What is the Collection Framework?

The Java Collections Framework provides a unified architecture for representing and manipulating groups of objects. It includes:

Interfaces — abstract types (Collection, List, Set, Queue, Map)
Classes — concrete implementations (ArrayList, HashMap, TreeSet...)
Algorithms — utility methods in Collections class (sort, search, shuffle...)

Hierarchy:
Iterable
  └── Collection
        ├── List      → ArrayList, LinkedList, Vector, Stack
        ├── Set       → HashSet, LinkedHashSet, TreeSet
        └── Queue     → PriorityQueue, ArrayDeque, LinkedList

Map (separate hierarchy)
  ├── HashMap, LinkedHashMap
  └── SortedMap → TreeMap

📌 Key PointBy default, collections hold heterogeneous objects (any type). Use Generics (e.g., List<String>) to restrict to a single type and get type safety.

Collection Interface

The root interface of the collection hierarchy. All collection types implement it. Key methods:

Method	Description
add(E e)	Adds element e
remove(Object o)	Removes specified element
size()	Returns number of elements
isEmpty()	Returns true if collection has no elements
contains(Object o)	Returns true if element is present
iterator()	Returns an Iterator over elements
clear()	Removes all elements
toArray()	Converts collection to array

List Interface

List is an ordered, index-based collection that allows duplicates.

Method	Description
get(int index)	Returns element at index
set(int index, E e)	Replaces element at index
add(int index, E e)	Inserts at specific position
remove(int index)	Removes element at index
indexOf(Object o)	Returns first index of element
subList(from, to)	Returns a view of portion [from, to)

import java.util.*;

List<String> names = new ArrayList<>();
names.add("Alice");
names.add("Bob");
names.add(0, "Zara");       // insert at front
names.set(1, "Arjun");       // replace index 1
System.out.println(names);     // [Zara, Arjun, Bob]
System.out.println(names.get(2)); // Bob

Set Interface

Set is a collection that does not allow duplicates. Order depends on implementation.

Implementation	Ordering	Null allowed	Performance
HashSet	No guaranteed order	One null	O(1) avg
LinkedHashSet	Insertion order	One null	O(1) avg
TreeSet	Sorted (natural/comparator)	No null	O(log n)

Set<String> fruits = new HashSet<>();
fruits.add("Apple");
fruits.add("Mango");
fruits.add("Apple"); // duplicate — ignored
System.out.println(fruits.size()); // 2

Queue Interface

Queue follows FIFO (First-In-First-Out) ordering. Key methods:

Method	Throws exception?	Returns special value?
add(e) / offer(e)	add() throws, offer() returns false	offer() returns false
remove() / poll()	remove() throws, poll() returns null	poll() returns null if empty
element() / peek()	element() throws, peek() returns null	peek() returns null if empty

Queue<String> queue = new LinkedList<>();
queue.offer("Task 1");
queue.offer("Task 2");
queue.offer("Task 3");
System.out.println(queue.peek());  // Task 1 (doesn't remove)
System.out.println(queue.poll());  // Task 1 (removes)
System.out.println(queue.size());  // 2

Map Interface

Map stores key-value pairs. Keys are unique; values can be duplicated. Map does NOT extend Collection.

Method	Description
put(K,V)	Adds key-value pair; overwrites if key exists
get(K)	Returns value for key, or null
remove(K)	Removes entry by key
containsKey(K)	true if key exists
keySet()	Returns Set of all keys
values()	Returns Collection of all values
entrySet()	Returns Set of Map.Entry pairs

Map<String, Integer> scores = new HashMap<>();
scores.put("Alice", 95);
scores.put("Bob", 88);
scores.put("Alice", 99); // overwrites Alice's score

for (Map.Entry<String,Integer> e : scores.entrySet())
    System.out.println(e.getKey() + " → " + e.getValue());

Iterator

Iterator provides a safe way to traverse collections and remove elements during iteration.

List<Integer> nums = new ArrayList<>(Arrays.asList(1,2,3,4,5));
Iterator<Integer> it = nums.iterator();

while (it.hasNext()) {
    int n = it.next();
    if (n % 2 == 0)
        it.remove(); // safe removal during iteration
}
System.out.println(nums); // [1, 3, 5]

⚠️ ImportantNever modify a collection using list.remove() directly while iterating with an iterator — it throws ConcurrentModificationException. Always use it.remove().

Collection Framework — Interfaces

What is the Collection Framework?

Collection Interface

List Interface

Set Interface

Queue Interface

Map Interface

Iterator

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