Java Collections Cheat Sheet with Examples

🧠 Java Collections Cheat Sheet with Examples

A concise but comprehensive reference for Java Collections — including Collection, List, Set, Queue, Map, and the Collections utility class.

🧩 1. Collection Interface (Common to List,…


This content originally appeared on DEV Community and was authored by ZeeshanAli-0704

🧠 Java Collections Cheat Sheet with Examples

A concise but comprehensive reference for Java Collections — including Collection, List, Set, Queue, Map, and the Collections utility class.

🧩 1. Collection Interface (Common to List, Set, etc.)

Method Description Example
add(E e) Add element list.add("Apple");
addAll(Collection<? extends E> c) Add all elements from another collection list.addAll(anotherList);
remove(Object o) Remove element set.remove("Banana");
removeAll(Collection<?> c) Remove all elements in another collection list.removeAll(anotherList);
retainAll(Collection<?> c) Keep only elements in another collection list.retainAll(anotherList);
contains(Object o) Check if element exists set.contains("Apple");
containsAll(Collection<?> c) Check if all elements exist list.containsAll(anotherList);
isEmpty() Check if empty list.isEmpty();
size() Number of elements list.size();
clear() Remove all elements list.clear();
toArray() Convert to array Object[] arr = set.toArray();

đź“‹ 2. List Interface Methods

Method Description Example
get(int index) Get element at index list.get(2);
set(int index, E element) Replace element at index list.set(1, "Orange");
add(int index, E element) Insert at index list.add(0, "Mango");
remove(int index) Remove element at index list.remove(2);
indexOf(Object o) First occurrence list.indexOf("Apple");
lastIndexOf(Object o) Last occurrence list.lastIndexOf("Apple");
subList(int from, int to) Get part of list list.subList(1, 3);
listIterator() Iterator in both directions ListIterator<String> it = list.listIterator();

Example:

List<String> list = new ArrayList<>();
list.add("Apple");
list.add("Banana");
list.add("Cherry");
System.out.println(list.get(1)); // Banana

🔢 3. Set Interface Methods

Method Description Example
add(E e) Add element (no duplicates) set.add("Apple");
remove(Object o) Remove element set.remove("Apple");
contains(Object o) Check existence set.contains("Apple");
size() Number of elements set.size();
isEmpty() Check if empty set.isEmpty();
clear() Remove all set.clear();

Example:


 Set<String> set = new HashSet<>();
 set.add("Apple");
 set.add("Banana");
 set.add("Apple"); // Ignored
 System.out.println(set.size()); // 2

đź•’ 4. Queue Interface Methods

Method Description Example
add(E e) Add element queue.add("Apple");
offer(E e) Add element, returns false if full queue.offer("Banana");
remove() Remove and return head queue.remove();
poll() Remove head, return null if empty queue.poll();
element() Return head, exception if empty queue.element();
peek() Return head, null if empty queue.peek();

Example:


Queue<String> queue = new LinkedList<>();
queue.add("Apple");
queue.add("Banana");
System.out.println(queue.poll()); // Apple
System.out.println(queue.peek()); // Banana

🗺️ 5. Map Interface Methods

Method Description Example
put(K key, V value) Add key-value map.put(1, "Apple");
putAll(Map<? extends K, ? extends V> m) Add all from another map map.putAll(otherMap);
get(Object key) Get value by key map.get(1);
remove(Object key) Remove key-value map.remove(1);
containsKey(Object key) Check key exists map.containsKey(1);
containsValue(Object value) Check value exists map.containsValue("Apple");
keySet() Get all keys map.keySet();
values() Get all values map.values();
entrySet() Get key-value pairs map.entrySet();
isEmpty() Check if empty map.isEmpty();
size() Number of entries map.size();
clear() Remove all entries map.clear();

Example:


Map<Integer, String> map = new HashMap<>();
map.put(1, "Apple");
map.put(2, "Banana");
System.out.println(map.get(1)); // Apple
System.out.println(map.keySet()); // [1, 2]

⚙️ 6. Collections Utility Class Methods

Method Description Example
sort(List<T> list) Sort list naturally Collections.sort(list);
sort(List<T> list, Comparator c) Sort with custom comparator Collections.sort(list, Comparator.reverseOrder());
reverse(List<?> list) Reverse list Collections.reverse(list);
shuffle(List<?> list) Random shuffle Collections.shuffle(list);
swap(List<?> list, int i, int j) Swap elements Collections.swap(list, 0, 1);
max(Collection c) Maximum element Collections.max(list);
min(Collection c) Minimum element Collections.min(list);
frequency(Collection c, Object o) Count occurrences Collections.frequency(list, "Apple");
binarySearch(List list, T key) Binary search (sorted list) Collections.binarySearch(list, "Banana");
fill(List list, T obj) Fill list with obj Collections.fill(list, "Empty");

Example:

List<String> list = new ArrayList<>(Arrays.asList("Banana", "Apple", "Cherry"));
Collections.sort(list);
System.out.println(list); // [Apple, Banana, Cherry]
Collections.reverse(list);
System.out.println(list); // [Cherry, Banana, Apple]

âś… Final Notes

  • Collection is the root interface for most data structures.
  • Use List when order matters.
  • Use Set to prevent duplicates.
  • Use Queue for FIFO (First-In-First-Out).
  • Use Map for key-value pairs.
  • Use Collections utility methods for sorting, searching, and manipulation.

đź’ˇ Save this cheat sheet and keep it handy for interviews, quick reference, or daily coding practice!


This content originally appeared on DEV Community and was authored by ZeeshanAli-0704


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