Hashmap vs. Map (in terms of their types)

[allDogsAreGood]

In the attached screenshot, we have a new "Hashmap" object being assigned to the variable called personMap which is of type Map<Integer, Person>.

Shouldn't the type of the personMap object variable be "Hashmap?" Why is it different?

[jim829]

It can be but it doesn't have to. HashMap implements the Map interface and it is usually best practice to assign the implementation to the
interface type. Exceptions are when the implementation provides additional methods one wants to use. The idea is that it makes writing code easier. If you start assigning everything to a HashMap type and decide you want to use a TreeMap, you need to change all HashMap types to TreeMap types. If you simply use the Map reference, you probably need to change it in fewer places.

In a related topic, it is best to pass the interface type to a method rather than the implementation.

Regards,
Jim