[java] How do I use the new computeIfAbsent function?

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.

multi-map

This is really helpful if you want to create a multimap without resorting to the Google Guava library for its implementation of MultiMap.

For example, suppose you want to store a list of students who enrolled for a particular subject.

The normal solution for this using JDK library is:

Map<String,List<String>> studentListSubjectWise = new TreeMap<>();
List<String>lis = studentListSubjectWise.get("a");
if(lis == null) {
    lis = new ArrayList<>();
}
lis.add("John");

//continue....

Since it have some boilerplate code, people tend to use Guava Mutltimap.

Using Map.computeIfAbsent, we can write in a single line without guava Multimap as follows.

studentListSubjectWise.computeIfAbsent("a", (x -> new ArrayList<>())).add("John");

Stuart Marks & Brian Goetz did a good talk about this https://www.youtube.com/watch?v=9uTVXxJjuco

Another example. When building a complex map of maps, the computeIfAbsent() method is a replacement for map's get() method. Through chaining of computeIfAbsent() calls together, missing containers are constructed on-the-fly by provided lambda expressions:

  // Stores regional movie ratings
  Map<String, Map<Integer, Set<String>>> regionalMovieRatings = new TreeMap<>();

  // This will throw NullPointerException!
  regionalMovieRatings.get("New York").get(5).add("Boyhood");

  // This will work
  regionalMovieRatings
    .computeIfAbsent("New York", region -> new TreeMap<>())
    .computeIfAbsent(5, rating -> new TreeSet<>())
    .add("Boyhood");