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parellel search with communications between threads

Suppose I have an int array, an element num and 4 threads.
I'm giving each thread 1/4 of the array to search for num.
(The search method is given below)
public static boolean contains(int[] array, int minIdx, int maxIdx, int num) { ...}
At my "top level", I can schedule 4 threads to search 4 quarters of the array, but how do I ensure ALL the threads stop searching as soon as one of them finds the element (assuming there is NO duplicate in the array, hence the element can appear at most once).
P.S: You see, suppose my 4th thread found the element at the first iteration, I want the top-level method to return immediately as opposed to wait for other 3 guys to complete.

You need explicit signaling. You might consider the built-in interruption mechanism, or you may roll your own because it's very simple.
One idea: share an AtomicBoolean among all the threads and let each one periodically check it. When a thread finds the answer, it flips the boolean. The best option to achieve periodic checking is a nested loop:
for (int i = start; i < end && !done.get();) {
for (int batchLimit = Math.min(i + BATCH_SIZE, end); i < batchLimit; i++) {
// your logic
}
}
This is the easiest for the JIT compiler to optimize.
The price of checking the value is very low whenever the value didn't change. It will be in the L3 cache. The case when the value did change is irrelevant because at that point you're done.

Use a flag to signal when you found the answer and share it between threads. AtomicBoolean is a good option.
Add the boolean to your loop end conditions for example
for (int i = minIdxs ; i < maxIdxs && found.get() == false; ++i){...}
Also share a CountDownLatch of size 4 and countDown() when you are returning from each thread.
Have your main thread await() and it'll mean all threads gracefully finish before you move on in your main thread.

You can write a class who will act like a controller. this class will know each thread and every thread knows the controller. (its like an observer pattern)
If one thread finds the answer, the thread can tell it to the controller which can inform the other threads to stop.
class ControllerOfAllTheThreads{
ArrayList<TheClassesWhichDoTheSearch> list = new ArrayList<TheClassesWhichDoTheSearch>();
public void tellThemWeFoundHim(){
for (TheClassesWhichDoTheSearch theThreads : list) {
if(theThreads.isAlive() && !theThreads.isInterrupted())
theThreads.interrupt();
}
}
}

Why is i++ not atomic?

Why is i++ not atomic in Java?
To get a bit deeper in Java I tried to count how often the loop in threads are executed.
So I used a
private static int total = 0;
in the main class.
I have two threads.
Thread 1: Prints System.out.println("Hello from Thread 1!");
Thread 2: Prints System.out.println("Hello from Thread 2!");
And I count the lines printed by thread 1 and thread 2. But the lines of thread 1 + lines of thread 2 don't match the total number of lines printed out.
Here is my code:
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.logging.Level;
import java.util.logging.Logger;
public class Test {
private static int total = 0;
private static int countT1 = 0;
private static int countT2 = 0;
private boolean run = true;
public Test() {
ExecutorService newCachedThreadPool = Executors.newCachedThreadPool();
newCachedThreadPool.execute(t1);
newCachedThreadPool.execute(t2);
try {
Thread.sleep(1000);
}
catch (InterruptedException ex) {
Logger.getLogger(Test.class.getName()).log(Level.SEVERE, null, ex);
}
run = false;
try {
Thread.sleep(1000);
}
catch (InterruptedException ex) {
Logger.getLogger(Test.class.getName()).log(Level.SEVERE, null, ex);
}
System.out.println((countT1 + countT2 + " == " + total));
}
private Runnable t1 = new Runnable() {
#Override
public void run() {
while (run) {
total++;
countT1++;
System.out.println("Hello #" + countT1 + " from Thread 2! Total hello: " + total);
}
}
};
private Runnable t2 = new Runnable() {
#Override
public void run() {
while (run) {
total++;
countT2++;
System.out.println("Hello #" + countT2 + " from Thread 2! Total hello: " + total);
}
}
};
public static void main(String[] args) {
new Test();
}
}

i++ is probably not atomic in Java because atomicity is a special requirement which is not present in the majority of the uses of i++. That requirement has a significant overhead: there is a large cost in making an increment operation atomic; it involves synchronization at both the software and hardware levels that need not be present in an ordinary increment.
You could make the argument that i++ should have been designed and documented as specifically performing an atomic increment, so that a non-atomic increment is performed using i = i + 1. However, this would break the "cultural compatibility" between Java, and C and C++. As well, it would take away a convenient notation which programmers familiar with C-like languages take for granted, giving it a special meaning that applies only in limited circumstances.
Basic C or C++ code like for (i = 0; i < LIMIT; i++) would translate into Java as for (i = 0; i < LIMIT; i = i + 1); because it would be inappropriate to use the atomic i++. What's worse, programmers coming from C or other C-like languages to Java would use i++ anyway, resulting in unnecessary use of atomic instructions.
Even at the machine instruction set level, an increment type operation is usually not atomic for performance reasons. In x86, a special instruction "lock prefix" must be used to make the inc instruction atomic: for the same reasons as above. If inc were always atomic, it would never be used when a non-atomic inc is required; programmers and compilers would generate code that loads, adds 1 and stores, because it would be way faster.
In some instruction set architectures, there is no atomic inc or perhaps no inc at all; to do an atomic inc on MIPS, you have to write a software loop which uses the ll and sc: load-linked, and store-conditional. Load-linked reads the word, and store-conditional stores the new value if the word has not changed, or else it fails (which is detected and causes a re-try).

