Duplicate keys are not allowed in a Map. Basically, Map Interface has two implementation classes HashMap and TreeMap the main difference is TreeMap maintains an order of the objects but HashMap will not. HashMap allows null values and null keys. Both HashSet and HashMap are not synchronized.

Can we have duplicate keys in map?

HashMap doesn't allow duplicate keys but allows duplicate values. That means A single key can't contain more than 1 value but more than 1 key can contain a single value. HashMap allows null key also but only once and multiple null values.

How do you duplicate a map key?

4 Answers. You can't have duplicate keys in a Map . You can rather create a Map<Key, List<Value>> , or if you can, use Guava's Multimap . And then you can get the java.