Collections

About

Collections in Java are part of the Java Collections Framework (JCF), a set of classes and interfaces designed to handle and manipulate groups of objects effectively. They provide data structures like lists, sets, and maps, along with algorithms to operate on these data structures, such as searching, sorting, and iterating. The Java Collections Framework is a vital tool for any Java developer, offering flexibility, scalability, and efficiency in managing large amounts of data.

Components

• Interfaces: Define abstract data types (e.g., List, Set, Map).

• Implementations: Concrete classes implementing these interfaces (e.g., ArrayList, HashSet, HashMap).

• Algorithms: Utility methods for common operations (e.g., Collections.sort()).

Hierarchy

• Collection Interface (base interface for List, Set, and Queue).

• Map Interface (key-value pairs, not part of Collection).

Core Interfaces

a. List

• Purpose: Ordered collection allowing duplicate elements.

• Common Implementations: ArrayList, LinkedList, Vector.

• Features:

  • Access elements by index.

  • Dynamic resizing.

List<String> list = new ArrayList<>();
list.add("A");
list.add("B");
System.out.println(list.get(0)); // Output: A

b. Set

• Purpose: Unordered collection that does not allow duplicate elements.

• Common Implementations: HashSet, LinkedHashSet, TreeSet.

• Features:

  • Ensures uniqueness of elements.

  • TreeSet maintains sorted order.

Set<String> set = new HashSet<>();
set.add("A");
set.add("B");
set.add("A"); // Duplicate ignored
System.out.println(set); // Output: [A, B]

c. Queue

• Purpose: Collection designed for holding elements prior to processing.

• Common Implementations: PriorityQueue, LinkedList.

• Features:

  • Follows FIFO (First-In-First-Out) order.

  • PriorityQueue orders elements based on their natural order or a comparator.

Queue<Integer> queue = new LinkedList<>();
queue.add(1);
queue.add(2);
System.out.println(queue.poll()); // Output: 1

d. Map

• Purpose: Key-value pair collection.

• Common Implementations: HashMap, LinkedHashMap, TreeMap.

• Features:

  • Keys are unique.

  • Allows null keys and values (except TreeMap).

Map<String, Integer> map = new HashMap<>();
map.put("A", 1);
map.put("B", 2);
System.out.println(map.get("A")); // Output: 1

Utility Classes

Collections

Utility class with static methods for operations like sorting and searching.

Collections.sort(list);

Arrays

Utility class for array operations.

Arrays.sort(array);

Choosing the Right Collection

Requirement	Recommended Collection
Maintain insertion order	ArrayList, LinkedHashSet
Fast lookup by key	HashMap, ConcurrentHashMap
Unique elements	HashSet, TreeSet
Thread-safe operations	Vector, ConcurrentLinkedQueue
Sorted order	TreeSet, TreeMap