Ever stumbled on HashMap vs Hashtable during an interview? Or hesitated when asked about ArrayList's internal working? This cheat sheet will bulletproof your Java Collections knowledge — with code, visuals, and interview-ready clarity.

If you're preparing for a Java interview, there's one thing you absolutely can't afford to skip: Java Collections Framework. From ArrayList to HashMap, these classes form the backbone of real-world Java applications—and interviewers know it.

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📦 1. Core Collection Interfaces

🔍 2. Most Commonly Asked Collections

🧰 ArrayList vs LinkedList

🔒 HashSet vs TreeSet vs LinkedHashSet

🗺️ 3. The Map Family

⚙️ HashMap vs TreeMap vs LinkedHashMap vs Hashtable

➡️ Pro Tip: Interviewers often ask about HashMap internal working: Explain hashCode(), equals(), buckets, collision resolution, and rehashing.

🧮 4. Big-O Time Complexities (At a Glance)

🎯 5. Real Interview Questions Based on Collections

1. 🔹 How does a HashMap work internally?
2. 🔹 What is the difference between ArrayList and LinkedList?
3. 🔹 How would you design an LRU cache in Java?
4. 🔹 How do Set and Map handle collisions?
5. 🔹 Explain fail-fast vs fail-safe iterators with examples.
6. 🔹 What is ConcurrentHashMap, and how is it different from Hashtable?

💡 Final Tips to Ace Collections Questions

• 🧠 Master internal implementations (especially HashMap).
• 👀 Understand memory trade-offs and performance implications.
• 🔁 Practice converting between different collections (List → Set, etc.)
• 🧪 Use LeetCode, HackerRank to practice collection-heavy problems.

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