Java Course

java mastery course welcome fundamentals oop collections spring boot microservices interview preparation enterprise development

java introduction history james gosling jvm bytecode write once run anywhere oop platform independent enterprise android spring

java architecture jdk jre jvm bytecode javac write once run anywhere execution engine class loader platform independent

jvm deep dive class loader bootstrap platform application parent delegation heap stack method area outofmemoryerror stackoverflowerror

jvm execution engine interpreter jit compiler garbage collection gc jni native method libraries hotspot performance optimization

java architecture best practices common mistakes performance tips cheat sheet heap stack gc jit profiling outofmemoryerror stackoverflowerror

java setup jdk ide intellij idea java home path javac java version install lts java 21 first project

java syntax semicolon curly braces identifiers keywords comments naming conventions camelCase PascalCase main method case sensitive

java variables data types primitive int double boolean char long float byte short string var type inference widening conversion

java operators arithmetic assignment relational logical ternary bitwise shift precedence increment decrement equals comparison

java control flow if else switch for while do-while break continue return sequential decision looping branching nested

java if statement condition boolean expression nested if equals logical operators && short-circuit voting eligibility login ATM withdrawal

java if else if ladder multiple conditions grading tax slab discount order matters switch vs if else if nested enterprise

java loops for while do-while for-each enhanced break continue nested infinite loop iteration array collection

java arrays fixed size zero-based indexing length multidimensional matrix ArrayIndexOutOfBoundsException for-each loop contiguous memory

java strings immutable string pool constant pool equals StringBuilder StringBuffer concat substring split trim interview

java methods parameters arguments return type void overloading pass by value recursion call stack static modular programming

java oop object oriented programming class object constructor this keyword static instance heap stack garbage collection fields methods

java encapsulation private public protected getters setters data hiding javabeans immutable validation access modifiers spring hibernate

java inheritance extends super override final class diamond problem single multilevel hierarchical IS-A composition method overriding

java polymorphism method overloading overriding dynamic method dispatch upcasting downcasting instanceof covariant return compile-time runtime

java abstraction abstract class interface default method static functional interface dependency injection spring abstract vs interface

java exception handling try catch finally throw throws checked unchecked runtime exception try with resources custom exception

java collections framework JCF ArrayList HashMap HashSet List Set Map Queue generics Collections sort

java list interface ArrayList LinkedList Vector Stack ListIterator fail-fast ConcurrentModificationException Deque

java set interface HashSet LinkedHashSet TreeSet hashCode equals red-black tree unique elements

java map interface HashMap LinkedHashMap TreeMap Hashtable hashCode equals load factor rehashing ConcurrentHashMap

java IO input output streams InputStream OutputStream FileReader BufferedReader NIO channels buffers serialization

java multithreading concurrency threads Runnable Callable synchronized volatile ReentrantLock ExecutorService CompletableFuture ConcurrentHashMap

java enums enumeration EnumSet EnumMap valueOf ordinal switch type-safe constants OrderStatus

java generics type safety wildcard extends super PECS type erasure bounded generic class method diamond operator

java optional NullPointerException ofNullable orElse orElseThrow map flatMap filter Spring Boot repository

java time API LocalDate LocalDateTime Instant ZonedDateTime Period Duration DateTimeFormatter java.time

java Comparator Comparable compareTo compare Collections.sort TimSort thenComparing reversed

java records immutable DTO compact constructor accessor methods POJO Spring Boot Java 16

java sealed classes permits final non-sealed sealed interface pattern matching records Java 17

java annotations @Override @Deprecated @Target @Retention custom annotation Spring Hibernate JUnit metadata

java reflection API Class.forName getDeclaredMethod invoke setAccessible annotations Spring Hibernate runtime inspection

java serialization deserialization Serializable serialVersionUID transient ObjectOutputStream ObjectInputStream

memory management (heap, stack, gc) you don't malloc in java — the jvm does it for you. but understanding where objects live

java streams API filter map flatMap collect reduce parallelStream lazy evaluation terminal intermediate operations

java lambda expressions functional interface Predicate Function Consumer Supplier method reference effectively final

concurrency & executors beyond raw threads, java provides a rich concurrency toolkit: executors, completablefuture, locks, concurrent collections and (java 21+) structured concurrency.

