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Java 2D Game slowing down without apparent reason

i created a 2D platformer style game in java in which some parts are simualted by a physics-engine that i programmed myself as well. The problem is that the game slows down to around 30 fps from the intended 60 fps at some points. The game loop looks like this:
#Override
public void run() {
init();
long startTime;
long elapsedTime;
long waitTime;
while(running) {
startTime = System.nanoTime();
update();
long updateTime = (System.nanoTime() - startTime) / 1000000;
draw();
long drawTime = (System.nanoTime() - startTime) / 1000000;
drawToScreen();
long drawscreenTime = (System.nanoTime() - startTime) / 1000000;
elapsedTime = System.nanoTime() - startTime;
waitTime = targetTime - elapsedTime / 1000000;
if(waitTime < 0) {
waitTime = 0;
}
try {
Thread.sleep(waitTime);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
I am measuring the time that is needed to update the game logic with updateTime and the time that is needed to draw with drawTime and drawscreenTime. At all times (during the slowdowns as well!) the updateTime is 0 ms and drawTime + drawscreenTime is around 10 ms, which is below the maximum of 16 ms (for 60 fps).
So far i have found out that changing the setup of potential collisions of my physical objects can reduce the slowdown. At first i wanted to seperate the objects by zones, so that the program doesnt need to check every object with every object. For example instead of 100x100 = 10000 objects only 33x33 + 33x33 + 33x33 = 3267 objects.
Seperating by zones looked like this:
private ArrayList<PhysicalObject> objects;
private ArrayList<ArrayList<PhysicalObject>> zones;
for(int i = 0; i < numberOfZones; i++) {
zones.get(i).clear();
}
for(PhysicalObject object : objects) {
for(int zone : object.getSimulationZones()) {
zones.get(zone).add(object);
}
}
for(ArrayList<PhysicalObject> list : zones) {
for(PhysicalObject object1 : list) {
for(PhysicalObject object2 : list) {
collision stuff happens
}
}
}
}
The "blunt" approach of chekcing every object with every other object looked like this:
private ArrayList<PhysicalObject> objects;
for(PhysicalObject object1 : objects) {
for(PhysicalObject object2 : objects) {
collision stuff happens
}
}
Now as you might expect the blunt approach is slower when looking at updateTime, but when i use this approach the slowdowns are far less significant.
I dont know how this is happening, since the time needed to do one game loop seems to be unrelated to the slowdowns, but when i change code that runs during the game loop, the slowdowns are varying.
At first i thought that Thread.sleep() might be at fault, since i heard that it is sometimes unreliable, so i tried using a Java.util.Timer. But when i use that with a delay of 16 ms (for 60 fps) the fps are jumping up and down around 30 fps.
I would really appreciate if someone could help me with this.
If you need more Code, I can post more, but since the game-code is already pretty big I figured it would be best to start small and not flood my question even more.
EDIT:
It seems that drawing the game-level has an influence on the slowdowns as well. But instead of slowing down when there are more images to draw, it slows down when there are less images. But again: Even though the slowdowns are decreasing, the drawTime is increasing when there are more things to draw. The code for drawing the level looks like this:
public void draw(Graphics2D g2d) {
for(int row = drawStartRow; row < drawStartRow + rowsToDraw; row++) {
if(row >= numRows) {
break;
}
for(int col = drawStartCol; col < drawStartCol + colsToDraw; col++)
{
if(col >= numCols) {
break;
}
//0 stands for no tile to draw
if(map[row][col] == 0) {
continue;
}
int rc = map[row][col];
int r = rc / numTilesAcross;
int c = rc % numTilesAcross;
g2d.drawImage(tiles[r][c].getImage(), (int)x + col * tileSize, (int)y + row * tileSize, null);
}
}
}
The images in the array were generated via BufferedImage.getSubImage() in the constructor.
EDIT 2:
I was finally able to get rid of the slowdowns by changing the drawing method of my level. Instead of iterating over every spot and drawing the corresponding image, i am now iterating over the different images first and then (for every image) over the spots and drawing them if they belong to the spot.
So overall my loop is times numberOfImages longer, but the slowdowns disappeared. The reason seems to be that I am now drawing every image as often as it needs to be drawn and then going on to the next image. But i dont understand how this is affecting my overall framerate so much, even though the time needed to draw is not changing?

