How do you map unique values from a list of maps to a new list of maps in Java?

Question

I have a list of maps

List<Map<String, Object>> myList = [{value1=3, value2=11},{value1=8, value2=16},{value1=3, value2=11},{value1=10, value2=11} ...]

and I want to extract value2 to a new list of maps but have only unique values there:

List<Map<String, Object>> res = [{value2=11},{value2=16} ...]

What id the best way to do it?

Answer 1

Try this. Here first I am creating a list of Entry<value2, anyUniqueValue> by using 2 functions. First filtering only those Entries which has key value2 by using function filterFunction

Secondly by filtering unique value by using function distinctByValue

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.concurrent.ConcurrentHashMap;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.stream.Collectors;

public class Demo{

public static void main(String[] args) {
    List<Map<String, Object>> myList = new ArrayList<>();
    Map<String, Object> map1 = new HashMap<>();
    map1.put("value1", 3);
    map1.put("value2", 11);

    Map<String, Object> map2 = new HashMap<>();
    map2.put("value1", 8);
    map2.put("value2", 16);

    Map<String, Object> map3 = new HashMap<>();
    map3.put("value1", 3);
    map3.put("value2", 11);

    Map<String, Object> map4 = new HashMap<>();
    map4.put("value1", 10);
    map4.put("value2", 11);

    myList.add(map1);
    myList.add(map2);
    myList.add(map3);
    myList.add(map4);

    List<Entry<String, Object>> list = myList.stream().flatMap(map -> map.entrySet().stream())
            .filter(s -> filterFuction(s)).filter(distinctByValue(entry -> entry.getValue()))
            .collect(Collectors.toList());

    List<Map<String, Object>> list1 = list.stream().map(t -> {
        Map<String, Object> map = new HashMap<>();
        map.put(t.getKey(), t.getValue());
        return map;
    }).collect(Collectors.toList());
    System.out.println(list1);
}

public static <T> Predicate<T> distinctByValue(Function<? super T, ?> keyExtractor) {

    Map<Object, Boolean> seen = new ConcurrentHashMap<>();
    return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
}

public static boolean filterFuction(Entry<String, Object> entry) {
    if (entry.getKey().equals("value2"))
        return true;
    return false;
 }

}