SCJP Java Collections Framework

Question 1

Which Java collection class allows duplicate elements and maintains insertion order?

Accepted Answer

ArrayList

Answer

`ArrayList` is an ordered, index-based list that allows duplicate elements and preserves insertion order.

Question 2

Which interface does HashMap implement?

Accepted Answer

Map

Answer

`HashMap` implements the `Map` interface, storing key-value pairs where keys must be unique.

Question 3

What is the time complexity of retrieving an element from a HashMap by key (average case)?

Accepted Answer

O(1)

Answer

HashMap provides O(1) average-case performance for `get()` and `put()` due to hashing.

Question 4

Which collection maintains elements in a sorted natural order and does not allow duplicates?

Accepted Answer

TreeSet

Answer

`TreeSet` is a sorted Set implementation that stores unique elements in their natural order or by a Comparator.

Question 5

What happens when you add a duplicate key to a HashMap?

Accepted Answer

The old value is replaced by the new value

Answer

When a duplicate key is inserted into a HashMap, the existing value associated with that key is overwritten.

SCJP - Sun Certified Java Programmer Practice Test