1Z0-819 Core APIs & Data Manipulation 5

Question 1

Which method on `Optional<T>` executes an action if a value is present and does nothing otherwise?

Accepted Answer

ifPresent()

Answer

`ifPresent(Consumer)` calls the consumer with the value when present and is a no-op when the Optional is empty.

Question 2

What does `TreeSet` use to order its elements by default?

Accepted Answer

Natural ordering via Comparable

Answer

`TreeSet` relies on the elements' natural ordering (their `Comparable` implementation) unless a custom `Comparator` is provided at construction.

Question 3

Which statement about `StringJoiner` is true?

Accepted Answer

It can be constructed with a delimiter, optional prefix, and optional suffix

Answer

`StringJoiner(delimiter, prefix, suffix)` builds a joined string with optional surrounding tokens, and returns the empty-value string (not an exception) when no elements are added.

Question 4

What is the output of: `List list = new ArrayList<>(); list.add(1); list.add(2); list.remove(1);` — what remains?

Accepted Answer

[1]

Answer

`list.remove(1)` uses the index overload (not the object overload) because the argument is an `int`, removing the element at index 1 (value 2), leaving [1].

Question 5

Which `Stream` collector groups elements into a `Map>` based on a predicate?

Accepted Answer

Collectors.partitioningBy()

Answer

`Collectors.partitioningBy(Predicate)` always produces a two-partition map with `true` and `false` keys.

(1Z0-819) Oracle Java SE 11 Developer Practice Test

(1Z0-819) Oracle Java SE 11 Developer Practice Test

1Z0-819 Core APIs & Data Manipulation 5