Parsing map to get list of objects based on Key

Question

Is there an easy way to extract the list of objects from a map containing list of objects? For ex, below is an example map that I need to deal with.

map<Id, List<Contact>> accountWithContacts = new map<Id, List<Contact>>();

Assuming that this map is populated with an accountId and the contacts that belong to the accountId, how to extract the List from the map? I could not find any method in the map class that returns a list.

Can someone help?

Answer 1

There is no simple one-line method (as in JavaScript, where we can e.g. Array.prototype.flat), its most simple form would be:

List<Contact> allContacts = new List<Contact>();
for(List<Contact> contacts: accountWithContacts.values()) {
  allContacts.addAll(contacts);
}