duplicate key in hashmap code example

Example 1: how to insert duplicate key in hashmap

Map<String, List<String>> 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");

Example 2: does key repeats in hashmap

HashMap does not allow duplicate keys however it allows to have duplicate values. HashSet permits to have a single null value. HashMap permits single null key and any number of null values.