SCJP Java Collections Framework 5

Question 1

What is the difference between `HashSet` and `LinkedHashSet`?

Accepted Answer

LinkedHashSet maintains insertion order; HashSet does not

Answer

LinkedHashSet uses a linked list internally to maintain the order in which elements were inserted, while HashSet has no ordering guarantee.

Question 2

Which method of the `Collections` class returns an unmodifiable view of a specified list?

Accepted Answer

Collections.unmodifiableList(list)

Answer

Collections.unmodifiableList() wraps a list in a view that throws UnsupportedOperationException on any mutating operation.

Question 3

What does the `NavigableMap` interface add over `SortedMap`?

Accepted Answer

Methods like floorKey(), ceilingKey(), higherKey(), and lowerKey()

Answer

NavigableMap extends SortedMap with navigation methods that return the closest matches for given search targets, such as floor, ceiling, higher, and lower.

Question 4

Which of the following is NOT a characteristic of `ConcurrentHashMap`?

Accepted Answer

It allows null keys

Answer

ConcurrentHashMap does not permit null keys or null values, unlike HashMap which allows one null key.

Question 5

What is the time complexity of `contains()` on a `HashSet` in the average case?

Accepted Answer

O(1)

Answer

HashSet.contains() is O(1) on average because it computes the hash code to directly locate the bucket, then checks equality.

SCJP - Sun Certified Java Programmer Practice Test

SCJP - Sun Certified Java Programmer Practice Test

SCJP Java Collections Framework 5