hashMap mergeについて

ME.java

import java.util.HashMap;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class ME {

    public static void main(String[] args) {
        HashMap<Integer, Integer> hashMap = new HashMap<Integer, Integer>();
        hashMap.put(1, 100);
        hashMap.put(2, 200);
        hashMap.merge(1, 10, (a, b) -> a + b);
        System.out.println(hashMap.get(1));
    }
}