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Java Handwritten Notes, Interview Questions, CheatSheet, Spring Boot & Hibernate Resources

Java Handwritten Notes, Interview Questions, CheatSheet, Spring Boot & Hibernate Resources

Lets Code•October 4, 2025

Java has been a backbone of software development for years. Whether you’re new to programming or getting ready for interviews, Java has a lot to offer. In this post, we’ll cover the essentials , key concepts, architecture, useful resources like handwritten notes and cheat sheets, plus a set of curated interview questions to help you prepare.

1. Why Java?

Java is a general-purpose, statically typed, object-oriented language compiled to bytecode that runs on the Java Virtual Machine (JVM).
Its mantra is Write Once, Run Anywhere (WORA) — meaning your compiled bytecode can run on any JVM-enabled system without recompilation.

Key Strengths

  • Platform independence via JVM + bytecode.
  • Rich ecosystem: standard libraries, frameworks (Spring, Hibernate, etc.).
  • Strong tooling: IDEs, build tools, debugging, profiling.
  • Large community, abundant learning resources, and demand in the industry.
  • Robustness features: memory management, exception handling, security model.

Challenges / Trade-offs

  • Performance overhead vs low-level languages due to abstraction.
  • Verbose syntax compared to newer languages.
  • JVM tuning and garbage collection quirks for large systems.
  • Complexity in large enterprise systems (dependency hell, versioning, etc.).

2. Java Platform Architecture

Java’s architecture has two major dimensions: the platform architecture (JVM, JRE, JDK) and the application architecture (how your code is organized).

Platform Components

  • JDK (Java Development Kit): compiler (javac), tools, APIs.
  • JRE (Java Runtime Environment): JVM + standard libraries needed to run Java programs.
  • JVM (Java Virtual Machine): executes bytecode, manages memory (heap, stack), class loading, JIT, garbage collection.

How It Works: From Code to Execution

  1. You write .java source files.
  2. javac compiles them into .class files (bytecode).
  3. JVM loads classes, verifies, JIT-compiles hotspots.
  4. Bytecode executes on the target platform with memory + GC management.

Application Architecture Patterns

  • Layered (presentation / service / data access)
  • MVC (Model-View-Controller) for web apps
  • Microservices & modularization
  • Event-driven / message-based architectures
  • Domain-driven design, hexagonal architecture

3. Core Java Fundamentals

3.1 Syntax, Types & Variables

  • Primitive types: int, long, float, boolean, char, etc.
  • Reference types: objects, arrays, interfaces.
  • Scope: local, instance, static.

3.2 Control Flow & Operators

  • if-else, switch, for, while, do-while.
  • Operators: arithmetic, relational, logical, bitwise.

3.3 Methods / Functions

  • Overloading: compile-time polymorphism.
  • Overriding: runtime polymorphism via inheritance.

3.4 OOPS Principles

  • Encapsulation – private fields, public getters/setters.
  • Abstraction – interfaces, abstract classes.
  • Inheritance – extends, “is-a” relationships.
  • Polymorphism – overriding, interface polymorphism.

3.5 Exception Handling

  • Checked vs unchecked exceptions.
  • try-catch-finally, throw, throws, try-with-resources.

3.6 Collections & Generics

  • Collections Framework: List, Set, Map, Queue.
  • Generics provide compile-time type safety.

3.7 Concurrency & Threads

  • Built-in threading (Thread, Runnable).
  • java.util.concurrent utilities: locks, executors, atomics.

3.8 Miscellaneous

  • I/O (java.io, java.nio).
  • Streams & lambdas (Java 8+).
  • Serialization, reflection, annotations.

4. Advanced Topics & Modern Enhancements

Garbage Collection & Memory

  • Algorithms: G1, ZGC, Shenandoah, etc.

JIT Compilation

  • Hotspot JIT, optimizations like inlining, loop unrolling, escape analysis.

Java Module System

  • Introduced in Java 9: requires, exports for modular packaging.

Future of Java

  • Project Valhalla: value classes, null-restricted types, primitive generics.

App Architecture Trends

  • Microservices, reactive programming, event-driven design, cloud-native Java.

5. Helpful Java Resources

6. Sample Java Interview Questions (with hints)

  1. Difference between == and equals()?
    == → reference equality.
    equals() → logical equality (overridable).
  2. Explain Java memory model (heap, stack, method area, PC).
    • Heap: objects
    • Stack: method frames
    • Method area: class metadata
    • PC register: thread execution pointer
  3. What is the diamond problem?
    • Java prevents multiple inheritance of implementation.
    • Solved via interfaces (default methods).
  4. How do generics work with type erasure?
    • Type info removed at runtime, replaced with raw types/bounds.
  5. HashMap vs TreeMap vs LinkedHashMap?
    • HashMap: unordered.
    • TreeMap: sorted keys.
    • LinkedHashMap: insertion-order.
  6. synchronized vs ReentrantLock?
    • ReentrantLock → fairness, tryLock, interruptible locks.
  7. How does GC find live objects?
    • Reachability from GC roots (strong/weak/soft/phantom refs).
  8. Checked vs Unchecked exceptions?
    • Checked → must declare/catch.
    • Unchecked → runtime exceptions/errors.

7. Tips for Learning & Revision

  • Practice coding daily (mini projects, coding platforms).
  • Revise with your cheat sheet regularly.
  • Use flashcards for OOPS & Collections concepts.
  • Teach/blog concepts to cement learning.
  • Mock interviews with a timer.
  • Stay updated with latest Java features (JDK 17, 21, etc.).

At Last

Java is vast, but structured learning + continuous practice + solid revision materials (like your handwritten notes, cheat sheets, and interview question bank) make mastery achievable.

👉 Use the resources linked above to build your foundation, strengthen your preparation, and ace interviews.

Join Telegram group for more resources & discussions!

🧰 Useful Resources for Your Placement Prep

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