1Z0-819 Collections Framework 4

Question 1

What is the behavior of `TreeMap.subMap(fromKey, toKey)` with respect to the boundary keys?

Accepted Answer

Inclusive on fromKey, exclusive on toKey

Answer

`TreeMap.subMap(fromKey, toKey)` returns a view with `fromKey` inclusive and `toKey` exclusive, matching the convention of standard range methods.

Question 2

Which class provides a thread-safe variant of `HashMap` without synchronizing the entire map?

Accepted Answer

ConcurrentHashMap

Answer

`ConcurrentHashMap` uses segment-level or bucket-level locking to allow concurrent reads and partial concurrent writes without locking the full map.

Question 3

Which `Map` method was added in Java 8 and computes a new value only if the key is absent?

Accepted Answer

computeIfAbsent()

Answer

`computeIfAbsent(key, mappingFunction)` invokes the function to compute a value only when the key is not already present, and stores the result.

Question 4

What exception is thrown when you attempt to add a null element to a `TreeSet`?

Accepted Answer

NullPointerException

Answer

`TreeSet` uses natural ordering or a `Comparator`, and comparing `null` causes a `NullPointerException` when the first comparison is attempted.

Question 5

Which of the following collection factory methods allows duplicate elements?

Accepted Answer

List.of(1, 1)

Answer

`List.of()` permits duplicate elements, whereas `Set.of()` and `Map.of()` with duplicate keys/elements throw `IllegalArgumentException` at construction time.

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

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

1Z0-819 Collections Framework 4