Java · Theory

Deque & ArrayDeque

Theory

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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.

Exam tip

Contrast array vs ArrayList in one line.

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.

Short notes

DefDeque & ArrayDeque — List = order + duplicates OK. Set = unique. Map = key → value.
RuleDeque & ArrayDeque — pick it only when that structure matches the problem.
RememberDeque & ArrayDeque — generics List<String>. ArrayList for daily lists. HashMap for lookup.
TrapDeque & ArrayDeque — using arrays for everything that must grow, or mixing List/Set/Map.

// 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

Example

Short notes

Questions

Deque & ArrayDeque

Example

Short notes

Questions