thought I was good. Do you guys know what could be going on? Here's part of the source code (you really need to look at the run() method):
package jeffrey_ryan.game2d;
public class GameLoop implements Runnable {
private boolean running = false;
private boolean paused = false;
private float gameHertz = 30.0f;
private long timeBetweenUpdates = (long) (1_000_000_000 / gameHertz);
private int maxUpdates = 5;
private LoopListener loopListener;
public void run() {
long lastUpdateTime = System.nanoTime();
running = true;
while (running) {
long now = System.nanoTime();
if (!paused) {
int updates = 0;
while (now - lastUpdateTime >= timeBetweenUpdates && updates < maxUpdates) {
if (loopListener != null) {
loopListener.update((double) timeBetweenUpdates / 1_000_000_000);
}
lastUpdateTime += timeBetweenUpdates;
updates++;
}
if (loopListener != null) {
float interpolation = Math.min(1.0f, (float) (now - lastUpdateTime) / timeBetweenUpdates);
loopListener.render(interpolation);
}
long timeRemaining = (timeBetweenUpdates - (now - lastUpdateTime)) / 1_000_000;
try {
Thread.sleep(Math.max(timeRemaining - 5, 0)); // HERE'S THE OFFENDING LINE ******************
}
catch (InterruptedException ie) {
ie.printStackTrace();
}
}
else {
try {
Thread.sleep(25);
}
catch (InterruptedException ie) {
ie.printStackTrace();
}
}
}
}
public void start() {
running = true;
}
public void stop() {
running = false;
}
public void pause() {
paused = true;
}
public void play() {
paused = false;
}
public float getSpeed() {
return gameHertz;
}
public void setSpeed(float hertz) {
gameHertz = hertz;
timeBetweenUpdates = (long) (1_000_000_000 / gameHertz);
}
public int getMaxUpdates() {
return maxUpdates;
}
public void setMaxUpdates(int updates) {
maxUpdates = updates;
}
public void setLoopListener(LoopListener listener) {
loopListener = listener;
}
}
In my subclass of JPanel, here's the code that runs this loop (Where the loop variable is an instance of the above class):
#Override
public void addNotify() {
super.addNotify();
addKeyListener(this);
addMouseListener(this);
Thread thread = new Thread(loop, "GameLoop");
thread.start();
}
If you guys could help me I would love it, I'm really stumped. Thanks!

You should use SwingWorker instead of a Thread to manipulate Swing components asynchronously. When I discovered this guy my life changed =). The SwingWorker gives you a method "process" which you can use to make actions gradually, and a "done" method to finish your processing, both of these methods are safe to handle the event dispatch thread. The background process you should make on "doInBackground".

Calling 'Thread.sleep(n)' causes the whole thread to become unresponsive, if this thread is tied to your JFrame thread then that thread will also become unresponsive and cause the whole frame and component to freeze and stop responding -- probably the reason for the flickering. So make sure the sleep is in game loop and not on the frame, one way to do this is create two threads at initialization, one for the frame and the other for the logic, then just let the game loop handle input and output while the display thread simply displays (i believe this how most game engines work). Also make sure neither thread is linked in any or the sleeping thread will affect the display thread.

I found the answer to my problem. The class that was calling the loop, which was a JPanel, didn't repaint when resized, only when the loop told it to, which caused some periods where the JPanel wasn't painted too. I fixed this by overriding paintComponent.

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.

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.

To calculate the total time elapsed using threads in java

I have this java code using Threads to calculate the time elapsed once the start button is hit till the stop button is not hit.
I want to do this using Threads only
import javax.swing.*;
import java.awt.event.*;
// This will count the elapsed time between running time of two threads.
class ThreadGame {
JButton button;
MyAction my_action;
public static void main(String[] args) {
ThreadGame tg = new ThreadGame();
}
public ThreadGame() {
JFrame frame = new JFrame("Calculate time - Game");
button = new JButton("Start");
button.addActionListener(new MyAction());
frame.add(button);
frame.setSize(400, 400);
frame.setVisible(true);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
class MyAction implements ActionListener {
public void actionPerformed(ActionEvent e) {
String text = (String) e.getActionCommand();
final Timer timer = new Timer();
if (text.equals("Start")) {
button.setText("Stop");
Thread start_time = new Thread() {
public void run() {
timer.startTime();
}
};
start_time.start();
try {
start_time.join();
} catch (Exception e1) {
}
} else {
Thread stop_time = new Thread() {
public void run() {
timer.stopTime();
}
};
Thread get_time = new Thread() {
public void run() {
timer.getElapsedTime();
System.out.println(timer.elapsed);
}
};
stop_time.start();
try {
stop_time.join();
} catch (Exception e2) {
}
get_time.start();
button.setText("Start");
}
}
}
class Timer {
public double startTime = 0.0;
public double stopTime = 0.0;
public boolean running = false;
public double elapsed = 0.0;
public void startTime() {
this.startTime = System.nanoTime();
this.running = true;
}
public void stopTime() {
this.stopTime = System.nanoTime();
this.running = false;
}
// Elasped time in seconds
public double getElapsedTime() {
// double elapsed;
if (running) {
elapsed = ((System.nanoTime() - startTime) / 1000);
} else {
elapsed = ((stopTime - startTime) / 1000);
}
return elapsed;
}
}
}
EDIT: I have understand the problem: timer scope was the problem.
EDIT 2: Ok, it looks like I have to use suspend and resume in one thread only.

