Using Collectors.toMap with map supplier and mergeFunction

example_using_collectors_toMap.java

import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.TreeMap;
import java.util.stream.Collectors;

class Scratch {

  public static void main(String[] args) {
    final TreeMap<Object, Object> results = List.of(
        Map.entry("one", 1),
        Map.entry("one", "I"),
        Map.entry("one", "first"),
        Map.entry("two", 2)
    ).stream()
        .collect(Collectors.toMap(
            Entry::getKey,
            Entry::getValue,
            (o, o2) -> {
              System.out.printf("mergeFunction called with %s, %s%n", o, o2);
              return o;
            },
            TreeMap::new
        ));

    System.out.println(results);
  }
}

Output:

mergeFunction called with 1, I
mergeFunction called with 1, first
{one=1, two=2}