i++ involves two operations :
read the current value of i
increment the value and assign it to i
When two threads perform i++ on the same variable at the same time, they may both get the same current value of i, and then increment and set it to i+1, so you'll get a single incrementation instead of two.
Example :
int i = 5;
Thread 1 : i++;
// reads value 5
Thread 2 : i++;
// reads value 5
Thread 1 : // increments i to 6
Thread 2 : // increments i to 6
// i == 6 instead of 7

Java specification
The important thing is the JLS (Java Language Specification) rather than how various implementations of the JVM may or may not have implemented a certain feature of the language.
The JLS defines the ++ postfix operator in clause 15.14.2 which says i.a. "the value 1 is added to the value of the variable and the sum is stored back into the variable". Nowhere does it mention or hint at multithreading or atomicity.
For multithreading or atomicity, the JLS provides volatile and synchronized. Additionally, there are the Atomic… classes.

Why is i++ not atomic in Java?
Let's break the increment operation into multiple statements:
Thread 1 & 2 :
Fetch value of total from memory
Add 1 to the value
Write back to the memory
If there is no synchronization then let's say Thread one has read the value 3 and incremented it to 4, but has not written it back. At this point, the context switch happens. Thread two reads the value 3, increments it and the context switch happens. Though both threads have incremented the total value, it will still be 4 - race condition.

i++ is a statement which simply involves 3 operations:
Read current value
Write new value
Store new value
These three operations are not meant to be executed in a single step or in other words i++ is not a compound operation. As a result all sorts of things can go wrong when more than one threads are involved in a single but non-compound operation.
Consider the following scenario:
Time 1:
Thread A fetches i
Thread B fetches i
Time 2:
Thread A overwrites i with a new value say -foo-
Thread B overwrites i with a new value say -bar-
Thread B stores -bar- in i
// At this time thread B seems to be more 'active'. Not only does it overwrite
// its local copy of i but also makes it in time to store -bar- back to
// 'main' memory (i)
Time 3:
Thread A attempts to store -foo- in memory effectively overwriting the -bar-
value (in i) which was just stored by thread B in Time 2.
Thread B has nothing to do here. Its work was done by Time 2. However it was
all for nothing as -bar- was eventually overwritten by another thread.
And there you have it. A race condition.
That's why i++ is not atomic. If it was, none of this would have happened and each fetch-update-store would happen atomically. That's exactly what AtomicInteger is for and in your case it would probably fit right in.
P.S.
An excellent book covering all of those issues and then some is this:
Java Concurrency in Practice

In the JVM, an increment involves a read and a write, so it's not atomic.

If the operation i++ would be atomic you wouldn't have the chance to read the value from it. This is exactly what you want to do using i++ (instead of using ++i).
For example look at the following code:
public static void main(final String[] args) {
int i = 0;
System.out.println(i++);
}
In this case we expect the output to be: 0
(because we post increment, e.g. first read, then update)
This is one of the reasons the operation can't be atomic, because you need to read the value (and do something with it) and then update the value.
The other important reason is that doing something atomically usually takes more time because of locking. It would be silly to have all the operations on primitives take a little bit longer for the rare cases when people want to have atomic operations. That is why they've added AtomicInteger and other atomic classes to the language.

There are two steps:
fetch i from memory
set i+1 to i
so it's not atomic operation.
When thread1 executes i++, and thread2 executes i++, the final value of i may be i+1.

In JVM or any VM, the i++ is equivalent to the following:
int temp = i; // 1. read
i = temp + 1; // 2. increment the value then 3. write it back
that is why i++ is non-atomic.

Concurrency (the Thread class and such) is an added feature in v1.0 of Java. i++ was added in the beta before that, and as such is it still more than likely in its (more or less) original implementation.
It is up to the programmer to synchronize variables. Check out Oracle's tutorial on this.
Edit: To clarify, i++ is a well defined procedure that predates Java, and as such the designers of Java decided to keep the original functionality of that procedure.
The ++ operator was defined in B (1969) which predates java and threading by just a tad.

