Map> map = new HashMap<>();map.computeIfAbsent("key1", k -> new ArrayList<>()).add("value1");map.computeIfAbsent("key1", k -> new ArrayList<>()).add("value2"); assertThat(map.get("key1").get(0)).isEqualTo("value1");assertThat(map.get("key1").get(1)).isEqualTo("value2");