🧠 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.

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🧩 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();

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📋 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

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🔢 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

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🕒 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

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🗺️ 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]

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⚙️ 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]

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✅ 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.

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