Java collections: which one to use, and when

Most Java coding questions come down to picking the right collection. Ask two questions: do I need order, and do I need duplicates?

import java.util.*;
public class CollectionsDemo {
  public static void main(String[] args) {
    List<String> subjects = new ArrayList<>(List.of("DBMS", "OS", "CN", "OS"));
    Set<String> unique = new LinkedHashSet<>(subjects);
    Map<String, Integer> credits = new TreeMap<>();
    credits.put("OS", 4); credits.put("DBMS", 4); credits.put("CN", 3);
    Deque<String> history = new ArrayDeque<>();
    history.push("page1"); history.push("page2");
    Queue<Integer> pq = new PriorityQueue<>(List.of(30, 10, 20));
    System.out.println(subjects + " " + unique);
    System.out.println(credits);
    System.out.println(history.pop() + " " + pq.poll());
  }
}

[DBMS, OS, CN, OS] [DBMS, OS, CN]
{CN=3, DBMS=4, OS=4}
page2 10

Quick chooser

• ArrayList: an ordered list with duplicates. Get by index is O(1); adding at the end is O(1) on average; inserting or removing in the middle is O(n). Your default list.
• HashSet / LinkedHashSet: no duplicates, contains() is O(1) on average. LinkedHashSet also keeps insertion order, as above.
• HashMap: key to value, O(1) average lookups. Iteration order is not guaranteed, so never print a HashMap and expect a fixed order.
• TreeMap / TreeSet: kept sorted by key, O(log n) operations, plus firstKey, floorKey and ceilingKey for range questions.
• ArrayDeque: use it as a stack (push/pop) or a queue (offer/poll). It is faster than the old Stack class and than LinkedList.
• PriorityQueue: always gives you the smallest element first (a min-heap). Pass Comparator.reverseOrder() for a max-heap. Ideal for "top k" questions.

Habits that save marks

• Declare with the interface (List, Map) and create with the class (ArrayList, HashMap), so you can switch later.
• List.of and Map.of create collections that cannot be changed. Wrap them in new ArrayList<>(...) if you need to add items.
• Do not remove items from a list inside a for-each loop over it; you will get a ConcurrentModificationException. Use removeIf instead.
• Practise more in UniCircle interview prep: Java.