As per the specs of drawImage :
https://docs.oracle.com/javase/7/docs/api/java/awt/Graphics2D.html#drawImage(java.awt.Image,%20java.awt.geom.AffineTransform,%20java.awt.image.ImageObserver)
The drawing may be done asynchronously, which means that the image is not rendered when the function returns. By blocking the thread with Thread.sleep, you may be slowing the rendering. You can do a quick test to check if the image is fully rendered by checking the return value.

Loop throttled with Thread.sleep() runs more than twice as often as intended

I use the following loop (cut some stuff out) as the main loop for a game, but I can't get it to throttle down to a speed I want it to, it keeps running about twice as fast as I intend.
private void myLoop() throws InterruptedException {
long timer = TimeUtils.getMillis();
int achievedLoops = 0;
long currTime = 0l;
long loopTime = 0l;
long lastTime = TimeUtils.getNano();
while(!isRequestedToStop) {
currTime = TimeUtils.getNano();
loopTime = currTime - lastTime;
lastTime = currTime;
if(TimeUtils.getDeltaMillis(timer) > 1000) {
timer += 1000;
logger.debug(achievedLoops + " Loops");
achievedLoops = 0;
}
achievedLoops++;
if(loopTime < TIME_PER_LOOP) {
Thread.sleep( (TIME_PER_LOOP - loopTime) / 1000000l);
}
}
}
Alternative implementation of the sleeping, gets slightly better results (loop runs only 1.9 times too often):
while(loopTime < TIME_PER_LOOP) {
Thread.sleep(1l);
loopTime += 1000000l;
}
Another alternative:
while(loopTime < TIME_PER_LOOP) {
Thread.sleep(1l);
loopTime = TimeUtils.getNano() - lastTime;
}
Why does that happen?
Are there any other ways to throttle a thread down?
I basically could run it uncontrolled as the logic is tied to timed steps, but I would like to reduce the total runs of the loop as otherwise there's a marginal chance of it doing damage to a CPU..

LockSupport can disable the scheduling of a thread for a specified number of nanoseconds.
LockSupport.parkNanos(TIME_PER_LOOP - loopTime);
But as others have mentioned there are better ways to control timing (e.g. ScheduledExecutorService).

Each time you perform the division
(TIME_PER_LOOP - loopTime) / 1000000l
You are truncating the result and sleeping up to 1ms less than you expect (average 0.5ms). Given your 4ms loop time, this would easily cause your loop to run twice as fast as you expect. As others have mentioned, there are much better ways to control timing.

Java Game Loop Explanation

Recently, I've been getting into Java Game programming. I have learned Java before, but this is my first time with Graphics, Game Loops etc.
I followed this tutorial for a Game Loop, and it's working pretty well.
There is nothing wrong with it, but I just can't figure out why I am thread.sleeping the code, and whats the point of it.
If anyone could explain what start, elapsed and running do, and why I am doing thread.sleep(wait) then I would be super appreciative!
GAME LOOP (Remember, it works, I just don't know why):
while (running) {
start = System.nanoTime();
update();
draw();
drawToScreen();
elapsed = System.nanoTime() - start;
wait = targetTime - elapsed / 1000000;
if (wait < 0) {
wait = 5;
}
try {
Thread.sleep(wait);
} catch (Exception e) {
Game.logger.log("ERROR! Printing Stacktrace...");
e.printStackTrace();
}
}
}

Well, running is a flag that can be set to false to terminate the game.
start and elapsed are used to measure the time one round in the loop took. You are waiting to not make the game run too fast. If you are moving elements with a set speed, then having the game loop run too fast makes them also move too fast. That can lead to a bad user experience, since the user might not be fast enough to play the game.
Additionally, any animation doesn't run smooth anymore without the waiting. It will speed up and slow down depending on how fast your code executes.
Just continue with the tutorial, and when are at the point where you make something move, remove the waiting and see what happens.
EDIT
The code makes a round in the loop take about targetTime. It's unit is milliseconds. So to calculate the frame rate, just divide 1000 / targetTime.

start is the time before the game logic runs. elapsed is the amount of time that it takes for the game logic to run. running is a boolean (true/false) variable that determines whether the game should continue. Thread.sleep(wait) is a method that delays for a certain amount of time.
The goal is to keep the amount of time between frames roughly constant by delaying, so that the animations don't run faster or slower depending on how fast your computer processor is running.

