Java map entry

A simple solution is to iterate through the map and use put(Key,Value) once for each mapping key and value in the another Map.

Java HashMap merge() The Java HashMap merge() method inserts the specified key/value mapping to the hashmap if the specified key is already not present . If the specified key is already associated with a value, the method replaces the old value with the result of the specified function.

Merging two Maps in Java