design patterns design patterns are recurring solutions to recurring problems — a vocabulary that lets senior engineers communicate. knowing them is interview

performance optimization measure, don't guess. java performance work without a profiler is folklore. once you measure, the wins are usually in algorithms,

logging logs are how a running java service tells you what it's doing. the de-facto stack is slf4j (the api) + logback

security basics java security spans crypto, input validation, dependency hygiene and authentication. you don't need to be a cryptographer — you need

java memory model (jmm) the java memory model defines how threads interact through memory — what guarantees exist for visibility, ordering and

volatile keyword a volatile field guarantees that reads and writes go directly to main memory — every thread sees the latest value.

locks (reentrantlock) beyond synchronized, java provides explicit lock implementations — reentrantlock, readwritelock, stampedlock — with try-lock, timeouts, fairness and interruptibility.

atomic variables the java.util.concurrent.atomic package provides lock-free, cas-based counters and references — atomicinteger, atomicreference, longadder. they're the right tool for high-contention counters

fork/join framework the fork/join framework (java 7+) splits large tasks into smaller subtasks, executes them in parallel on a work-stealing pool, and

structured concurrency structured concurrency (java 21+, preview → final in 25) treats groups of related tasks as a single unit of work

spring boot basics spring boot is the most popular framework for building java backends. it bundles spring + auto-configuration + an embedded

rest apis with spring a rest api exposes resources over http using verbs (get, post, put, delete). spring's annotation model makes mapping

crud application almost every backend job interview asks you to build a crud app: create, read, update, delete. master the pattern with

microservices architecture microservices split a monolith into many small, independently-deployable services that talk over the network. done well it scales teams; done

database with jpa / hibernate jpa (java persistence api) is the spec; hibernate is the most-used implementation. together they map java objects

authentication with jwt a jwt (json web token) is a signed, base64-encoded json blob. it's the standard token format for stateless rest

deployment with docker docker packages your java app + jre into a single, reproducible image you can run on any host. it's

dependency injection dependency injection (di) is the cornerstone of spring — objects receive their collaborators from the container instead of creating them.

spring bean lifecycle every spring bean goes through a well-defined lifecycle: instantiation → dependency injection → initialization → use → destruction. hooks

spring security spring security is the de-facto auth framework for java backends. it provides filter chains for authentication, authorization, csrf protection, session

bean validation jakarta bean validation (@notnull, @size, @email) declaratively validates request dtos. spring boot auto-configures hibernate validator — just annotate your fields

global exception handling a @controlleradvice class centralises exception-to-http-response mapping across all controllers. no more try/catch in every endpoint — one place handles

transactions (@transactional) spring's @transactional wraps methods in a database transaction — commit on success, rollback on runtime exception. understanding propagation, isolation and

hibernate internals under spring data jpa lies hibernate — the orm that maps objects to sql. understanding the persistence context, lazy loading,

pagination returning 100,000 rows in one api call will crash your app and your database. spring data's pageable interface provides standard offset/limit

jpa specifications jpa specifications (spring data jpa) let you build dynamic, type-safe queries programmatically — perfect for search endpoints with optional filters

redis caching redis is an in-memory data store used for caching, session storage, rate limiting and pub/sub. in spring boot, spring-boot-starter-data-redis +

apache kafka apache kafka is a distributed event streaming platform — the backbone of event-driven microservices. producers publish to topics; consumers in

rabbitmq rabbitmq is a message broker implementing amqp — great for task queues, work distribution and reliable delivery with acknowledgements. simpler than

