1Z0-819 Collections Framework 5

Question 1

What is the default initial capacity of a `HashMap` when constructed with no arguments?

Accepted Answer

16

Answer

The default initial capacity of `HashMap` is 16, and it resizes by doubling when the load factor threshold (default 0.75) is exceeded.

Question 2

Which statement is TRUE about `EnumSet`?

Accepted Answer

It is backed by a bit vector for high performance

Answer

`EnumSet` is implemented as a bit vector (one or two `long` values), making it extremely fast and memory-efficient for enum-typed elements.

Question 3

What does the `Map.merge(key, value, remappingFunction)` method do if the key already exists with a non-null value?

Accepted Answer

Applies the remapping function to the old and new values

Answer

`merge()` calls `remappingFunction.apply(oldValue, newValue)` when the key exists; if the function returns null, the key is removed.

Question 4

Which of the following correctly uses `Comparator.comparing()` to sort a list of strings by their length?

Accepted Answer

Comparator.comparing(String::length)

Answer

`Comparator.comparing(String::length)` creates a `Comparator<String>` that extracts the length as the sort key using a method reference.

Question 5

After calling `iterator.next()` on a `ListIterator`, which method moves the cursor backward?

Accepted Answer

iterator.previous()

Answer

`ListIterator.previous()` returns the previous element and moves the cursor backward, enabling bidirectional traversal of a list.

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