The problem is that the start button press is starting a different Timer object than the stop-button button press is stopping because the Timer object is created every time when the actionPerformed(...) method is called.
The Timer needs to be a field in your MyAction class. You also don't need all of the thread start/joins because the Timer object is very simple and fast.
Really, you can just use a startTimeMillis long field instead of a timer. Something like:
class MyAction implements ActionListener {
private long startTimeMillis;
public void actionPerformed(ActionEvent e) {
String text = (String) e.getActionCommand();
if (text.equals("Start")) {
startTimeMillis = System.currentTimeMillis();
...
} else {
System.out.println(System.currentTimeMillis() - startTimeMillis);
}
}
}

Your problem is caused by the scope of timer. This should be a private instance variable, not a local method variable. Further, wrapping calls to startTime and endTime in a thread's run method isn't gaining you anything, because these are incredibly short-lived calls. But that's not the real problem here.
There's no reason to be running Timer in its own thread. That is, without using a specialized real-time operating system, using threads to solve the problem of measuring the duration between two events is just plain wrong.
You might think that you could create a thread with a loop that increments a msec variable after a Thread.sleep(1). Don't do this! This kind of timing is also just plain wrong. Your computer uses a preemptive multitasking model which means there's no guarantee that your thread will execute on a regular interval. That is, there is nothing requiring that Thread.sleep(1) will sleep for some maximum duration. The guarantee is that your thread will sleep for a minimum of 1ms. This means there's no way to determine clock error if you're managing a clock yourself in software, and this error is not insignificant. Just don't do it!! Time should always be measured by your operating system, preferably using an interrupt-based timer (which is how System.nanoTime works on most, if not all platforms).
Instead of using a thread, just call your startTime and stopTime methods directly from your original thread.
Try this:
class ThreadGame {
JButton button;
MyAction my_action;
private final Timer timer = new Timer();
public static void main(String[] args) {
ThreadGame tg = new ThreadGame();
}
public ThreadGame() {
JFrame frame = new JFrame("Calculate time - Game");
button = new JButton("Start");
button.addActionListener(new MyAction());
frame.add(button);
frame.setSize(400, 400);
frame.setVisible(true);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
class MyAction implements ActionListener {
public void actionPerformed(ActionEvent e) {
String text = (String) e.getActionCommand();
if (text.equals("Start")) {
timer.startTime();
button.setText("Stop");
} else {
timer.stopTime();
button.setText("Start");
}
}
}
class Timer {
public double startTime = 0.0;
public double stopTime = 0.0;
public boolean running = false;
public double elapsed = 0.0;
public void startTime() {
this.startTime = System.nanoTime();
this.running = true;
}
public void stopTime() {
this.stopTime = System.nanoTime();
this.running = false;
}
// Elasped time in seconds
public double getElapsedTime() {
return (this.startTime-this.stopTime)*1000000000.0;
}
}
}
If you want to learn how to use threads, try writing an application that solves a problem for which threads are a good fit. For example, write a small Swing application that lets you download a file from the web. As the file is downloading, update a progress bar in your UI. The download should happen in a worker thread separately from the UI thread, otherwise the UI will block during the download's progress.
Another example problem is to write a threaded ray tracer (here's an example tutorial written in C++).
If you want something simpler, write a Swing clock. Use a loop within a separate thread to update the UI at a periodic interval, but do not use the loop to manage what time it is. Again, don't try to keep time in the loop, just let the OS keep the time, and use the loop to schedule when the UI gets updated with the current OS time.

You're never calling getElapsedTime() that updates elapsed field.

You are creating a new Timer when ever you click the button. Make timer a class variable of your MyAction
The code below should be sufficient to get elapsed time.
class MyAction implements ActionListener {
final Timer timer = new Timer();
public void actionPerformed(ActionEvent e) {
String text = (String) e.getActionCommand();
if (text.equals("Start")) {
button.setText("Stop");
timer.startTime();
} else {
timer.stopTime();
System.out.println(timer.elapsed);
button.setText("Start");
}
}
}

Simply do this...
- Call System.currentTimeMillis() at the Starting of threads.
- Then again call System.currentTimeMillis() at the end of threads.
- Subtract the end with starting value to get the Elapsed time...
/////////////////EDITED///////////////////////
Make sure that the method trying to manipulate(ie. read and write) the variable holding the System.currentTimeMillis() must be synchronized, with synchronized keyword, so that Race Condition doesn't occur according to Brian's Law...
If you are writing a variable that might next be read by another thread, or reading a variable that might have last been written by another thread, you must use synchronization, and further, both the reader and the writer must synchronize using the same monitor lock.