Lesson: Java Collections Framework

1. Introduction (Aashray)

Overview:
Java Collections are reusable data structures that enable developers to store, retrieve, and manipulate groups of objects efficiently. Instead of manually managing arrays (which have a fixed size and limited operations), the Collections Framework provides ready-to-use implementations (e.g., ArrayList, HashSet, HashMap) with useful methods such as sorting, shuffling, searching, and more. This lesson will introduce the core concepts, key terms, and practical usage with code examples and interactive exercises.

Objectives:

  • Understand what collections are and why we use them.
  • Learn the differences between collections and arrays.
  • Identify and describe the main types of collection interfaces (List, Set, Queue, Map) and their implementations.
  • Learn key methods and how to iterate through collections.
  • Try out simple code experiments to reinforce the hands-on concepts.

2. Key Definitions & Terminology

  • Collection:
    An object that groups multiple elements into a single unit. Collections can grow and shrink dynamically, and (using generics) are type-safe.

  • Framework:
    A set of interfaces, classes, and methods that provide standardized ways to manipulate data structures. In Java, the Collections Framework is found in the java.util package.

  • Generics:
    A language feature (added in Java 5) that allows a collection to specify the type of its elements, ensuring compile-time type safety and eliminating the need for explicit casts.

  • Interface vs. Implementation:
    The framework defines interfaces (e.g., List, Set, Map) as blueprints of what operations a collection should offer. Concrete classes (like ArrayList, HashSet, HashMap) implement these interfaces with specific details.

  • Iterator:
    An object for traversing collections one element at a time. There are standard iterators (from the Iterator interface) as well as list-specific iterators (ListIterator).

  • Fail-fast vs. Fail-safe:
    Iterators from most collections (like those of ArrayList) throw a ConcurrentModificationException if the collection is modified during iteration (fail-fast). Some concurrent collections (such as those in java.util.concurrent) allow modifications during traversal (fail-safe).

3. Why Use Collections?

Advantages over Arrays:

  • Dynamic sizing: Unlike arrays, collections automatically grow or shrink to fit the number of elements.
  • Predefined methods: Framework classes provide utility methods like sort(), shuffle(), search(), and many more.
  • Flexibility: Collections support advanced operations (iterators, streams, lambda expressions) and can hold objects of any type (using generics).
  • Interoperability: The framework is designed with standard interfaces so you can switch implementations (for example, from ArrayList to LinkedList) with minimal changes in your code.
  • Thread safety options: Classes like ConcurrentHashMap and utilities (e.g., Collections.synchronizedList()) help when developing concurrent applications.

4. Types of Collections

Java Collections fall broadly into two categories: Collection interfaces and Map interfaces.

A. Collection Interfaces

These include interfaces that represent groups of objects:

  1. List
    • Definition: An ordered collection that allows duplicate elements.
    • Common Implementations:
      • ArrayList (backed by a dynamically resized array; best for random access)
      • LinkedList (a doubly linked list; best for frequent insertions/deletions)
    • Key Methods: add(), get(), set(), remove(), size()
  2. Set
    • Definition: A collection that does not allow duplicate elements. Not inherently ordered.
    • Common Implementations:
      • HashSet (unordered, uses hashing for fast lookup)
      • LinkedHashSet (preserves insertion order)
      • TreeSet (stores elements in sorted order)
    • Key Methods: add(), contains(), remove(), size()
  3. Queue
    • Definition: A collection used to hold multiple elements prior to processing. Typically follows FIFO (first-in-first-out).
    • Common Implementations:
      • LinkedList (can function as a queue)
      • PriorityQueue (orders elements by priority)
    • Key Methods: offer(), poll(), peek()
  4. Deque (Double-Ended Queue)
    • Definition: Supports element insertion and removal at both ends.
    • Common Implementations:
      • ArrayDeque
      • LinkedList
    • Key Methods: addFirst(), addLast(), removeFirst(), removeLast()

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B. Map Interfaces

Unlike the collection interfaces above, a Map stores key-value pairs. Note that Map is not a subtype of Collection.

  • Map
    • Definition: An object that maps keys to values with no duplicate keys.
    • Common Implementations:
      • HashMap (unsorted, fast access via hashing)
      • LinkedHashMap (preserves insertion order)
      • TreeMap (sorted by keys)
    • Key Methods: put(), get(), remove(), containsKey(), keySet()