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Why not use a scheduledExcecutor for a game loop

I'm currently reading about game development, every time I see a game loop implemented it is always the same way.
A while(true) block, with a sleep inside for the FPS.
And I'm asking myself, why shouldn't I use a scheduledExcecutor, it seems like the obvious choice?
I mean, I'm sure I'm wrong here, it's unlikely that I'm the first one to think of this, but WHY not ?

First of all, look at some of the evolution of the traditional game loop. Your while(true) + sleep example is the simplest one.
Then people noticed that games were affected by the speed of the computer they were running on. Look up "variable time step game loop" on Google, for examples of how this is dealt with.
Now read over the docs on the ScheduledExecutorService. Neither scheduleAtFixedRate nor scheduleWithFixedDelay are quite what you'd want for a game.
Also, scrolling down in the docs, you'll notice "If any execution of the task encounters an exception, subsequent executions are suppressed." This can cause your game loop to abruptly terminate if you don't handle every possible exception.
There's nothing to stop you from extending the ScheduledExecutorService and modifying it to suit your needs, but I wouldn't recommend it as-is.

The main reason is probably simply that most of the code you are reading was coded before (or following patterns that were coded before) the concurrent package came out/was in heavy use.
A secondary might be trust. When you are coding something that critical to your app you want to be sure exactly how it behaves. The while(true) loop makes it very clear exactly what's going on... You'd have to spend some time dealing with the various executors before you became truly comfortable with one.
There should be a solution that solves your problem, I'm sure you can find something that fires regularly, and has an adjustment in case one iteration takes a little longer than expected.
You may even want to build your own executor. The pattern itself is fine, it's just finding a way to make it work for you.

Java make wrong optimization

I have this code
while(!decks.get(0).isEmpty()){
SingleCardWindow obj = new SingleCardWindow(decks.get(0).take());
while(obj.isVisible()){
}
System.out.println("Closed");
}
SingleCardWindow it is inherit-class from JFrame. This code displays all cards from the deck. Displays one card, wait until I close this window, and displays next card. In Windows it works well.
In linux (java-oracle-7) "Closed" never writing!
But if I make
while(!decks.get(0).isEmpty()){
SingleCardWindow obj = new SingleCardWindow(decks.get(0).take());
while(obj.isVisible()){
System.out.println("SOMETHING");
}
System.out.println("Closed");
}
program works right. So, i think that compiler optimize "while(obj.isVisible())" like "while(true)". What I should do with this? I dont needs any code into the loop.

In the second case you added this line:
System.out.println("SOMETHING");
PrintStream#println is a synchronized method, so if that helped your program to start working, it is a quite clear indication that the method obj.isVisible is not synchronized, and since you don't use any explicit synchronization, you are effectively causing a data race in your program. The reading thread never observes the change in the value of the isVisible property.
But, as many others have already noticed, this just answers the question as you have asked it; the code you have posted has many more issues with an inappropriate programming model (busy waiting) used to solve an essentially event-based problem.

how to find the place where a Thread was started from a breakpoint

This is my situation:
I am studying a large codebase, running on Java1.7, not very easy to move around, lots of interfaces, deep inheritance trees, lots of threads etc.
I put a breakpoint in some place, but this object is running in a Thread that was spawned somewhere. I need to find that place.
there are too many .run() and .start() hits to look for individually (and to narrow down by the class is difficult too as there are many classes/inheritance (and I don't know the codebase yet)).
So my questions is, is there a way, having a Thread stopped in a breakpoint (intelliJ, but I can use eclipse too) to find out where it was started??
thanks

Maybe you can put breakpoint into Thread.start().
To avoid mutltiple invocation of breakpoint, maybe it make sense to place breakpoint with conditional logic, for example checking global boolean flag. For example, you suspect, that your code invokes right before some event, when event happens, put global flag to true.

No.
I don't think there's a way out of this without some brute force effort.
I would trace back to the Runnable that was started (through the stack trace), then get that class' inheritance and interface hierarchy, then look for run() and start() methods on all those classes. Unless someone has just gone nuts with inheritance, it shouldn't take that long.

Breaking on the code in your object tells you which thread it is and its call stack can tell you which Runnable you should be looking for. I'm assuming you've already gotten this far and that it's not enough to find all the references to this Thread/Runnable. In that case you can write a wrapper class for java.lang.Thread that does an instanceof/type check in the run() and setting your breakpoint there

I think first you have to get the Runnable that is run. That's simple as it's always the first line of your stack trace. (Of course you need the concrete class and not the one that defines the run method.) Once you have the class it should be easy to find the instantiation. Then it should simple to follow to the point where the thread is started. Did I miss something?

