1Z0-819 Functional Interfaces 4

Question 1

Which method reference form is used in `Arrays.sort(names, String::compareToIgnoreCase)`?

Accepted Answer

Instance method reference on an arbitrary instance of a type

Answer

String::compareToIgnoreCase is an instance method reference on an arbitrary instance — the first argument becomes the receiver of the method call.

Question 2

What functional interface does a constructor reference like `ArrayList::new` satisfy?

Accepted Answer

Supplier<ArrayList>

Answer

ArrayList::new with no arguments matches Supplier<ArrayList> because it takes no input and produces a new ArrayList instance.

Question 3

Given `Function<String, Integer> f = Integer::parseInt;`, what kind of method reference is this?

Accepted Answer

Static method reference

Answer

Integer.parseInt is a static method, so Integer::parseInt is a static method reference.

Question 4

What does `Comparator.comparing(Person::getName)` return?

Accepted Answer

A Comparator<Person> that orders persons by their name

Answer

Comparator.comparing accepts a key extractor Function and returns a Comparator that orders by the extracted key's natural order.

Question 5

Which code correctly creates a Comparator that sorts strings by length, then alphabetically?

Accepted Answer

Comparator.comparing(String::length).thenComparing(Comparator.naturalOrder())

Answer

thenComparing chains a secondary Comparator used only when the primary comparison yields equal results.

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