Java Collections Framework Flashcards

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

Answer: LinkedHashSet maintains insertion order; HashSet does not

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

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

Answer: Collections.unmodifiableList(list)

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

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

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

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

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

Answer: It allows null keys

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

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

Answer: O(1)

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

Given `TreeSet ts = new TreeSet<>(); ts.add(5); ts.add(1); ts.add(3);`, what does `ts.first()` return?

Answer: 1

TreeSet stores elements in ascending natural order, so first() returns the smallest element, which is 1.

Which statement accurately describes `Collections.sort()` for a `List` where T does not implement `Comparable`?

Answer: A ClassCastException is thrown at runtime

If T does not implement Comparable and no Comparator is supplied, Collections.sort() throws a ClassCastException at runtime when it tries to cast elements to Comparable.