Merge maps java

@Test
public void testMapMerge() {
    // given
    Map<String, Integer> mutableMap = new HashMap<>(Map.of("a", 1, "b", 11));
    Map<String, Integer> otherMap = Map.of("b", 22, "c", 111);

    // when
    otherMap.forEach((k, v) -> mutableMap.merge(k, v, (v1, v2) -> v1 + v2));

    // then
    Assert.assertEquals(mutableMap, Map.of("a", 1, "b", (11 + 22), "c", 111));

    // when
    mutableMap.merge("b", 44, (v1, v2) -> v1 + v2);
    mutableMap.merge("d", 1111, (v1, v2) -> v1 + v2);

    // then
    Assert.assertEquals(mutableMap, Map.of("a", 1, "b", (11 + 22 + 44), "c", 111, "d", 1111));
}