Is there a more concise way to do this in Java 8 - Map manipulation

Question

A very common operation on maps of collections is to create a new collection with an initial value when the key is not present, or if the key is present, do some function on the

Answer 1

Here's another alternative:

Set set = m.computeIfAbsent (key , k -> new HashSet<> ());
set.add(set.size() + 1);

The only reason this is a two liner (instead of one) is the need to obtain the current size of the Set in order to decide which value to add to it.