Functional Java: Hate the player not the game

We've all heard it over and over, Java 8 is not really functional programming. I admit that features like tail call optimization, closure over the context, the hideous boxed() method call when mapping a primitive type to a boxed type might make functional programming in java a little verbose, and maybe even bitter. But that doesn't prevent Java 8 from providing a set of tools and constructs that produce very elegant solutions. In what follows, I have compiled a (very short) set of very elegant solutions that I've encountered, and I couldn't help but admire their beauty.

Recursive traversal of a tree: by Fahd Shariff

Assuming a tree made of a root node, where each node has associated children. Please admire this one liner:

public Stream <TreeNode<E>> stream() {
         return Stream.concat(Stream.of(this), children.stream().flatMap(TreeNode::stream));
} 

Count the occurrence of specific words in a larger passage: by Ken Kousen

public Map<String,Integer> countWords(String passage, String... strings) {
            Map<String, Integer> wordCounts = new HashMap<>();
            Arrays.stream(strings).forEach(s -> wordCounts.put(s, 0));
            Arrays.stream(passage.split(" ")).forEach(
                word -> wordCounts.computeIfPresent(word, (key, val) -> val + 1));
            return wordCounts;
}

Fibonacci Sequence with memoization: by Ken Kousen

private Map<Long, BigInteger> cache = new HashMap<>();
        public BigInteger fib(long i) {
        if (i == 0) return BigInteger.ZERO;
        if (i == 1) return BigInteger.ONE;
       return cache.computeIfAbsent(i, n -> fib(n - 2).add(fib(n - 1)));
}