SCJP Java Collections Framework 2

Question 1

Which method must be overridden in a class for its objects to work correctly as HashMap keys?

Accepted Answer

equals() and hashCode()

Answer

HashMap uses `hashCode()` to find the bucket and `equals()` to confirm key equality, so both must be correctly overridden.

Question 2

Which List implementation provides O(1) add/remove at the ends but O(n) for random access?

Accepted Answer

LinkedList

Answer

`LinkedList` uses a doubly-linked list, giving O(1) insertions at head/tail but O(n) for index-based access.

Question 3

What is the difference between `Iterator` and `ListIterator` in Java?

Accepted Answer

ListIterator can traverse in both directions; Iterator is forward-only

Answer

`ListIterator` extends `Iterator` with the ability to traverse a list in both forward and backward directions and also supports `add()` and `set()`.

Question 4

Which collection class is synchronized and considered the thread-safe version of ArrayList?

Accepted Answer

Vector

Answer

`Vector` is a synchronized, thread-safe dynamic array, essentially the legacy thread-safe counterpart to `ArrayList`.

Question 5

What does the `Collections.sort()` method require from the objects it sorts?

Accepted Answer

Objects must implement Comparable

Answer

`Collections.sort()` requires list elements to implement `Comparable` so their natural ordering can be used.

SCJP - Sun Certified Java Programmer Practice Test