Java equals() vs hashCode() for HashMap and HashSet

🚀 100 Days of Java Tips – Day 3 Topic: equals() and hashCode() 💡 Java Tip of the Day If you override equals(), you must override hashCode() ⚠️ This is not just a best practice — it’s a core contract in Java that affects how objects behave in collections. 🤔 Why does this matter? Hash-based collections like HashMap, HashSet, and ConcurrentHashMap rely on both: equals() → to check logical equality hashCode() → to decide where the object is stored Breaking this contract can lead to unexpected behavior 🚨 📌 The Rule If two objects are equal according to equals(), they must return the same hashCode(). ❌ What can go wrong? Duplicate keys in HashMap Missing data during lookup Bugs that are hard to debug in production 😓 ✅ Key Takeaway Always override hashCode() whenever you override equals() — especially for Entity and DTO classes. 👉 Save this for interview prep 📌 👉 Comment “Day 4” if this helped you #Java #100DaysOfJava #JavaDeveloper #BackendDeveloper #CleanCode #JavaInternals #LearningInPublic

🔹 Understanding HashSet in Java – Internals Simplified HashSet is widely used in Java for storing unique elements, but do you know how it works under the hood? Internally, HashSet is backed by a HashMap, where each element is stored as a key and a dummy object as its value. When an element is added: hashCode() determines the bucket location. equals() ensures no duplicates are stored. From Java 8 onwards, if collisions in a bucket exceed a threshold, the linked list transforms into a Red-Black Tree, improving retrieval performance from O(n) → O(log n). Properly overriding hashCode() and equals() is crucial to maintain HashSet’s correctness. HashSet remains an efficient choice for fast lookups, uniqueness, and performance-sensitive applications. Diagram Caption: HashSet Internal Structure: Element → Key in HashMap → Bucket (Array) → LinkedList/Red-Black Tree for collisions. #Java #HashSet #CollectionsFramework #BackendDevelopment #CleanCode #InterviewPrep #JavaInternals

#InterviewQuestions - 3 ❓ Can we overload the main method in Java? ✅ Yes, the main method can be overloaded in Java. ✔However, the Java Virtual Machine (JVM) will only ever recognize and execute the method with the exact signature: >>> public static void main(String[] args). ✔Overloaded main methods are treated just like any other method in the class and will not run automatically when you execute the program. ✔We must explicitly call the overloaded methods from within the standard main method if you want them to be executed. 📊 Examples and output are shared in the slides below ⬇️ #Java #MainMethod #MethodOverloading #JavaInterview #OOP #SoftwareEngineering #LearningEveryday

Hello Java Developers, 🚀 Day 16 – Java Revision Series Today’s topic: Internal Working of LinkedHashMap — the perfect blend of HashMap performance and predictable iteration order. 💡 LinkedHashMap = Hash table + Doubly Linked List 🔹 Internals: Uses the same bucket array as HashMap for O(1) average access Each entry also participates in a doubly-linked list (before / after) This maintains insertion-order or access-order 🔹 How put() works: Compute hash → find bucket Handle collision like HashMap Insert node into bucket and append it to the linked list tail 🔹 How get() works: Compute hash → search bucket using equals() If access-order is enabled, the accessed node is moved to the tail 🎯 Why it matters: Predictable iteration order Great for LRU cache implementations Slightly more memory than HashMap, but same average O(1) performance 📄 I’ve shared a PDF with diagrams + step-by-step flow for quick revision. #Java #CoreJava #LinkedHashMap #JavaInternals #DataStructures #InterviewPreparation #LearningInPublic

Java☕ — Collections changed everything 📦 Before collections, my code looked like this: #Java_Code int[] arr = new int[100]; Fixed size. No flexibility. Painful logic. Then I met the Java Collections Framework. #Java_Code List<Integer> list = new ArrayList<>(); Suddenly I could: ✅Grow data dynamically ✅Use built-in methods ✅Write cleaner logic The biggest lesson for me wasn’t syntax, it was choosing the right collection. 📌ArrayList → fast access 📌LinkedList → frequent insert/delete 📌HashSet → unique elements 📌HashMap → key-value data Java isn’t powerful because of loops. It’s powerful because of its collections. #Java #CollectionsFramework #ArrayList #HashMap #LearningInPublic

