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?
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.