Understanding Collection Framework Hierarchy in Java

If you're working with Java, you've likely used collections—ArrayList, HashSet, or HashMap. But have you ever wondered how these classes are structured or related? That’s where understanding the Java Collection Framework hierarchy becomes crucial.

Think of the framework as a family tree of data structures, designed for storing, processing, and retrieving objects efficiently. It helps developers choose the right data structure for the task, boosting performance, readability, and maintainability.

---

🌳 Core Definition and Purpose

The Java Collections Framework (JCF) is a group of interfaces, abstract classes, and concrete classes designed to store and manipulate groups of data.

• Interfaces: Define behavior (List, Set, Map, Queue)
• Abstract Classes: Provide partial implementations (AbstractList, AbstractSet)
• Concrete Classes: Full implementations ready to use (ArrayList, HashSet, TreeMap)

---

🧬 Java Collection Framework Hierarchy

High-Level Overview

          Iterable
              |
           Collection
        /     |      \
     List    Set    Queue
      |       |        |
  ArrayList HashSet PriorityQueue

Map Is Special

Map is not a subtype of Collection. It’s a parallel hierarchy for key–value data.

               Map
         /      |       \
   HashMap  TreeMap  LinkedHashMap

---

🔍 Interface Breakdown

Collection Interface

The root of the JCF, extended by List, Set, and Queue.

List

• Ordered, allows duplicates
• Implementations: ArrayList, LinkedList

Set

• No duplicates, unordered or ordered (via TreeSet, LinkedHashSet)
• Implementations: HashSet, TreeSet, LinkedHashSet

Queue

• Supports FIFO operations
• Implementations: PriorityQueue, ArrayDeque

Map

• Stores key-value pairs
• Implementations: HashMap, TreeMap, LinkedHashMap

---

💻 Syntax and Class Examples

List Example

List<String> list = new ArrayList<>();
list.add("Apple");
list.add("Banana");

Set Example

Set<String> set = new HashSet<>();
set.add("One");
set.add("One"); // Duplicate, ignored

Map Example

Map<Integer, String> map = new HashMap<>();
map.put(1, "A");
map.put(2, "B");

---

⚙️ Internal Working & Performance

ArrayList Resizing

• Backed by an array
• Grows by ~50% when full

HashMap Hashing

• Uses hashCode() and equals()
• Collisions handled via chaining and treeification

Performance Table

Structure	Lookup	Insert	Delete
ArrayList	O(1)*	O(1)*	O(n)
LinkedList	O(n)	O(1)	O(1)
HashMap	O(1)*	O(1)*	O(1)*
TreeMap	O(log n)	O(log n)	O(log n)

* Amortized time

---

🔄 Functional Programming Support

Stream Example

List<String> result = list.stream()
  .filter(e -> e.startsWith("A"))
  .map(String::toUpperCase)
  .collect(Collectors.toList());

Predicate & Function

Predicate<String> startsWithA = s -> s.startsWith("A");
Function<String, Integer> length = String::length;

---

🔁 Comparisons

Feature	List	Set	Map
Duplicates	Allowed	Not Allowed	Keys not allowed
Order	Preserved	Depends	Based on implementation
Null Support	Yes	Yes	Key/value varies

---

🚨 Anti-patterns and Misuse

• Using Vector or Hashtable in new code
• Forgetting equals()/hashCode() overrides in key objects
• Modifying collections while iterating

---

🧼 Refactoring Legacy Code

Before:

Hashtable<String, Integer> table = new Hashtable<>();

After:

Map<String, Integer> table = new HashMap<>();

✅ Modern, fast, thread-safe when needed via ConcurrentHashMap.

---

✅ Best Practices

• Program to interfaces, not implementations
• Use generics (List<String> not List)
• Initialize with appropriate capacity
• Prefer Collections.unmodifiableList() when immutability is needed

---

📌 What's New in Java?

Java 8

• Streams API
• Lambdas
• Functional interfaces

Java 9

• List.of(), Map.of() for immutable collections

Java 10+

• var for local variable type inference

Java 21

• Record patterns, enhanced performance
• Incubator for sequenced collections

---

📘 Conclusion and Key Takeaways

• Java Collections are organized into a flexible hierarchy
• Understanding the hierarchy helps in making better data structure choices
• Java 8+ features make collections more expressive and functional
• Keep performance, immutability, and maintainability in mind

---

❓ FAQ – Collection Framework Hierarchy

1. Why is Map not a part of Collection?
   Because it doesn't fit the single-element model. It stores key–value pairs.

2. Which collection maintains order and allows duplicates?
   ArrayList and LinkedList.

3. What’s the difference between Set and List?
   List allows duplicates; Set doesn’t.

4. How is HashMap different from TreeMap?
   HashMap is unordered and faster; TreeMap is sorted.

5. Can I sort a HashSet?
   Not directly. Convert it to a List or use TreeSet.

6. What is the initial capacity of HashMap?
   16, with load factor 0.75.

7. Is LinkedList faster than ArrayList?
   For frequent inserts/deletes, yes. For random access, no.

8. Which is thread-safe: HashMap or ConcurrentHashMap?
   Only ConcurrentHashMap is thread-safe.

9. Can a Map have duplicate values?
   Yes, but not duplicate keys.

10. When to use LinkedHashMap?
    When you need predictable iteration order.

---