"map.getOrDefault" equivalent in Dart - flutter

Question

I'm working on a friend suggestion algorithm for a flutter social media application. I'm still an amateur when it comes to Dart so I'm stuck on the following line of code:
class FriendSuggestionAlgorithm {
User friendSuggestion(User user) {
int max = -1;
User suggestion;
Map<User, int> map = new HashMap();
for (User friend in user.friends) {
for (User mutualFriend in friend.friends) {
if (mutualFriend.id != user.id && !user.friends.contains(mutualFriend)) {
map.putIfAbsent(mutualFriend, map.getOrDefault(mutual, 0) + 1);
}
}
}
for (MapEntry<User, int> mutualFriend in map.entries) {
if (mutualFriend.value > max) {
max = mutualFriend.value;
suggestion = mutualFriend.key;
}
}
return suggestion;
}
}
map.getOrDefault is underlined (I know the method doesn't exist in Dart). Do you know what the equivalent is in Dart? (PS, I'm just translating Java code into Dart.
Any help is appreciated!

Answer 1

Your code doesn't make sense. map.putIfAbsent will do work only if the key doesn't exist, so the hypothetical map.getOrDefault call with the same key would always return the default value anyway. That is, your logic would be the equivalent of map.putIfAbsent(mutual, () => 1), where nothing happens if the key already exists.
Map.putifAbsent takes a callback as its argument to avoid evaluating it unless it's actually necessary. I personally prefer using ??= when the Map values are non-nullable.
I presume that you actually want to increment the existing value, if one exists. If so, I'd replace the map.putIfAbsent(...) call with:
map[mutual] = (map[mutual] ?? 0) + 1;
Also see: Does Dart have something like defaultdict in Python?

Answer 2

You could do it like this:
map.putIfAbsent(mutual, (map.containsKey(mutual) ? map[mutual] : 0) + 1)
Maybe take a look at this for more info: https://dart.dev/guides/language/language-tour#conditional-expressions
Edit:
This code should work
class FriendSuggestionAlgorithm {
User? friendSuggestion(User user) {
int max = -1;
User? suggestion;
Map<User, int> map = {};
for (User friend in user.friends) {
for (User mutualFriend in friend.friends) {
if (mutualFriend.id != user.id && !user.friends.contains(mutualFriend)) {
map.putIfAbsent(mutualFriend, () => (map[mutualFriend] ?? 0) + 1);
}
}
}
for (MapEntry<User, int> mutualFriend in map.entries) {
if (mutualFriend.value > max) {
max = mutualFriend.value;
suggestion = mutualFriend.key;
}
}
return suggestion;
}
}
Note that suggestion nullable because it could happen that suggestion is never assigned. And therefore friendSuggestion(user) can return null;
To come back to your question
the correct code is
map.putIfAbsent(mutualFriend, () => (map[mutualFriend] ?? 0) + 1);
My mistake on my original answer, the ifAbsent part of this is a function. In the function the value of mutualFriend is retrieved. If that is null use 0.