handle duplicate key in Collectors.toMap() function

I am creating a map which its (key,value) will be (name, address) in my Person object:

Map<String, String> myMap = persons.stream.collect(Collector.toMap(person.getName(), person.getAddress(), (address1, address2) -> address1));

In the duplicate key situation, I would like to skip to add the second address to the map and would like to log the name also. Skipping the duplicate address I can do already using mergeFunction, but in oder to log the name I need in this mergeFunction the person object, something like:

(address1, address2) -> {
                           System.out.println("duplicate "+person.name() + " is found!");
                           return address1;
                        }

I am getting stuck by passing person object to this merge function.

Answer

if you want to access the whole person object in the merge function then pass Function.identity() for the valueMapper:

Map<String, Person> myMap = 
        persons.stream()
               .collect(toMap(p -> p.getName(), 
                      Function.identity(), // or p -> p
                     (p1, p2) -> { /* do logic */ }));

But as you can see the resulting map values are Person objects, if you still want a Map<String, String> as a result and still access the whole Person object in the mergeFunction then you can do the following:

 persons.stream()
         .collect(toMap(p -> p.getName(), Function.identity(),(p1, p2) -> { /* do logic */ }))
         .entrySet()
         .stream()
         .collect(toMap(Map.Entry::getKey, p -> p.getValue().getAddress()));