🚀 Java Collections – The Ultimate Cheat Sheet Every Developer Needs! If you’re preparing for interviews or strengthening your Java fundamentals, understanding the Java Collection Framework is non-negotiable. Here’s a quick breakdown 👇 🔹 List ✔ Ordered ✔ Allows duplicates ✔ Examples: ArrayList, LinkedList, Vector ✔ Best when you need indexed access 🔹 Queue ✔ FIFO (First-In-First-Out) ✔ Useful for scheduling & processing tasks ✔ Examples: PriorityQueue, ArrayDeque 🔹 Set ✔ No duplicates ✔ Ideal for unique elements ✔ Examples: HashSet, LinkedHashSet, TreeSet 🔹 Map ✔ Key–Value pairs ✔ No duplicate keys ✔ Examples: HashMap, LinkedHashMap, TreeMap 💡 Pro Tip: Use ArrayList for fast retrieval. Use LinkedList for frequent insertions/deletions. Use HashSet when uniqueness matters. Use HashMap for fast key-value lookups. Use TreeMap/TreeSet when sorting is required. Mastering Collections = Writing Efficient Java Code 🔥 Which Java Collection do you use the most in your projects? 👇 Let’s discuss! #Java #JavaDeveloper #Programming #Coding #TechCareers #SoftwareDevelopment #InterviewPreparation

📘 Day 17 – Java Concept | static vs instance Members Today I revised one of the most fundamental OOP concepts in Java that interviewers use to test understanding of memory, class loading, and object behavior. What I Learned 👇 🔹 static Members Belong to the class, not to individual objects Single copy shared by all instances Loaded into memory once when the class is loaded Accessed using the class name Example: class Demo {   static int count = 0; } System.out.println(Demo.count); 🔹 Instance Members Belong to the object Each object has its own copy Created when an object is instantiated Accessed using the object reference Example:class Demo {   int value = 10; } Demo d = new Demo(); System.out.println(d.value); 🔹 Golden Rule in Java Use static → For shared data and utility methods Use instance → For object-specific state and behavior Why This Matters Understanding this concept helps in building memory-efficient, scalable Java applications and answering system-level interview questions confidently. #Day17 #Java #CoreJava #OOP #JavaInterview #ProgrammingJourney #PlacementPrep

Java doesn’t store strings randomly — it stores them smartly 💡. Java strings look simple… until memory comes into play 🧠 The String Constant Pool is one of the most important (and most asked) Java concepts in interviews. This post breaks it down visually and clearly. ✔ Why strings are shared ✔ new String() vs literals ✔ Heap vs SCP 👉 Save for revision 👉 Share with a Java learner . . Thanks to Harshita Mittal for the design touch! . . #Java #CoreJava #JavaMemory #StringConstantPool #JavaInternals #LearnJava #JavaDeveloper #JavaStrings #Programming #MadeEasy #JavaExplainedSimply #InterviewReady

Hello Java Developers, 🚀 Day 15 – Java Revision Series Today’s topic is one of the most important and most asked questions in Java interviews: ❓ How does HashMap work internally in Java? HashMap stores data in key-value pairs using an array of buckets and hashing. 🔹 When you call put(key, value): hashCode() of the key is calculated An index (bucket) is derived from that hash The entry is stored in that bucket 🔹 If two keys map to the same bucket (collision): Before Java 8 → Stored in a LinkedList Since Java 8 → Converted to a Red-Black Tree after a threshold for better performance 🔹 When you call get(key): HashMap again calculates the index using hashCode() Then uses equals() to find the exact key inside the bucket ⚠️ That’s why the equals() and hashCode() contract is critical. ⏱️ Time Complexity: Average: O(1) Worst case (Java 8+): O(log n) due to tree structure 📄 I’ve summarized the full internal working in a PDF for quick revision. #Java #CoreJava #HashMap #DataStructures #JavaInternals #InterviewPreparation #LearningInPublic

🚀 Java Core Concepts – Interview Questions & Answers 📌 Question: Why can’t we create a generic array in Java? Generic arrays cannot be created in Java because of Type Erasure. 🔹 Arrays are reified – they retain their type information at runtime 🔹 Generics are implemented using Type Erasure – type information is removed at compile time 🔹 Arrays perform a runtime type check and can throw ArrayStoreException 🔹 If generic arrays were allowed, the runtime type safety of arrays would break 💡 Since arrays check types at runtime and generics lose type information after compilation, combining both would cause type safety issues. That’s why Java does not allow: new T[] new List<String>[] 👉 Follow Ashok IT School for daily Java interview questions 👉 Comment “JAVA” to get Core Java, Collections & Advanced interview prep 👉For Java Course Details  Visit:https://lnkd.in/gwBnvJPR . #Java #CoreJava #Generics #TypeErasure #JavaInterviewQuestions #JavaCollections #Programming #AshokIT #AshokITSchool