It appears the code aims to be executed targetTime seconds. So you count how much time has already passed (elapsed), and then calculate how much to wait (divide it to get your remaining time in miliseconds, required for methon sleep(wait)).
One purpose of such waiting is often forcing some time between repainting to the screen.
To get your FPS, one way would be to slightly change your code:
long prevStart = 0; //just initialize. First FPS is of course wrong, next get fine.
double FPS = 0;
while (running) {
start = System.nanoTime();
FPS = 1 / double(start - prevStart);
prevStart = start;
update();
draw();
drawToScreen();
elapsed = System.nanoTime() - start;
wait = targetTime - elapsed / 1000000;
if (wait < 0) {
wait = 5;
}
try {
Thread.sleep(wait);
} catch (Exception e) {
Game.logger.log("ERROR! Printing Stacktrace...");
e.printStackTrace();
}
}

Updating and rendering for a 2D game in Java

So, I am working on making a 2D game in Java, and I really don't have too much experience with Java. I currently use a very simple loop using a swing timer running every 10ms or so that looks something like:
public void actionPerformed(ActionEvent e) {
update();
repaint();
}
However, I need something more practical for obvious reasons. These reasons include the fact that more lag means less FPS AND slower movement/other updating.
I found the following code in a tutorial for a 3D Java game here. It would begin running when the program starts, and I understand enough to know it would work. However, I do not fully understand it: (tick() is the updater, render() renders the screen)
long currenttime;
long previoustime = System.nanoTime();
long passedtime;
int frames = 0;
double unprocessedseconds = 0;
double secondspertick = 1 / 60.0;
int tickcount = 0;
boolean ticked = false;
while (gameIsRunning) {
currenttime = System.nanoTime();
passedtime = currenttime - previoustime;
previoustime = currenttime;
unprocessedseconds += passedtime / 1000000000.0;
while (unprocessedseconds > secondspertick) {
tick();
unprocessedseconds -= secondspertick;
ticked = true;
tickcount++;
System.out.println(tickcount);
if (tickcount % 60 == 0) {
System.out.println(frames + " FPS");
previoustime += 1000;
frames = 0;
}
}
if (ticked) {
render();
frames++;
}
render();
frames++;
}
This code was not explained in the tutorial I found it in. Could someone please break this down and explain it?
I have also looked here for ideas, and the final piece of code on that page with a render thread and an update thread makes a lot of sense to me.
Which method should I use? One of the above, or something totally different? Also, you can probably tell that this is my first question here on stackoverflow.
Thanks in advance,
Josh

tick() is probably updating the game object's physical properties (position, velocity, etc.) tick() is called multiple times each update because some simulations can't handle too large a timestep without becoming unstable.
There's a popular article online which explains why this is the case, and why using a fixed timestep is the proper procedure. Check it out.
Each update the game is advanced in 1/60th second (so 60 frames a second) increments. This is repeated until there's less than 1/60th a second remaining in the aggregate. Aggregate is just a fancy word for sum.
Then a snapshot of the game's current state is rendered to the screen.
I won't get too deep into it, but really this code should be inerpolating each object's position by the remaining time in the aggregate during render().
long currenttime;
long previoustime = System.nanoTime();
long passedtime;
int frames = 0;
//this is an aggregate, games usually step in fixed units of time.
//this is usually because physics simulations can't handle too large of time steps.
double unprocessedseconds = 0;
double secondspertick = 1 / 60.0;
int tickcount = 0;
boolean ticked = false;
while (gameIsRunning) {
//get elapsed nano seconds from the epoch (january 1st, 1970)
currenttime = System.nanoTime();
//take difference of current time in nanos and previous time in nanos
passedtime = currenttime - previoustime;
previoustime = currenttime;
//divide to get the elapsed time in seconds.
unprocessedseconds += passedtime / 1000000000.0;
while (unprocessedseconds > secondspertick) {
tick();
unprocessedseconds -= secondspertick;
ticked = true;
tickcount++;
System.out.println(tickcount);
if (tickcount % 60 == 0) {
System.out.println(frames + " FPS");
previoustime += 1000;
frames = 0;
}
}
if (ticked) {
render();
frames++;
}
render();
frames++;
}
Good luck Josh.
Edit:
I have no experience with games using one thread for updates, and one for rendering. I can't give advice on those for that reason. If you have little or no experience with multithreading I'd avoid it as only complex games are likely to require this approach, and multithreading will add a multitude of issues you probably don't want to deal with.
Multithreaded game engines will consume more memory between rendering and updating than a single threaded game, or will wind up being depend on one another anyway. This is because the two threads can't manipulate the same data simultaneously. Therefor the only way for the two threads to operate is with synchronization on those data structures, or by the update thread suppling the render thread with immutable data to render.
Writing a multithreaded game engine would be a good introduction to threading. It could teach you quite a lot. Depends on what you want to get out of this.
If you are making a 2D game I feel even more confident that you will not need one thread for updating and one for rendering.
If you really want to pursue this, here's the approach I'd take.
You don't need more than a while loop to control rendering.