Java code seems to only use two concurrent threads

I have approximately 40000 objects which might need to be repainted.
Most of them are not on the screen, so it seems that I could save a lot of work by doing the checks concurrently. But, my CPU never goes above 15% usage, so it seems that it is still only using one core. Have I implemented the threads correctly? If so, why aren't all my cores being used? And is there a better way which does utilize all my cores?
public void paintComponent(Graphics g)
{
super.paintComponent(g);
if (game.movables.size() > 10000)
{
final int size = game.drawables.size();
final Graphics gg = g;
Thread[] threads = new Thread[8];
for (int j = 0; j < 8; ++j)
{
final int n = j;
threads[j] = new Thread(new Runnable()
{
public void run()
{
Drawable drawMe;
int start = (size / 8) * n;
int end = (size / 8) * (n + 1);
if (n == 8) end = game.drawables.size(); // incase size
// % 8 != 0
for (int i = start; i < end; ++i)
{
drawMe = game.drawables.get(i);
if (drawMe.isOnScreen())
{
synchronized (gg)
{
drawMe.draw(gg);
}
}
}
}
});
threads[j].start();
}
try
{
for (int j = 0; j < 8; ++j)
threads[j].join();
}
catch (InterruptedException e)
{
// TODO Auto-generated catch block
e.printStackTrace();
}
}
else
{
for (Drawable drawMe : game.drawables)
{
if (drawMe.isOnScreen())
{
drawMe.draw(g);
}
}
}
}

As has been pointed out, the synchronized (gg) is effectively serializing all the drawing, so you're probably going slower than single-threaded code due to thread creation and other overhead.
The main reason I'm writing however is that Swing, which this presumably is, is not thread safe. So the behavior of this program is not only likely to be bad, it's undefined.
Threading errors like this turn up as screwy behavior on some machines with some java runtime parameters and some graphics drivers. Been there. Done that. Not good.
JOGL will give you direct access to the GPU, the surest way to speed rendering.

To do this right, you might start by putting each drawMe in a (properly synchronized) list, then actually draw them in a loop after the joins are done. You can't speed the drawing (though if you've knocked out 99% of the drawMe's you've cut down the time needed dramatically), but if isOnScreen() is somewhat complicated, you'll get some real work out of your cores.
A ConcurrentLinkedQueue would save you the need to synchronize adds to the list.
The next step might be to use a blocking queue instead of a list, so the paint code could run in parallel with the visibility checks. With eight checks running, they should keep well ahead of the drawing. (But I think all the blocking queues either need synchronizing or do synching themselves. I'd skip this and stick with the CLQ and the first solution. Simpler and possibly faster.)
And (as Gene pointed out), everything Swing related starts on the EventQueue. Keep it there or life will get strange. Only your own code, not referencing the UI, should run in your threads.

Since you're already not drawing any objects that are off-screen, you're probably gaining very very little by doing what you're doing above.
I would also go as far as to say you're making it worse, but introducing synchronize which is slow and also introducing threads that cause context switches, which are expensive.
To improve performace you should perhaps look into using different drawing libraries, such as the Java2D drawing library, which is part of the JDK: http://java.sun.com/products/java-media/2D/index.jsp

I'm not sure how java will handle this, but other languages will blow up horribly and die if you reference something across scopes like you're doing with final int n (since it goes out of scope when the loop stops). Consider making it a field of the runnable object. Also, you're synchronizing on the graphics object while you're doing all of the real work. It's likely that you aren't getting any real performance increase from this. You might benefit from explicitly checking if the object is on the screen in parallel which is a read only operation, adding on-screen objects to a set or collection of some other sort, and then rendering sequentially.

Is a static counter thread safe in multithreaded application?

public class counting
{
private static int counter = 0;
public void boolean counterCheck(){
counter++;
if(counter==10)
counter=0;
}
}
Method counterCheck can be accessed by multiple threads in my application. I know that static variables are not thread safe. I would appreciate if someone can help me with example or give me reason why I have to synchronize method or block. What will happen if I don't synchronize?

It's clearly not thread-safe. Consider two threads that run in perfect parallel. If the counter is 9, they'll each increment the counter, resulting in the counter being 11. Neither of them will then see that counter equal to 10, so the counter will keep incrementing from then on rather than wrapping as intended.

This is not thread safe, AND this pattern of updating a count from multiple threads is probably the #1 way to achieve negative scaling (it runs slower when you add more threads) of a multi-threaded application.
If you add the necessary locking to make this thread safe then every thread will come to a complete halt while counting. Even if you use atomic operations to update the counter, you will end up bouncing the CPU cache line between every thread that updates the counter.
Now, this is not a problem if each thread operation takes a considerable amount of time before updating the counter. But if each operation is quick, the counter updates will serialize the operations, causing everything to slow down on all the threads.

Biggest danger? Two increments to counter before the counter == 10 check, making the reset to 0 never happen.

