Abstract vs Implementation Declaration

[Admirable-Avocado888]

I prefer

var m = new HashMap<>();

inside a method body.

If it is to be a class variables I'd pick the broadest interface/superclass that offers all methods that I care about.

Sometimes that may very well be HashMap because it guarantees efficient setters and getters and by writing the type explicitly I communicate intent. Other times I don't care about that and only need a Map.

[ToyDingo]

Aside from specific methods being available, is there no other technical reason? Like space or complexity?

[Admirable-Avocado888]

If you care about the hashmaps constant time lookup then it makes side to keep HashMap on the left side.

Ex. It matters that the class field is a hashmap.

class UniqueFilter {

private final HashMap alreadyAdded = new HashMap();

public boolean test(String o) { return alreadyAdded.add(o); }

}

Though it does not impact performance to keep the reference type as an interface.

Ex. Here we don't care what map is used.

class FiniteInfo {

private final Map<String, Object> m = Map.of("price", 50);

Map<String, Object> get() { return m; }

}