Question

Why does java.util.Map.values() allow you to delete entries from the returned Collection when it makes no sense to remove a key value pair based on the value? The code which does this would have no idea what key the value(and hence a key) being removed is mapped from. Especially when there are duplicate values, calling remove on that Collection would result in an unexpected key being removed.

Answer 1

it makes no sense to remove a key value pair based on the value

I don’t think you’re being imaginative enough. I’ll admit there probably isn’t wide use for it, but there will be valid cases where it would be useful.

As a sample use case, say you had a Map<Person, TelephoneNumber> called contactList. Now you want to filter your contact list by those that are local.

To accomplish this, you could make a copy of the map, localContacts = new HashMap<>(contactList) and remove all mappings where the TelephoneNumber starts with an area code other than your local area code. This would be a valid time where you want to iterate through the values collection and remove some of the values:

Map<Person, TelephoneNumber> contactList = getContactList();
Map<Person, TelephoneNumber> localContacts = new HashMap<Person, TelephoneNumber>(contactList);

for ( Iterator<TelephoneNumber> valuesIt = localContacts.values().iterator(); valuesIt.hasNext(); ){
    TelephoneNumber number = valuesIt.next();
    if ( !number.getAreaCode().equals(myAreaCode) ) {
        valuesIt.remove();
    }
}

Especially when there are duplicate values, calling remove on that Collection would result in an unexpected key being removed.

What if you wanted to remove all mappings with that value?