1. Collection Hierarchy

Java Collections Framework provides a set of interfaces and classes for working with collections.

Core Interfaces

1. Collection (root interface)
   • Common methods: add(), remove(), size(), iterator(), etc.
2. List (ordered collection, allows duplicates)
   • Implementations: ArrayList, LinkedList, Vector
3. Set (unordered, no duplicates)
   • Implementations: HashSet, LinkedHashSet, TreeSet
4. Queue (FIFO or LIFO order)
   • Implementations: PriorityQueue, ArrayDeque
5. Map (key-value pairs, no duplicate keys)
   • Implementations: HashMap, LinkedHashMap, TreeMap, Hashtable

2. Classes Overview

3. Key Methods in Collections

• Adding Elements: add(element), addAll(collection)
• Removing Elements: remove(element), clear()
• Accessing Elements:
  • For Lists: get(index)
  • For Maps: get(key)
• Iterating:
  • Using Iterator
  • Using forEach loop
  • Using Streams (stream().forEach())

4. Important Classes

ArrayList

• Dynamic array.
• Faster access (get() method).
• Slower at add()/remove() operations in the middle.

LinkedList

• Doubly linked list.
• Efficient insertions/removals but slower random access.

HashSet

• Stores unique elements.
• Backed by a HashMap.

TreeSet

• Elements are sorted.
• Slower than HashSet due to sorting.

HashMap

• Key-value pairs.
• Fast lookup (constant time on average).

TreeMap

• Keys are sorted.
• Slower than HashMap.

5. Key Features

1. Generics
   • Collections can be type-safe.
   • Example: List<String> list = new ArrayList<>();
2. Thread-Safety
   • Not all collections are thread-safe.
   • Use Collections.synchronizedList() or ConcurrentHashMap.
3. Streams and Lambda
   • Functional programming support via Streams.
   • Example: list.stream().filter(e -> e.startsWith("A")).collect(Collectors.toList());

6. Differences Between Key Classes

7. Advanced Concepts

1. Fail-Fast Iterators
   • Iterators of most collections throw ConcurrentModificationException if the collection is modified while iterating.
2. Fail-Safe Iterators
   • Concurrent collections like CopyOnWriteArrayList allow modification during iteration.

8. Real-Time Use Cases

• List: Maintaining ordered lists like students, tasks.
• Set: Storing unique IDs, tags.
• Queue: Implementing task scheduling.
• Map: Managing key-value pairs like dictionaries or caching.

Code Examples

Tips for Using Collections

1. Choose ArrayList when frequent random access is needed.
2. Use HashSet for unique, unordered collections.
3. For thread-safe operations, prefer ConcurrentHashMap over Hashtable.
4. Use TreeMap or TreeSet for sorted data.

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