How do I use the new computeIfAbsent function?

Question

I very much want to use Map.computeIfAbsent but it has been too long since lambdas in undergrad.

Almost directly from the docs: it gives an example of the old way to do things:

Map<String, Boolean> whoLetDogsOut = new ConcurrentHashMap<>();
String key = "snoop";
if (whoLetDogsOut.get(key) == null) {
  Boolean isLetOut = tryToLetOut(key);
  if (isLetOut != null)
    map.putIfAbsent(key, isLetOut);
}

And the new way:

map.computeIfAbsent(key, k -> new Value(f(k)));

But in their example, I think I'm not quite "getting it." How would I transform the code to use the new lambda way of expressing this?

Answer 1

Recently I was playing with this method too. I wrote a memoized algorithm to calcualte Fibonacci numbers which could serve as another illustration on how to use the method.

We can start by defining a map and putting the values in it for the base cases, namely, fibonnaci(0) and fibonacci(1):

private static Map<Integer,Long> memo = new HashMap<>();
static {
   memo.put(0,0L); //fibonacci(0)
   memo.put(1,1L); //fibonacci(1)
}

And for the inductive step all we have to do is redefine our Fibonacci function as follows:

public static long fibonacci(int x) {
   return memo.computeIfAbsent(x, n -> fibonacci(n-2) + fibonacci(n-1));
}

As you can see, the method computeIfAbsent will use the provided lambda expression to calculate the Fibonacci number when the number is not present in the map. This represents a significant improvement over the traditional, tree recursive algorithm.