About Question enthuware.ocpjp.v11.2.3064 :

[dimitrilc]

Hello, for option B, it is not clear why x is typed to Object.

This explanation

Map has a method named keySet(), which returns Set<K>.

does not make a lot of sense here. I think it is clearer to say either

1:
Properties extends Hashtable<Object, Object> (note the concrete generic type of Object), so all references to K, V declared in Hashtable<K, V> become Object, Object. Just saying that Properties extends Hashtable is not enough; It should be Hashtable<Object, Object>.

2:
Properties overrides keySet() (and a bunch of other methods) to explicitly return Set<Object> in its source code

Code: Select all

    @Override
    public Set<Object> keySet() {
        return Collections.synchronizedSet(map.keySet(), this);
    }

[admin]

It is explained in the explanation to the first option (just above the second one). Even so, the explanation has now been enhanced to make it clearer.
thank you for your feedback!

[Deleted User 6318]

How is this _ used as a keyword? What is purpose of that?

[admin]

How is this _ used as a keyword? What is purpose of that?

It has no use or special meaning but it is now in the list of keywords. You can verify it here: https://docs.oracle.com/javase/specs/jl ... ml#jls-3.9