The way I do my engines is just as explained before, I multi-thread. Basically, if you split the job of processing and drawing the game into two segments it becomes quicker at the expense of more resources in use. I do a little something like this:
public class Engine implements Runnable {
//Sets classes
Engine tick = new Engine(true);
Engine render = new Engine(false);
Thread tickThread = new Thread(tick);
Thread renderThread = new Thread(render);
boolean job;
boolean isRunning = false;
long sleepTime = 5L;
public Engine(boolean job) {
//Sets what the thread does
this.job = job;
}
public static void startEngine() {
//Starts Engine
isRunning = true;
tickThread.start();
renderThread.start();
}
public void tick() {
//Process things
}
public void render() {
//Draw things
}
public void run() {
//Do engine base things
while(isRunning) {
if(job) {
tick();
} else {
render();
}
Thread.sleep(sleepTime);
}
}
}
This is by no means advanced. This is just an example of what a simple multi-thread game engine would be like. Honestly, I used this exact code when I was starting off making games. This could be used but some adjustments should be made depending on what you use it for. What I mean is that lets say you have an object that's moving and its being rendered at the same time. If the objects position is 50 and increasing and the render method is drawing it then the object could go to 51 then 52 before being rendered again. Normally, the processing is faster than the drawing. Another example: Lets say you have an ArrayList and are constantly removing and adding objects to it. Sometimes you can remove an object just as the render method is about to draw it and cause a null pointer exception because it's trying to draw something that doesn't exist. (I used "if(object.get(i) != null)" and worked around it that way)
I hope this helped at least a little (two years later, lol) and helped you get a basis of what multi-threading is like (if you didn't already).

Why is my game lagging?

I'm making a basic 2D game, yet I'm having problems with lag. The character on the game moves fine, yet sometimes slows right down, mainly when near an enemy. The enemy is not causing the lag though, as I've had 50 of them on the screen at once and it runs fine.
I've done the timing almost identically to how it's done in the Killer Game Programming in Java book, yet it doesn't seem to work too well. The FPS also seems to change a lot, which it shouldn't, because it should always take 15ms to complete an update-render-paint-sleep cycle, can anyone explain this?
Also, I think it lags more when my computer is doing things in the background (antivirus scanning, etc.), although I can run the game at about 550FPS while it's running other programs, so I don't think it's my computer.
Here's how I do the timing:
int waitTime = 1000 / 65; //65 being the desired FPS
while(running){
if(!loading){
gameUpdate();
gameRender();
gamePaint();
afterTime = System.nanoTime();
timeDiff = afterTime - beforeTime;
sleepTime = waitTime - (timeDiff / 1000000L);
if(sleepTime > 0){
try{
mainThread.sleep(sleepTime);
} catch(InterruptedException ie){}
}
else{
sleepSkipped++;
if(sleepSkipped == maxSleepSkipped){
Thread.yield();
sleepSkipped = 0;
}
}
fpsTime += (System.nanoTime() - beforeTime) / 1000000;
fps++;
if(fpsTime > 1000){
System.out.println("FPS: " + fps);
fps = 0;
fpsTime = 0;
}
beforeTime = System.nanoTime();
}
}

I think your problem is similar to one very recently Weird behavior in Thread.sleep()
I see that in your loop you add the sleeptime to your timer, meaning it keeps increasing (lowering fps) until it becomes so large that substracting the waittime from it gives a negative number, making your loop go into the one that yields the thread where it adds to the skipcount and yields when the count is high enough.

As arynaq suggested, it was a problem with the enemy collision detection. Every cycle through the loop made each enemy call quite a few methods, however I have now removed and relocated many of the method calls, which has completely fixed the lag problem. Thank you, arynaq.