opentelemetry opentelemetry is the vendor-neutral standard for traces, metrics and logs. spring boot 3.x auto-instruments http, jdbc and kafka — export to

resilience4j resilience4j brings circuit breakers, retries, rate limiters and bulkheads to spring boot — preventing cascading failures when downstream services are slow

junit 5 junit 5 (jupiter) is the standard test framework for java. annotations like @test, @beforeeach, @parameterizedtest and @nested structure your test

mockito mockito creates test doubles — mocks that record interactions and stubs return values. combined with junit 5, it lets you unit-test

mockmvc (web layer tests) mockmvc tests spring mvc controllers without starting a real http server. it dispatches requests through the full filter

testcontainers testcontainers spins up real docker containers (postgresql, redis, kafka) during tests — giving you integration tests against the same software you

integration testing integration tests verify that multiple layers work together — controller → service → repository → database. spring boot's test slices

test-driven development (tdd) tdd is red → green → refactor: write a failing test, write the minimum code to pass, then improve

project: calculator cli build a command-line calculator that parses expressions like 2 + 3 * 4 and evaluates them. you'll practice parsing,

project: atm simulator simulate an atm machine with account balance, deposit, withdraw, transfer and pin validation. practice oop (account, atm, transaction), concurrency

project: expense tracker build an expense tracker with spring boot — rest api for adding expenses, categorising by tag, monthly summaries and

project: task manager api create a task manager backend — users, projects, tasks with status (todo/in_progress/done), due dates and assignees. a classic

project: e-commerce backend build a full e-commerce backend — product catalog, shopping cart, checkout, order processing, inventory and jwt authentication. this is

project: url shortener build a url shortener like bit.ly — encode long urls to short codes, redirect on access, track click counts.

java interview questions answers bank platform jvm jre jdk wrapper classes strings oop polymorphism inheritance interfaces abstract modifiers final static volatile exceptions

java interview — 4 years experience at the 4-year mark interviewers expect you to be solid on core java + collections +

java interview — 10 years experience at 10 years the bar shifts to concurrency, performance, debugging stories and system-design fundamentals. interviewers want

java interview — 12+ years experience at 12+ years coding still matters but architecture, microservice trade-offs, scaling decisions and cost dominate. you're

java interview — 15+ years experience at 15+ years you're interviewing for principal / staff+ / architect roles. the questions become open-ended:

java practice hub: stream api coding questions welcome to the java code practice hub for stream api coding questions. this track contains

find even numbers practice a focused java stream api coding question: find even numbers. solve it first with a stream pipeline, then

find odd numbers practice a focused java stream api coding question: find odd numbers. solve it first with a stream pipeline, then

square each number practice a focused java stream api coding question: square each number. solve it first with a stream pipeline, then

remove duplicates practice a focused java stream api coding question: remove duplicates. solve it first with a stream pipeline, then compare it

count elements practice a focused java stream api coding question: count elements. solve it first with a stream pipeline, then compare it

find first element practice a focused java stream api coding question: find first element. solve it first with a stream pipeline, then

find last element practice a focused java stream api coding question: find last element. solve it first with a stream pipeline, then

convert to uppercase practice a focused java stream api coding question: convert to uppercase. solve it first with a stream pipeline, then

sort ascending practice a focused java stream api coding question: sort ascending. solve it first with a stream pipeline, then compare it

sort descending practice a focused java stream api coding question: sort descending. solve it first with a stream pipeline, then compare it

find max salary employee practice a focused java stream api coding question: find max salary employee. solve it first with a stream

find min salary employee practice a focused java stream api coding question: find min salary employee. solve it first with a stream

group employees by department practice a focused java stream api coding question: group employees by department. solve it first with a stream

count employees by department practice a focused java stream api coding question: count employees by department. solve it first with a stream

find duplicate characters practice a focused java stream api coding question: find duplicate characters. solve it first with a stream pipeline, then

find frequency of words practice a focused java stream api coding question: find frequency of words. solve it first with a stream

