I wrote a program for calculating fibonacci numbers recursively, with a ConcurrentHashMap and computeIfAbsent() method:
Program works absolutely fine when i used small values like 8,9,10 but stuck in endless loop when value increased from 10 to 20 program never halts
public class Test {
static Map<Integer, Integer> concurrentMap = new ConcurrentHashMap<>();
public static void main(String[] args) {
System.out.println("Fibonacci result for 20 is" + fibonacci(20));
}
static int fibonacci(int i) {
if (i == 0)
return i;
if (i == 1)
return 1;
return concurrentMap.computeIfAbsent(i, (key) -> {
System.out.println("Value is " + key);
return fibonacci(i - 2) + fibonacci(i - 1);
});
}
}
Can some one tell me why it is being stuck forever?
You are hitting a deadlock.
computeIfAbsent on a ConcurrentHashMap will lock the bucket in which the corresponding key will go to. If you are attempting to calculate a Fibonacci that is higher than the number of buckets in your map, then the recursive calls will attempt to lock a bucket that is already locked further up the call stack. But of course, that lock cannot be released until all of the recursive calls have completed. Thus, your deadlock.
I would reconsider your decision to use a ConcurrentHashMap here.