Because Map requires a key and a value, it is incompatible with Collection. For example, if we wish to add a key-value pair, we will use put (Object key, Object value).
To add an element to a HashMap object, you must provide two arguments. Add(Object o) has only one argument in the Collection interface.
The other reason is that Map offers valueSet, keySet, and other appropriate methods that merely have different views from the Collection interface.