Ignore duplicates when producing map using streams

Question

Map<String, String> phoneBook = people.stream()
                                      .collect(toMap(Person::getName,
                                                     Person::getAddress));

I get java.lang.IllegalStateException: Duplicate key when a duplicated element is found.

Is it possible to ignore such exception on adding values to the map?

When there is duplicate it simply should continue by ignoring that duplicate key.

Answer 1

This is possible using the mergeFunction parameter of Collectors.toMap(keyMapper, valueMapper, mergeFunction):

Map<String, String> phoneBook = 
    people.stream()
          .collect(Collectors.toMap(
             Person::getName,
             Person::getAddress,
             (address1, address2) -> {
                 System.out.println("duplicate key found!");
                 return address1;
             }
          ));

mergeFunction is a function that operates on two values associated with the same key. adress1 corresponds to the first address that was encountered when collecting elements and adress2 corresponds to the second address encountered: this lambda just tells to keep the first address and ignores the second.