# Java 26 Interview Questions & Answers - Comprehensive Guide

**Updated for Java 26** – Covers Collections, Concurrency, Design Patterns, System Design, Spring Boot, Redis, Kafka, Kubernetes, Microservices, and more.

## Table of Contents
- [Collections Framework](#collections-framework)
- [HashMap Internals](#hashmap-internals)
- [Set Implementations](#set-implementations)
- [Queue & Deque](#queue--deque)
- [Comparable vs Comparator](#comparable-vs-comparator)
- [Advanced Collections](#advanced-collections)
- [Concurrent Collections](#concurrent-collections)
- [Interview MCQs](#interview-mcqs)
- [Coding Problems](#coding-problems)
- [System Design & Architecture](#system-design--architecture)
- [Java 26 Features](#java-26-features)

---

## Collections Framework

### ArrayList vs LinkedList
**ArrayList:**
- Dynamic array implementation
- O(1) for get/set operations
- O(n) for add/remove at arbitrary positions
- Better for random access

**LinkedList:**
- Doubly-linked list implementation
- O(n) for get/set operations
- O(1) for add/remove at ends
- Better for frequent insertions/deletions

```java
// ArrayList - Best for frequent access
List<String> arrayList = new ArrayList<>();
arrayList.add("A");  // O(1)
arrayList.get(0);    // O(1)

// LinkedList - Best for frequent modifications at ends
List<String> linkedList = new LinkedList<>();
linkedList.add("A");    // O(1)
linkedList.addFirst("B"); // O(1)
linkedList.removeLast();  // O(1)
```

### Vector vs ArrayList

| Feature              | Vector                  | ArrayList              |
|----------------------|-------------------------|------------------------|
| Synchronized         | Yes (thread-safe)       | No (not thread-safe)   |
| Performance          | Slower                  | Faster                 |
| Capacity Increment   | Doubles by default      | Increases by 50%       |
| Legacy               | Java 1.0                | Java 1.2               |

---

## HashMap Internals

- Default initial capacity: 16
- Default load factor: 0.75
- Collision resolution: Chaining (linked list → tree when ≥8 nodes)
- Java 26 enhancements: Better hashing and concurrency integration

**Key Operations Flow:**
1. Calculate hash
2. Find bucket index
3. Handle collisions

```java
public V put(K key, V value) {
    int hash = hash(key);
    int index = hash & (n - 1);
    // ... chaining or tree logic
}
```

### LinkedHashMap
- Maintains insertion or access order
- Excellent for LRU Cache

### TreeMap
- Red-Black tree
- Sorted keys
- O(log n) operations

---

## Set Implementations

| Feature       | HashSet     | LinkedHashSet     | TreeSet       |
|---------------|-------------|-------------------|---------------|
| Order         | None        | Insertion         | Sorted        |
| Performance   | O(1)        | O(1)              | O(log n)      |

---

## Queue & Deque

**PriorityQueue:** Min-heap by default  
**ArrayDeque:** Preferred for stack/queue operations

```java
// Max-heap
PriorityQueue<Integer> maxHeap = new PriorityQueue<>(Comparator.reverseOrder());
```

**Queue vs Deque:**
- Queue: Single-ended FIFO
- Deque: Double-ended (stack + queue)

---

## Comparable vs Comparator

**Comparable:** Natural ordering (`compareTo`)  
**Comparator:** External flexible ordering

---

## Advanced Collections

### Reverse HashMap, Merge Maps, Frequency Counter, Remove Duplicates
Detailed implementations using Streams, `merge()`, `computeIfAbsent()`, etc.

### Top-K Elements
Min-heap or Bucket sort approaches.

### LRU Cache (3 Implementations)
1. LinkedHashMap
2. Custom Doubly Linked List + HashMap
3. Thread-safe version

### WeakHashMap, IdentityHashMap, EnumMap, EnumSet

---

## Concurrent Collections

- **CopyOnWriteArrayList:** Read-heavy, snapshot on write
- **ConcurrentHashMap:** High concurrency with CAS
- **BlockingQueue:** Producer-consumer patterns

---

## Interview MCQs & JUnit 5 Tests
Multiple choice questions and comprehensive test suite included.

---

## Coding Problems
- Top K Frequent Elements
- LRU Cache full implementations
- Anagrams, Word Ladder, Median Finder, etc.
- Recursion, Backtracking, Linked Lists, Trees, Graphs, DP

---

## Java 26 Features

**New String methods, Scoped Values, Virtual Threads enhancements, Pattern Matching improvements.**

**Scoped Values Example:**
```java
ScopedValue<String> USER = ScopedValue.newInstance();
ScopedValue.where(USER, "admin").run(() -> { ... });
```

**Virtual Threads:** Better scalability with collections.

---

## System Design & Architecture
- Parking Lot
- URL Shortener
- Spring Boot REST + JPA
- Redis caching & rate limiting
- Kafka producers/consumers
- Kubernetes manifests
- Microservices patterns (Circuit Breaker, etc.)

**FAANG/Real Interview Questions** and solutions included.

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