FULL PRODUCT VERSION :
java version "1.8.0_131"
Java(TM) SE Runtime Environment (build 1.8.0_131-b11)
Java HotSpot(TM) 64-Bit Server VM (build 25.131-b11, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
MINGW64_NT-10.0 DESKTOP-M49HTNK 2.5.0(0.295/5/3) 2016-03-31 18:47 x86_64 Msys

A DESCRIPTION OF THE PROBLEM :
As per the documentation, "ConcurrentHashMap()" creates a new, empty map with the default initial table size (16).
However, I believe that the default size of it is much smaller.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
http://stackoverflow.com/questions/43774947/concurrenthashmap-and-fibonacci-numbers-inconsistent-result


REPRODUCIBILITY :
This bug can be reproduced rarely.

---------- BEGIN SOURCE ----------
static Map<Integer, Integer> concurrentMap = new ConcurrentHashMap<>();

public static void main(String[] args) {
int num = 16;
System.out.println("Fibonacci result for " + num + " is " + fibonacci(num));
}

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);
});
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
static Map<Integer, Integer> concurrentMap = new ConcurrentHashMap<>(16);

SUPPORT :
YES