Java Collections Framework

1. List

List is an ordered collection that allows duplicate elements. You can access elements using an index.

ArrayList Example


List<String> list = new ArrayList<>();
list.add("Apple");
list.add("Banana");
System.out.println(list.get(0)); // Apple

LinkedList Example


List<String> list = new LinkedList<>();
list.add("A");
list.add("B");
list.remove(0); // removes "A"

Stack Example


Stack<Integer> stack = new Stack<>();
stack.push(1);
stack.push(2);
System.out.println(stack.pop()); // 2

2. Set

Set does not allow duplicate elements and is mostly used when uniqueness is required.

HashSet Example


Set<String> set = new HashSet<>();
set.add("Java");
set.add("Java"); // Ignored

LinkedHashSet Example


Set<String> set = new LinkedHashSet<>();
set.add("A");
set.add("B");

TreeSet Example


Set<Integer> set = new TreeSet<>();
set.add(10);
set.add(5);
System.out.println(set); // [5, 10]

3. Queue

Queue stores elements in First-In-First-Out (FIFO) order. It is commonly used in scheduling or task queues.

PriorityQueue Example


Queue<Integer> queue = new PriorityQueue<>();
queue.add(20);
queue.add(10);
System.out.println(queue.poll()); // 10

4. Deque

Deque (Double-Ended Queue) allows inserting and removing elements from both front and rear.

ArrayDeque Example


Deque<String> deque = new ArrayDeque<>();
deque.addFirst("A");
deque.addLast("B");
System.out.println(deque.removeFirst()); // A

5. Map

Map stores key-value pairs. Keys must be unique, but values can be duplicated.

HashMap Example


Map<Integer, String> map = new HashMap<>();
map.put(1, "A");
map.put(2, "B");
map.put(1, "C"); // Replaces "A"

LinkedHashMap Example


Map<String, String> map = new LinkedHashMap<>();
map.put("A", "Apple");
map.put("B", "Banana");

TreeMap Example


Map<Integer, String> map = new TreeMap<>();
map.put(2, "B");
map.put(1, "A");
System.out.println(map); // Sorted by key

Collection Type Summary

Collection Type Ordered Allows Duplicates Sorted Examples
List Yes Yes No ArrayList, LinkedList
Set No No TreeSet only HashSet, TreeSet
Queue Yes Yes PriorityQueue LinkedList, PriorityQueue
Map No Keys: No, Values: Yes TreeMap only HashMap, TreeMap
References