It's NOT thread-safe, for multiple reasons. The most obvious one is that you could have two threads going from 9 to 11, as mentioned by other answers.
But since counter++ is not an atomic operation, you could also have two threads reading the same value and incrementing to the same value afterwards. (meaning that two calls in fact increment only by 1).
Or you could have one thread make several modifications, and the other always seeing 0 because due to the Java memory model the other thread might see a value cached in a register.
Good rule of thumb: each time some shared state is accessed by several threads, and one of them is susceptible to modify this shared state, all the accesses, even read-only accesses must be synchronized using the same lock.

Imagine counter is 9.
Thread 1 does this:
counter++; // counter = 10
Thread 2 does this:
counter++; // counter = 11
if(counter==10) // oops
Now, you might think you can fix this with:
if(counter >= 10) counter -= 10;
But now, what happens if both threads check the condition and find that it's true, then both threads decrement counter by 10 (now your counter is negative).
Or at an even lower level, counter++ is actually three operations:
Get counter
Add one to counter
Store counter
So:
Thread 1 gets counter
Thread 2 gets counter
Both threads add one to their counter
Both threads store their counter
In this situation, you wanted counter to be incremented twice, but it only gets incremented once. You could imagine it as if this code was being executed:
c1 = counter;
c2 = counter;
c1 = c1 + 1;
c2 = c2 + 1;
counter = c1; // Note that this has no effect since the next statement overrides it
counter = c2;
So, you could wrap it in a synchronized block, but using an AtomicInteger would be better if you only have a few threads:
public class counting {
private static AtomicInteger counter = new AtomicInteger(0);
public static void counterCheck() {
int value = counter.incrementAndGet();
// Note: This could loop for a very long time if there's a lot of threads
while(value >= 10 && !counter.compareAndSet(value, value - 10)) {
value = counter.get();
}
}
}

first of counter++ by itself is NOT threadsafe
hardware limitations make it equivalent to
int tmp = counter;
tmp=tmp+1;
counter=tmp;
and what happens when 2 threads are there at the same time? one update is lost that's what
you can make this thread safe with a atomicInteger and a CAS loop
private static AtomicInteger counter = new AtomicInteger(0);
public static boolean counterCheck(){
do{
int old = counter.get();
int tmp = old+1;
if(tmp==10)
tmp=0;
}
}while(!counter.compareAndSet(old,tmp));
}

Java atomic classes in compound operations

Will the following code cause race condition issue if several threads invoke the "incrementCount" method?
public class sample {
private AtomicInteger counter = new AtomicInteger(0);
public int getCurrentCount {
int current = counter.getAndIncrement();
if (counter.compareAndSet(8, 0)) current = 0;
return current;
}
}
If it causes race condition, what are the possible solution other than using synchronized keyword?

You probably don't want to let the counter exceed 8 and this won't work. There are race conditions.
It looks like you want a mod 8 counter. The easiest way is to leave the AtomicInteger alone and use something like
int current = counter.getAndIncrement() & 7;
(which is fixed and optimized version of % 8). For computations mod 8 or any other power of two it works perfectly, for other number you'd need % N and get problems with int overflowing to negative numbers.
The direct solution goes as follows
public int getCurrentCount {
while (true) {
int current = counter.get();
int next = (current+1) % 8;
if (counter.compareAndSet(current, next))) return next;
}
}
This is about how getAndIncrement() itself works, just slightly modified.

Yes, it probably does not do what you want (there is a kind of race condition).
One thread may call getAndIncrement() and receive a 8
A second thread may call getAndIncrement() and receive a 9
The first thread tries compareAndSet but the value is not 8
The second thread tries compareAndSet but the value is not 8
If there's no risk of overflowing, you could do something like
return counter.getAndIncrement() % 8;
Relying on that something does not overflow seems like a poor idea to me though, and I would probably do roughly what you do, but let the method be synchronized.
Related question: Modular increment with Java's Atomic classes

What are you trying to achieve? Even if you use the fixes proposed by ajoobe or maartinus you can end up with different threads getting the same answer - consider 20 threads running simultaneously. I don't see any interesting significance of this "counter" as you present it here - you may as well just pick a random number between 0 and 8.

Based on the code for getAndIncrement()
public int getCurrentCount() {
for(;;) {
int courrent = counter.get();
int next = current + 1;
if (next >= 8) next = 0;
if (counter.compareAndSet(current, next))
return current;
}
}
However a simpler implementation in your case is to do
public int getCurrentCount() {
return counter.getAndIncrement() & 0x7;
}

I assume that the what you want is to have a counter form 0 to 7.
If that is the case then a race condition can possibly happen and the value of counter can become 9.
Unless you are ok to use % soln. as said by others, you micht have to use synchronized.