Server problems with a counter

Question

My clients are given jobs by a server thread, the clients create a thread to work on the problem, once the client is finished it makes a call to the giveResult(String result,int resultIndex) method. This method will put the results(there are 100 hundred of them) into an array that size at the given index (that was passed into the method), it will then increment (resultIndex++). This works fine when it`s getting a reasonable amount of requests, but if i test it with request every .001secs it sometimes forgets to increment the counter. This causes it to hang (while resultIndex <100). This i would have put down to synchronisation but when i tried to implement it with a vector array and made the break condition (array.size() < 100),
it works fine, and it is putting all the client reults in the right place, it just seems that sometimes it forgets to increment...any ideas?
a simplified version.....
void giveResult(String result, int ResultIndex)
boolean writing = True;
resultArray[resultIndex] = result;
System.out.println("successfully outputted: "+result+" at array index:"+resultIndex);
resultIndex=resultIndex+1;
writing = false;

Answer 1

You probably have a synchronization problem in incrementing the index. My best guess is that
process a grabs the index value - say 0.
process b grabs the index value - also zero
process a completes the increment - 1
process b completes the increment - 1 // No fun - should be 2!
Synchronize the whole upfdate method.