Core Java

🧩 Java Topics Classification (Interview Roadmap)

1️⃣ Java Fundamentals (Beginner Level)

👉 Freshers / junior interviews

  • What is Java?
  • Features of Java (OOP, platform-independent, secure)
  • JDK vs JRE vs JVM
  • Java program structure
  • main() method
  • Data types (primitive & non-primitive)
  • Variables & scope
  • static keyword
  • Control statements (if, switch, loops)
  • Operators
  • Type casting (implicit / explicit)
  • Command line arguments

📌 Interview focus: core syntax & JVM basics

2️⃣ Object-Oriented Programming (OOP)

👉 Most important section

  • Class & Object
  • Encapsulation
  • Inheritance
  • Polymorphism
  • Abstraction
  • Interface vs abstract class
  • this & super
  • Method overloading
  • Method overriding
  • Access modifiers
  • Composition vs inheritance

📌 Interview focus: OOP concepts + real examples

3️⃣ Java Memory Management & JVM Internals

👉 Very frequently asked

  • JVM architecture
  • ClassLoader hierarchy
  • Heap vs Stack
  • Method Area / Metaspace
  • Garbage Collection basics
  • Types of GC (Young, Old)
  • finalize() (deprecated conceptually)
  • OutOfMemoryError
  • StackOverflowError

📌 Interview focus: how Java works internally

4️⃣ Data Types, Wrappers & Strings

👉 Tricky interview questions

  • Primitive data types
  • Wrapper classes
  • Autoboxing & unboxing
  • String, StringBuilder, StringBuffer
  • String immutability
  • String pool
  • equals() vs ==
  • hashCode() contract

📌 Interview focus: immutability & performance

5️⃣ Exception Handling

👉 Common interview favorite

  • Checked vs unchecked exceptions
  • try-catch-finally
  • throw vs throws
  • Custom exceptions
  • Exception hierarchy
  • Best practices
  • try-with-resources

📌 Interview focus: real-world error handling

6️⃣ Collections Framework

👉 Almost guaranteed questions

  • List, Set, Map hierarchy
  • ArrayList vs LinkedList
  • HashSet vs TreeSet
  • HashMap vs Hashtable vs ConcurrentHashMap
  • Internal working of HashMap
  • Iterators & fail-fast vs fail-safe
  • Comparable vs Comparator
  • Collections vs Arrays utility classes

📌 Interview focus: internals & performance

7️⃣ Generics

👉 Frequently misunderstood

  • Why generics?
  • Generic classes & methods
  • Bounded types
  • Wildcards (? extends, ? super)
  • PECS principle
  • Type erasure
  • Generics limitations

📌 Interview focus: type safety

8️⃣ Multithreading & Concurrency (Advanced 🔥)

👉 Senior-level must-have

  • Process vs Thread
  • Thread lifecycle
  • Creating threads
  • Synchronization
  • volatile
  • Deadlock, livelock, starvation
  • wait, notify, notifyAll
  • Locks (ReentrantLock)
  • ExecutorService
  • Callable vs Runnable
  • Future & CompletableFuture
  • Atomic variables
  • Java Memory Model

📌 Interview focus: thread safety & performance

9️⃣ Java 8+ Features (Modern Java 🔥)

👉 Mandatory in interviews

  • Lambda expressions
  • Functional interfaces
  • Streams API
  • map, filter, reduce
  • Optional
  • Method references
  • Default & static methods in interface
  • New Date/Time API

📌 Interview focus: functional programming

🔟 Input / Output (I/O & NIO)

👉 Backend & system interviews

  • File handling
  • Streams (byte vs character)
  • Serialization & deserialization
  • transient
  • NIO basics
  • Path, Files, Channels, Buffers

1️⃣1️⃣ Java Reflection & Annotations

👉 Framework-related interviews

  • Reflection API
  • Class object
  • Dynamic method invocation
  • Custom annotations
  • Runtime vs compile-time annotations

📌 Interview focus: Spring internals

1️⃣2️⃣ Design Patterns in Java

👉 System design rounds

  • Singleton
  • Factory
  • Builder
  • Observer
  • Strategy
  • Decorator
  • Template method
  • Proxy

📌 Interview focus: real-world usage

1️⃣3️⃣ Java Security

👉 Enterprise interviews

  • Immutability
  • Secure coding practices
  • final keyword
  • Serialization attacks
  • Reflection attacks
  • Singleton breaking

1️⃣4️⃣ JVM Performance & Tuning (Senior / Architect)

👉 Advanced roles

  • JVM options
  • GC tuning
  • Memory leaks
  • Profiling tools
  • Throughput vs latency

1️⃣5️⃣ Java + Framework Ecosystem

👉 Real-world interviews

  • Java with Spring / Spring Boot
  • Hibernate / JPA basics
  • REST APIs
  • Microservices concepts
  • Transaction management

🎯 How to Create Interview Questions Using This

LevelSections
Beginner1–2
Intermediate3–6
Advanced7–9
Senior10–15

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