How to Memorize the Java Collections Framework: 60 Flashcards from ArrayList to ConcurrentHashMap

The java.util package looks like a heap of unrelated classes until you see the grid underneath it: a handful of interfaces, three storage strategies (hash table, linked list, balanced tree), and one utility class. Deck UX7077 — Java: Collections Framework (60 cards) lays that grid out card by card. Here is how to turn it into recall that survives an interview whiteboard.

Learn the grid, not the class list

Every general-purpose implementation in the framework is the intersection of what contract and what data structure. The deck teaches both axes separately, then lets you cross them:

• Contracts: Set (no duplicates), List (ordered, duplicates allowed), Queue (FIFO holding area), Deque (both ends), Map (keys to values, no duplicate keys).
• Structures: hash table → HashSet / HashMap, balanced tree → TreeSet / TreeMap, hash table plus insertion order → LinkedHashSet / LinkedHashMap, resizable array → ArrayList / ArrayDeque, doubly-linked nodes → LinkedList.

Once the axes are automatic, 20 class names collapse into two lists you multiply together.

Nine cards that carry the deck

What the 60 cards cover

Interfaces and views: Collection, Set, List, Queue, Deque, Map, SortedSet, NavigableSet, SortedMap, NavigableMap, the RandomAccess marker, keySet(), values(), entrySet(), subList(). Implementations: HashSet, TreeSet, LinkedHashSet, ArrayList, LinkedList, HashMap, TreeMap, LinkedHashMap, ArrayDeque, PriorityQueue, plus the legacy Vector, Stack and Hashtable and why you should prefer their modern replacements. Concurrency: ConcurrentHashMap, CopyOnWriteArrayList, BlockingQueue, Collections.synchronizedList(). Operations and helpers: add(), remove(), contains(), size(), isEmpty(), clear(), iterator(), toArray(), offer(), put(), get(), Collections.sort(), reverse(), shuffle(), unmodifiableList(), emptyList(), singletonList(), Arrays.asList(), immutable collections and UnsupportedOperationException.

Two tips for this specific deck

Fix the capitalisation as you review. The card fronts are sentence-cased by the deck generator, so you will see Add() method, Contains() method, IsEmpty() method and ToArray() method. In real Java those are add(), contains(), isEmpty() and toArray() — lowercase first letter, camelCase after. Say the correct form out loud when you flip the card; the capital is an artefact, not Java.

Drill the comparison cards in both directions. ArrayList vs LinkedList, HashMap vs Hashtable and Collection vs Collections are the ones interviewers actually ask. Don't stop at recognising the answer: given "unsynchronised, allows one null key", name the class. Same for complexity — given O(1) random access, name the list.

Deck UX7077 — Java: Collections Framework is in the Flip It library at flip-it.app. If you haven't covered the language fundamentals yet, start with our study guide to Java core concepts and OOP.