Why place a reference to an object with an implementation of an interface in a variable of the type of this interface?

Question

Java

Hello!
To solve one problem, I was shown some code that used collections. I began to read about them, formed a general picture, but some points remained unclear. The HashSet class is, after all, an implementation of the Set interface, i.e. it contains implementations of the methods contained in the Set interface. (do I understand this correctly?) Why do we create an interface type variable so that it refers to an object whose class implements this same interface? Why we can't and shouldn't write simply I 've already read a lot about collections, watched video tutorials, but still everything is in a fog.
final Set<KeyCode> codes = new HashSet<>();
final HashSet<KeyCode> codes = new HashSet<>();

Answer 1

In order to pass not a concrete class, but an interface. Then we will not depend on the specific implementation of the interface. In principle, if inside the method we use only the implementation of the class, then the interface can not be pulled.

Answer 2

SOLID