ConcurrentHashMap and Fibonacci Numbers - Inconsistent result
concurrenthashmap, fibonacci, java
Solution
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.
Problem
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?