Collections Framework

Java Developer Mapping

Overview

The C# counterpart to java.util.* is System.Collections.Generic. It provides high-performance, strongly-typed data structures, implementing core interfaces like IEnumerable<T>, ICollection<T>, IList<T>, and IDictionary<TKey, TValue>.

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🗺️ Architectural Mapping: Java vs. C#

graph TD
    subgraph Java Collections
        J_List["List (ArrayList / LinkedList)"]
        J_Set["Set (HashSet / TreeSet)"]
        J_Map["Map (HashMap / TreeMap)"]
        J_Queue["Queue / Deque"]
    end

    subgraph C# Collections
        C_List["List<T> / LinkedList<T>"]
        C_Set["HashSet<T> / SortedSet<T>"]
        C_Map["Dictionary<TKey, TValue> / SortedDictionary"]
        C_Queue["Queue<T> / ConcurrentQueue<T>"]
    end

    J_List --> C_List
    J_Set --> C_Set
    J_Map --> C_Map
    J_Queue --> C_Queue

📋 Comprehensive Collections Comparison Table

Direct Equivalents Reference

Use this table to map your day-to-day Java collections to their C# equivalents.

Concept / Role	Java (java.util)	C# Equivalent (System.Collections.Generic)	Key Difference / Note
Dynamic Array	ArrayList / Vector	List<T>	C#‘s primary sequential list; uses indexers (list[0]).
Linked List	LinkedList	LinkedList<T>	Doubly linked list supporting fast insertions/deletions.
Key-Value Map	HashMap / LinkedHashMap	Dictionary<TKey, TValue>	Hash-table based; keys require proper GetHashCode() and Equals().
Sorted Map	TreeMap	SortedDictionary<TKey, TValue> / SortedList<TKey, TValue>	Maintains keys in sorted order using a binary search tree.
Unique Elements	HashSet / LinkedHashSet	HashSet<T>	Eliminates duplicate elements with lookup performance.
Sorted Set	TreeSet	SortedSet<T>	Maintains elements in a sorted, unique sequence.
FIFO Queue	Queue / LinkedList	Queue<T>	Standard First-In-First-Out data structure.
LIFO Stack	ArrayDeque / Stack	Stack<T>	Standard Last-In-First-Out structure (Java’s Stack is legacy).

⚡ Key Behavioral Differences to Remember

1. Printing & ToString() Behavior

• Java: Printing a map or list directly (System.out.println(map)) prints its formatted contents ({key=value}).

• C#: Printing a collection directly prints its type name (System.Collections.Generic.Dictionary'2[...]), because collection classes do not override ToString() for performance reasons. You must iterate or query them.

2. Querying via LINQ vs. Streams

• Java: Uses Java Streams (stream().filter(...).collect(...)) for functional pipelines.

• C#: Built-in language features called LINQ (Language-Integrated Query) offer direct syntax or extension methods on any IEnumerable<T>:

  C#

  var results = numbers.Where(n => n > 5).ToList();
  

3. Indexing Syntax

• Java: Uses .get(index) for lists and maps.

• C#: Uses clean bracket indexers (list[index] or dict[key]) natively across almost all collections.