An implementation of Fibonacci

Fibonacci.java

private BigInteger f(int n, Map<Integer, BigInteger> cache) {
    BigInteger result = cache.putIfAbsent(n, RESERVED);
    if (result == null) {
        int half = (n + 1) / 2;

        CompletableFuture<BigInteger> completableFuture = new CompletableFuture<>();

        CompletableFuture<BigInteger> futureResult = CompletableFuture.supplyAsync(() -> f(half - 1, cache));

        futureResult.thenAcceptBothAsync(CompletableFuture.supplyAsync(
            () -> f(half, cache)),
            (s1, s2) -> {
                BigInteger result1 = null;
                if (n % 2 == 1) {
                    result1 = s1.multiply(s1).add(s2.multiply(s2));
                } else {
                    result1 = s1.shiftLeft(1).add(s2).multiply(s2);
                }

                synchronized (RESERVED) {
                    cache.put(n, result1);
                    RESERVED.notifyAll();
                    completableFuture.complete(result1);
                }
            });

        try {
            return completableFuture.get();
        } catch (InterruptedException | ExecutionException e) {
            e.printStackTrace();
        }

    } else if (result == RESERVED) {
        try {
            ReservedBlocker blocker = new ReservedBlocker(n, cache);
            ForkJoinPool.managedBlock(blocker);
            result = blocker.result;
        } catch (InterruptedException e) {
            throw new CancellationException("interrupted");
        }
    }

    return result;
}