How to use the Java Collections Cheat Sheet
- 1. Search or browseType a word like the function or task you need, or pick a section.
- 2. CopyEach snippet has its own Copy button, so you can paste it straight into your editor.
- 3. DownloadSave the whole Java Collections cheat sheet as plain text or Markdown for offline revision.
What's in it
The Collections Framework comes up in nearly every Java interview, and this sheet focuses on choosing and using the right class. The 35 snippets are organised into 12 sections: Interfaces, List, Set, Map, Queue, Sorting, Utilities, Concurrency, Equality, Iteration, Complexity, Streams.
Example
Search "count" to find m.merge(word, 1, Integer::sum), the cleanest way to count words in a HashMap. Search "stack" and you'll see why ArrayDeque is recommended over the legacy Stack class. The Iteration section shows how to remove items safely with an iterator instead of hitting ConcurrentModificationException.
Tips for using it
- Know which collections keep order: LinkedHashMap and LinkedHashSet keep insertion order; TreeMap and TreeSet keep keys sorted.
- Remember to override equals() and hashCode() together for keys.
A quick look, one snippet per section
- Interfaces:
Collection → List, Set, Queue · Map is separate(the main hierarchy) - List:
List<String> l = new ArrayList<>();(resizable array, O(1) get) - Set:
Set<Integer> s = new HashSet<>();(unordered, O(1) average) - Map:
Map<String, Integer> m = new HashMap<>();(key→value, O(1) average) - Queue:
Queue<Integer> q = new ArrayDeque<>(); q.offer(1); q.poll();(fIFO queue) - Sorting:
Collections.sort(l);(natural order) - Utilities:
Collections.max(l), Collections.frequency(l, x)(max and count) - Concurrency:
ConcurrentHashMap, CopyOnWriteArrayList(concurrent collections) - Equality:
Override equals() and hashCode() together(required for keys in HashMap/HashSet)
Search the box above for the rest of the Java Collections snippets, or download the full sheet.
Frequently asked questions
Does it cover streams?
Two snippets show collecting to maps; the Java cheat sheet has more.
Which Java version?
Java 9+ for List.of and Map.of; the rest works on Java 8.
Is the Java Collections cheat sheet free?
Yes. You can search, read and copy every snippet in the Java Collections cheat sheet without signing up. The first time you download it, we ask once for your name and WhatsApp number; after that downloads and every other Uncodemy tool open straight away.