Java Collections Framework Tutorial

Introduction

• Presenter: Riti Datta
• Focus: Java Collections Framework for DSA (Data Structures and Algorithms)
• **Goals: **
  • Learn to use Java Collections for DSA problems.
  • Prepare for Java coding interviews at top tech companies.
  • Understand both theoretical and implementation aspects of Collections.
  • Cover essential interfaces and classes in Java Collections.
• Target Audience: Beginners with basic Java knowledge (loops, inheritance).

Collections Overview

• Collections: Group of objects treated as a single unit.
• **Java Collections Framework: **
  • Provides standard utility classes for managing collections.
  • Part of java.util package.
  • Comprises core interface, implementations, and static utility classes.

Core Interfaces

• Iterable: At the top of the collection hierarchy.
  • Part of java.lang package.
  • Core interface for collections to allow iteration.
• Collection: Extends Iterable and cannot be instantiated directly.
  • Examples: List, Set, Queue.
• List: Extends Collection (e.g., ArrayList, LinkedList, Vector).
• Queue: Extends Collection (sub-interface Deque).
• Set: Extends Collection (e.g., HashSet, TreeSet).
• Map: Separate from Collection interface (not iterable).
  • Examples: HashMap, TreeMap, LinkedHashMap.

List Interface and Implementations

• List: Ordered and allows duplicates.
• **ArrayList: **
  • Dynamic array with resizable capacity.
  • Preferred for random access due to constant time complexity.
  • Inefficient for frequent insertions/deletions at arbitrary positions.
• LinkedList:
  • Doubly linked list implementation.
  • Efficient for insertions/deletions due to pointer adjustments.
  • Implements both List and Deque interfaces.

**List Iterator: **

• Allows bi-directional iteration.
• Extends Iterator.
• Methods: hasPrevious, previous.

Queue Interface and Implementations

• Queue: Follows FIFO (First-In-First-Out) principle.
• **ArrayDeque: **
  • Implements Deque, can function as both stack and queue.
• LinkedList:
  • Implements Deque, use for FIFO queues.
• PriorityQueue:
  • Orders elements based on priority.
  • Requires elements to be comparable or provided with a comparator.

Set Interface and Implementations

• Set: Collection of unique elements (no duplicates).
• HashSet:
  • Unordered collection with constant time complexity for basic operations.
  • Requires correct implementation of hashCode and equals methods.
• LinkedHashSet:
  • Maintains insertion order using linked list internally.
• TreeSet:
  • Ordered collection based on a binary search tree (Red-Black tree).
  • Provides guaranteed log(n) time cost for basic operations.

Map Interface and Implementations

• Map: Collection of key-value pairs (entries).
• HashMap:
  • Provides constant time complexity for basic operations.
  • Allows one null key and multiple null values.
• LinkedHashMap:
  • Maintains insertion order using linked list internally.
• TreeMap:
  • Ordered map based on a Red-Black tree.
  • Implements NavigableMap for additional navigation methods.
  • Methods: ceilingEntry, floorEntry, higherEntry, lowerEntry.
• HashTable:
  • Thread-safe alternative to HashMap.
  • Does not allow null keys/values.

Comparing and Sorting

• Comparable vs Comparator:
  • Comparable: Implement in class, provides natural ordering via compareTo method.
  • Comparator: Implement separately, used for total ordering via compare method.
• Usage Scenarios:
  • Custom types should implement Comparable for natural ordering.
  • Use Comparator for custom total orderings, such as reverse order.

Additional Techniques and Utilities

• Wrapper Classes: Required for generics (e.g., Integer instead of int).
• Java Generics: Allow parameterized types.
• Boxing/Unboxing: Automatic conversion between primitives and their wrapper classes.
  • Example: Integer to int and vice versa.
• Common Operations:
  • Sorting: Arrays.sort, Collections.sort.
  • Conversion: Arrays.asList, List.toArray.
  • Stream API: Modern approach for collection processing.

Summary and Best Practices

Key Takeaways:

• Understand the hierarchy and relationships between collection interfaces/classes.
• Properly implement hashCode and equals for use in sets and maps.
• Use correct collection types based on performance needs and ordering requirements.
• Apply Comparable and Comparator for custom object sorting.
• Explore Java Generics and Stream API for efficient, modern collection processing.

Next Steps:

• Practice implementing and using various collections.
• Explore detailed tutorials on Java Generics and Stream API.
• Solve DSA problems using collections to solidify understanding.