handle duplicate key in Collectors.toMap() function

user_9563846 0
Viewed 14817

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.

java-stream java-8 hashmap java
4 Answers

@Aomine: solution looks good and works for me too. Just wanted to confirm that with this it iterates twice right ?? Cause with simple solution like below it iterates only once but achieve what is required.

Map<String, String> myMap = new HashMap<>();
persons.forEach(item -> {
    if(myMap.containsKey(item.getName()))
        {/*do something*/}
    else 
        myMap.put(item.getName(), item.getAddress());
});
user_108771410

I believe the forEach approach along with Map.merge would be much simpler and appropriate for the current use case :

Map<String, String> myMap = new HashMap<>();
persons.forEach(person -> myMap.merge(person.getName(), person.getAddress(), (adrs1, adrs2) -> {
    System.out.println("duplicate " + person.getName() + " is found!");
    return adrs1;
}));

Note: Map.merge also uses BiFunction (parent of BinaryOperator as used in toMap), hence you could correlate the merge function here to your existing desired functionality easily.

user_17461180

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()));
user_58620710

Here is another possibility using peek

import java.util.*;
import java.util.stream.*;

class App {
    public static void main(String[] args) {
        List<Person> persons = Arrays.asList(new Person("foo", "bar"), new Person("baz", "qux"), new Person("foo", "zuz"));

        Set<String> names = new HashSet<>();
        Map<String, String> nameAddress = persons.stream().peek(p -> {
            if (names.contains(p.getName())) {
                System.out.println("Duplicate key being skipped: " + p);
            } else {
                names.add(p.getName());
            }
        }).collect(Collectors.toMap(person -> person.getName(), person -> person.getAddress(), (addr1, addr2) -> addr1));

    }
}

class Person {
    String name;
    String address;

    public Person(String name, String address) {
        this.name = name;
        this.address = address;
    }

    public String getName() {
        return name;
    }

    public String getAddress() {
        return address;
    }

    @Override
    public String toString() {
        return name + " " + address;
    }
}

Output for me will be as follows for the snippet above:

Duplicate key being skipped: foo zuz
user_11731120
Related