second highest number practice a focused java stream api coding question: second highest number. solve it first with a stream pipeline, then

second lowest number practice a focused java stream api coding question: second lowest number. solve it first with a stream pipeline, then

merge two lists practice a focused java stream api coding question: merge two lists. solve it first with a stream pipeline, then

partition even and odd numbers practice a focused java stream api coding question: partition even and odd numbers. solve it first with

top 3 highest salaries practice a focused java stream api coding question: top 3 highest salaries. solve it first with a stream

most frequent element practice a focused java stream api coding question: most frequent element. solve it first with a stream pipeline, then

longest string practice a focused java stream api coding question: longest string. solve it first with a stream pipeline, then compare it

first non-repeated character practice a focused java stream api coding question: first non-repeated character. solve it first with a stream pipeline, then

first repeated character practice a focused java stream api coding question: first repeated character. solve it first with a stream pipeline, then

flatten nested lists practice a focused java stream api coding question: flatten nested lists. solve it first with a stream pipeline, then

find common elements practice a focused java stream api coding question: find common elements. solve it first with a stream pipeline, then

find missing numbers practice a focused java stream api coding question: find missing numbers. solve it first with a stream pipeline, then

find anagram groups practice a focused java stream api coding question: find anagram groups. solve it first with a stream pipeline, then

remove null values practice a focused java stream api coding question: remove null values. solve it first with a stream pipeline, then

custom collectors practice a focused java stream api coding question: custom collectors. solve it first with a stream pipeline, then compare it

group by multiple fields practice a focused java stream api coding question: group by multiple fields. solve it first with a stream

nested grouping practice a focused java stream api coding question: nested grouping. solve it first with a stream pipeline, then compare it

concurrent grouping practice a focused java stream api coding question: concurrent grouping. solve it first with a stream pipeline, then compare it

window-like operations practice a focused java stream api coding question: window-like operations. solve it first with a stream pipeline, then compare it

stream performance optimization practice a focused java stream api coding question: stream performance optimization. solve it first with a stream pipeline, then

lazy evaluation examples practice a focused java stream api coding question: lazy evaluation examples. solve it first with a stream pipeline, then

parallel stream examples practice a focused java stream api coding question: parallel stream examples. solve it first with a stream pipeline, then

reduce examples practice a focused java stream api coding question: reduce examples. solve it first with a stream pipeline, then compare it

collector composition practice a focused java stream api coding question: collector composition. solve it first with a stream pipeline, then compare it

sales analytics dashboard practice a focused java stream api coding question: sales analytics dashboard. solve it first with a stream pipeline, then

e-commerce order analysis practice a focused java stream api coding question: e-commerce order analysis. solve it first with a stream pipeline, then

banking transaction summaries practice a focused java stream api coding question: banking transaction summaries. solve it first with a stream pipeline, then

log processing practice a focused java stream api coding question: log processing. solve it first with a stream pipeline, then compare it

student result analytics practice a focused java stream api coding question: student result analytics. solve it first with a stream pipeline, then

attendance reporting practice a focused java stream api coding question: attendance reporting. solve it first with a stream pipeline, then compare it

employee reporting practice a focused java stream api coding question: employee reporting. solve it first with a stream pipeline, then compare it

product inventory reports practice a focused java stream api coding question: product inventory reports. solve it first with a stream pipeline, then

social media trend analysis practice a focused java stream api coding question: social media trend analysis. solve it first with a stream

fraud detection preprocessing practice a focused java stream api coding question: fraud detection preprocessing. solve it first with a stream pipeline, then

java exercises reading code teaches you the syntax; writing code makes it stick. work through these exercises in the playground above each

try yourself java (playground) use the in-browser playground below to experiment with java. pick a starter template (beginner → advanced) or write

Test your Java knowledge: 10 questions covering syntax, OOP, collections, concurrency, Spring & security with instant feedback.