TemporelMap.java

import java.util.Optional;
import java.util.TreeMap;
import java.util.function.BinaryOperator;

public class TemporelMap<K extends Comparable<K>, V> {

    private TreeMap<K, V> treeMap = new TreeMap<>();
    private BinaryOperator<V> add;

    public TemporelMap(BinaryOperator<V> add) {

        this.add = add;
    }

    public Optional<V> get(K key) {
        return treeMap.headMap(key, true).values().stream().reduce(add);
    }

    public void add(K key, V value) {
        treeMap.put(key, value);
    }

    @Override
    public String toString() {
        return treeMap.toString();
    }

    public static void main(String[] args) {

        TemporelMap<Integer, Integer> map = new TemporelMap<>((n1, n2) -> n1 + n2);
        System.out.println(map.toString());

        map.add(1, 2);
        map.add(3, 4);
        map.add(4, 7);
        map.add(5, 6);

        System.out.println(map.toString());
        System.out.println("Value for 2 = " + map.get(2));
        System.out.println("Value for 4 = " + map.get(4));
        System.out.println("Value for 7 = " + map.get(7));

        map.add(2, 3);

        System.out.println(map.toString());
        System.out.println("Value for 2 = " + map.get(2));
        System.out.println("Value for 4 = " + map.get(4));
        System.out.println("Value for 7 = " + map.get(7));
    }
}