Top Java Collections Coding Interview Questions and Answers for Experienced Developers

As an experienced Java developer you know that collections are a critical part of the Java language. Mastering collections is key to passing Java coding interviews and landing your dream job.

In this comprehensive guide I will share the most frequently asked Java collections coding interview questions and expert answers to help you ace your next interview. With detailed explanations and code examples you’ll learn insider tips and tricks to tackle any collections question that comes your way.

Common Java Collections Interview Questions

Here are some of the most common Java collections interview questions experienced developers can expect

• Explain the List, Set, and Map interfaces and commonly used implementations of each.
• When would you use an ArrayList vs. a LinkedList?
• How can you remove duplicate elements from an ArrayList?
• What is the difference between a HashSet and a TreeSet?
• How can you sort a List or ArrayList?
• What is the difference between HashMap and Hashtable?
• How can you synchronize Map and List implementations?
• When would you use Queue vs. Deque?
• How can you convert between arrays and Lists?

Let’s explore the answers to these key collections questions in more detail:

ArrayList vs. LinkedList

Interviewers often ask about the difference between ArrayLists and LinkedLists and when to use each.

• ArrayList is implemented using a dynamic array. It supports fast random access but is slow for inserts/deletes in the middle.
• LinkedList stores nodes with links between them. It is efficient for inserts/deletes but slow to access a specific index.

So if your program needs fast random access, use an ArrayList. If you frequently add/remove from the middle of the list, use a LinkedList.

Removing ArrayList Duplicates

To remove duplicates from an ArrayList, convert it to a HashSet which only contains unique elements:

List<String> list = new ArrayList<>();// add elements to listSet<String> set = new HashSet<>(list); // pass list to HashSet constructorlist.clear();list.addAll(set); //list now contains only unique elements

HashSet vs. TreeSet

• HashSet stores elements in a hash table, providing O(1) add, remove, and search. Elements are unordered.
• TreeSet stores elements in a sorted tree structure. Access and traversal is slower but elements are sorted.

So if you need sorting, use a TreeSet. If order doesn’t matter, use a HashSet for faster performance.

Sorting Lists and ArrayLists

To sort a List or ArrayList, convert it to an array and use Arrays.sort():

List<String> list = new ArrayList<>();String[] array = list.toArray(new String[0]);Arrays.sort(array);list.clear();list.addAll(Arrays.asList(array));

Or you can use Collections.sort() and pass a Comparator.

HashMap vs. Hashtable

• Both implement Map using hash tables.
• HashMap is non-synchronized while Hashtable is synchronized for thread-safety.
• HashMap permits null keys/values unlike Hashtable.

So use HashMap unless you need synchronization.

Making Collections Thread-Safe

You can make collections thread-safe by:

• Using concurrent collections like ConcurrentHashMap.
• Wrapping with Collections.synchronizedMap/List.
• Using immutable collections like ImmutableList.

Queue vs. Deque

Queues and Deques are both designed for add/remove operations on both ends but they have some differences:

• Queue supports addition at the rear, removal from the front (FIFO).
• Deque supports addition/removal at both ends (double-ended queue).

So Deque provides more flexibility if you need to add/remove from both sides.

Converting Arrays and Lists

You can convert between arrays and Lists using:

String[] array = new String[3]; List list = Arrays.asList(array); //array to listString[] array2 = list.toArray(new String[0]); //list to array

But note that the List returned by Arrays.asList() is fixed size.

Key Takeaways

• Know when to use ArrayLists vs. LinkedLists. LinkedLists are better for frequent inserts/deletes in the middle.
• Remove ArrayList duplicates by converting to HashSet.
• Use TreeSet if sorting is needed, HashSet if sorting is not required.
• Make collections thread-safe via synchronization, concurrency, or immutability.
• Prefer HashMap over Hashtable unless you need synchronization.
• Use Deque if you need to add/remove from both ends.
• Collections can easily be converted to/from arrays.

Frequency of Entities:

LinkedList: 4
ArrayList: 4
HashSet: 2
TreeSet: 2
HashMap: 2
Hashtable: 2
Queue: 2
Deque: 2
List: 5
Set: 2
Map: 2
Arrays: 2
Collections: 3
ConcurrentHashMap: 1
ImmutableList: 1