Explain the timing causing HashMap.put() to execute an infinite loop

As a number of people have noted and encountered HashMap.put can go into an infinite execution loop when used concurrently (see GRIZZLY-1207, JGRP-525, possibly HHH-6414, and this SO answer).
HashMap is clearly documented as not thread safe. Obviously, the correct fix is to use a thread-safe implementation of Map, ConncurrentHashMap in particular. I'm more curious about the concurrent timing that causes the infinite loop. I encountered this loop recently with a Java 7 JRE and would like to understand the exact causes. For example, is this caused by multiple puts at the same time?
A look inside HashMap.put shows that HashMap.Entry contains a link to the next node (in the bucket?). I assume these links are getting corrupting to contain circular references, which is causing the infinite loop. However, I still don't understand exactly how that corruption is occurring.

To the contrary of what many people think, the main issue with multi-threading and HashMaps is not just a duplicate entry or a vanishing one... As you said, an infinite loop might occur when two or multiple Threads concurrently decide to resize the HashMap.
If the size of the HashMap passes a given threshold, several threads might end up trying to resize it at the same time, and if we are lucky enough (you deployed the code in production already) they will keep going forever...
The issue is caused by the way the void resize(int newCapacity); and void transfer(Entry[] newTable); are implemented, you can take a look at the openjdk source code yourself. A mix of bad luck, good timing, entries that get reversed (ordering is not required in this data structure) and that end up to mistakenly referring to each other while a thread keep going while(e != null)...
While I could try to give you an explanation myself, I want to give credit to Paul Tyma's post (I cannot do better than him anyway) where I learned how this worked the first time I decided to figure out why I wasn't hired for a job several months ago...
http://mailinator.blogspot.com/2009/06/beautiful-race-condition.html
As Paul says, the best word to describe this race is condition is: beautiful

Weird Java problem, while loop termination

I have a piece of code that looks like this:
Algorithm a = null;
while(a == null)
{
a = grid.getAlgorithm();
}
getAlgorithm() in my Grid class returns some subtype of Algorithm depending on what the user chooses from some options.
My problem is that even after an algorithm is selected, the loop never terminates. However, that's not the tricky bit, if I simply place a System.out.println("Got here"); after my call to getAlgorithm(), the program runs perfectly fine and the loop terminates as intended.
My question is: why does adding that magic print statement suddenly make the loop terminate?
Moreover, this issue first came up when I started using my new laptop, I doubt that's related, but I figured it would be worth mentioning.
Edit: The program in question is NOT multithreaded. The code for getAlgorithm() is:
public Algorithm getAlgorithm ()
{
return algorithm;
}
Where algorithm is initially null, but will change value upon some user input.

I believe the issue has to deal with how grid.getAlgorithm is executed. If there is very little cost associated with executing the method, then your while loop will cycle very quickly as long the method continues to return null. That is often referred to as a busy wait.
Now it sounds like your new laptop is encountering a starvation issue which didn't manifest on your old computer. It is hard to say why but if you look at the link I included above, the Wikipedia article does indicate that busy waits do have unpredictable behavior. Maybe your old computer handles user IO better than your new laptop. Regardless, on your new laptop, that loop is taking resources away from whatever is handling your user IO hence it is starving the process that is responsible for breaking the loop.

You are doing active polling. This is a bad practice. You should at least let the polling thread sleep (with Thread.sleep). Since println does some io, it probably does just that. If your app is not multithreaded it is unlikely to work at all.

If this loop is to wait for user input in a GUI then ouch. Bad, bad idea and even with Thread.sleep() added I'd never recommend it. Instead, you most likely want to register an event listener on the component in question, and only have the validation code fire off when the contents change.
It's more than likely you're program is locking up because you've reached some form of deadlock more than anything else, especially if your application is multithreaded. Rather than try to solve this issue and hack your way round it, I'd seriously consider redesigning how this part of the application works.

You should check getAlgorithm(), there must be something wrong in the method.

There are two scenarios:
Your code is really not meant to be multi-threaded. In this case you need to insert some sort of user input in the loop. Otherwise you might as well leave it as Algorithm a = grid.getAlgorithm(); and prevent the infinite loop.
Your code is multi-threaded in which case you have some sort of 'visibility' problem. Go to Atomicity, Visibility and Ordering or read Java Concurrency in Practice to learn more about visibility. Essentially it means that without some sort of synchronization between threads, the thread you are looping in may never find out that the value has changed due to optimizations the JVM may perform.
You did not mention any context around how this code is run. If it is a console based application and you started from a 'main' function, you would know if there was multi-threading. I am assuming this is not the case since you say there is no multithreading. Another option would be that this is a swing application in which case you should read Multithreaded Swing Applications. It might be a web application in which case a similar case to swing might apply.
In any case you could always debug the application to see which thread is writing to the 'algorithm' variable, then see which thread is reading from it.
I hope this is helpful. In any case, you may find more help if you give a little more context in your question. Especially for a question with such an intriguing title as 'Weird Java problem, while loop termination'.