Question

There is an ExecutorService that has 3 threads that each one add Pair<String,Integer> objects to a TreeSet<Pair<String,Integer>>. The addToSet() method is declared synchronized and everything works fine. But I must implement a new thread that execute a scheduled task that has to access this Set and print all the values. The problem is that sometimes the scheduled thread crashes(the other threads work fine). I think that it crashes because the Set is being modified by the other 3 threads during the for loop of the printer(scheduled thread).

Answer 1

You need mutual exclusion between the addToSet method and the method which prints all values. One way to do that:

Object setLock = new Object(); // Put this in a scope where all threads can access it

void addToSet( T element ){
    synchronized(setLock) {
       //add it
   }
}

void printAllValues(){
   synchronized(setLock) {
       //print the values
   }
}