Fibonacci - exception

[Martin Møller Skarbiniks Pedersen]

This code in chapter 5 is not correct:
public long fibonacci(int x) { return cache.computeIfAbsent(x, n -> fibonacci(n-1) + fibonacci(n-2)); }
because you are not allowed to change the cache when you are using computeIfAbsent()

Exception in thread "main" java.util.ConcurrentModificationException

[Richard Warburton]

See discussion on #27

[Martin Møller Skarbiniks Pedersen]

OK