Deque & ArrayDeque — Java Technical Interview Questions & Answers

Deque & ArrayDeque

Deque & ArrayDeque

  • Deque & ArrayDeque belongs to the Collections story. List = ordered, duplicates OK (ArrayList). Set = unique (HashSet). Map = key → value (HashMap). Pick by need, not by what you revised last.
  • For Deque & ArrayDeque: ArrayList is the daily List: get(i) is fast, add at end is fast. HashMap lookup by key is average O(1). HashSet is unique values, no duplicates.
  • Use Deque & ArrayDeque with generics: List<String>, Map<String, Integer>. Iterate with for-each or iterator. Don’t use a raw array when the size must grow.
  • Let's take this on the board with one tiny Main class — no extra files. Deque & ArrayDeque — prints length 3, middle value 20, then all values. Start from main, go line by line, and stop at each print. That output is the proof for Deque & ArrayDeque.

Technical Interview Tip: Contrast array vs ArrayList in one line.

Code Examples for Deque & ArrayDeque

Example

// Array demo
public class Main {
  public static void main(String[] args) {
    int[] nums = {10, 20, 30};
    System.out.println(nums.length);
    System.out.println(nums[1]);
    for (int n : nums) System.out.print(n + " ");
  }
}

Deque & ArrayDeque — prints length 3, middle value 20, then all values.

How to Explain Deque & ArrayDeque to an Interviewer

What is Deque & ArrayDeque?

Deque & ArrayDeque — List = order + duplicates OK. Set = unique. Map = key → value. Deque & ArrayDeque — pick it only when that structure matches the problem. Deque & ArrayDeque — generics List<String>. ArrayList for daily lists. HashMap for lookup.

Why do we use Deque & ArrayDeque in Java?

Makes the program clearer or safer Deque & ArrayDeque — List = order + duplicates OK. Set = unique. Map = key → value.

Write one small example of Deque & ArrayDeque.

Show syntax Say what prints

What mistake do beginners make with Deque & ArrayDeque?

Only the name, no example Deque & ArrayDeque — using arrays for everything that must grow, or mixing List/Set/Map.

Where is Deque & ArrayDeque used in a real program?

Day-to-day Java code