204 docs tagged with "intermediate"

A creational pattern that produces families of related objects without specifying their concrete classes.

Hide complexity behind a clean contract — when to use abstract classes vs. interfaces, default methods, and the design forces that govern the choice.

Hands-on code examples and step-by-step walkthroughs for Abstraction in Java.

Hands-on examples of the Adapter pattern — wrapping incompatible APIs, integrating third-party SDKs, and using Spring DI as an adapter.

The single REST endpoint, full request and response shapes, Bean Validation rules, and what happens when validation fails.

Principles and patterns for designing clear, versioned, and client-friendly REST, gRPC, and GraphQL APIs in Java/Spring Boot services.

Scenario-based walkthrough of designing a production-quality REST API with versioning, pagination, error handling, and rate-limiting headers in Spring Boot 3.

Hands-on examples for externalized configuration using profiles, @ConfigurationProperties, @Value, and startup validation in Spring Boot.

How Spring Boot externalizes configuration using application.properties/yml, profiles, @Value, and @ConfigurationProperties for structured, type-safe config binding.

Lock-free thread-safe operations using AtomicInteger, AtomicReference, LongAdder, and the compare-and-swap (CAS) CPU primitive that powers them.

Hands-on walkthroughs of AtomicInteger, AtomicReference, LongAdder, and CAS-based patterns including the ABA problem.

Hands-on code examples for Spring Security authentication — UserDetailsService, BCrypt, custom login endpoint, and accessing the current user.

Hands-on examples for Spring Security authorization — URL rules, @PreAuthorize, SpEL expressions, ownership checks, and testing access control.

Hands-on examples showing how Spring Boot auto-configuration works, how to observe it, override it, and write your own.

The complete lifecycle of a Spring bean — from instantiation and dependency injection, through initialization callbacks, to destruction — and how to hook into each stage.

How Spring's bean scopes — singleton, prototype, request, session, and application — control when beans are created, how many instances exist, and how they interact with each other.

A comparison of Git Flow, GitHub Flow, and trunk-based development — what each model is, when to use it, and how it shapes your CI/CD pipeline.

A creational pattern that separates the construction of a complex object from its representation, allowing the same construction process to produce different results.

Hands-on examples of the Builder pattern — from the classic Effective Java idiom to Lombok @Builder to a step-by-step Director.

Patterns for caching application data — cache-aside, write-through, write-behind — eviction policies, TTL design, and cache stampede prevention in Spring Boot with Redis.

Scenario-based walkthrough of implementing cache-aside with Redis in Spring Boot — including TTL configuration, cache eviction, and stampede prevention.

A behavioral pattern that passes a request along a chain of handlers, each deciding to process it or pass it to the next handler.

How the JVM loads, links, and initializes classes on demand — the parent-delegation model, the three standard classloaders, and how frameworks use custom classloaders for isolation and hot reload.

Consolidated interview Q&A for the Java Collections Framework — hierarchy, List, Set, Map, Queue, iterators, sorting, and immutability — from beginner through advanced.

Quick-reference summary of the Java Collections Framework — hierarchy, implementations, complexity, and top interview questions.

The complete interface tree of the Java Collections Framework — Iterable, Collection, List, Set, Queue, and why Map stands apart.

Hands-on examples showing how the Collection/Map interface tree works in practice.

Java's built-in Collector implementations — toList, groupingBy, partitioningBy, joining, toMap, counting, and how to build custom collectors.

Hands-on examples for groupingBy, toMap, joining, partitioningBy, and custom collectors.

A behavioral pattern that encapsulates a request as an object, enabling undo/redo, queuing, logging, and parameterization of operations.

A structural pattern that composes objects into tree structures to represent part-whole hierarchies, letting clients treat individual objects and compositions uniformly.

How Git detects and marks merge conflicts, what three-way merge means, how to resolve conflicts efficiently with rerere and merge tools, and strategies to avoid them in the first place.

Why connection pooling is essential, how HikariCP works, and how to configure it correctly in Spring Boot applications.

Hands-on Spring Boot HikariCP configuration examples, pool monitoring, leak detection, and Kubernetes multi-pod sizing.

Quick-reference summary of Java's core utility classes — Object, String, StringBuilder, Math, wrapper types, and Optional — with a learning path and top interview questions.

Consolidated interview Q&A for Core Java covering beginner through advanced topics — variables, types, operators, control flow, arrays, strings, methods, and packages.

Hands-on examples for CSRF protection and CORS configuration in Spring Boot — Cookie-based CSRF for SPAs, production CORS setup, fixing the preflight 401 problem, and testing with MockMvc.

How Spring Security prevents Cross-Site Request Forgery attacks with CSRF tokens, when to disable CSRF for REST APIs, and how to configure CORS for Single-Page Applications talking to Spring Boot backends.

How to create domain-specific exception classes in Java — checked vs. unchecked choice, adding context fields, exception chaining, and hierarchy design.

Hands-on examples for building domain-specific exception hierarchies, adding typed fields, chaining exceptions, and integrating with Spring Boot's @ControllerAdvice.

Quick-reference summary of SQL, indexes, transactions, connection pooling, NoSQL trade-offs, and schema migration for Java backend engineers.

A structural pattern that dynamically adds behavior to an object by wrapping it in decorator objects, without altering the original class.

How Spring's container wires beans together using constructor, setter, and field injection, and when to use @Autowired, @Qualifier, and @Primary.

The entities, DTOs (Java Records), and enums that represent the core concepts of a loan application — and why each was designed the way it was.

Production-proven rules for when to throw, catch, wrap, and log exceptions in Java — covering the checked vs. unchecked debate, layered architecture patterns, and common anti-patterns.

Hands-on code contrasting good and bad exception handling patterns — swallowing, double-logging, flow control misuse, and InterruptedException restoration.

How GlobalExceptionHandler uses @RestControllerAdvice to intercept validation failures and unexpected errors and convert them into structured JSON responses.

Hands-on Spring Boot demo of @ControllerAdvice, ProblemDetail error responses, validation error shaping, and the catch-all exception handler pattern.

How to centralise HTTP error responses in Spring Boot — @ExceptionHandler, @ControllerAdvice, ProblemDetail (RFC 7807), and mapping domain exceptions to the right status codes.

Consolidated interview Q&A for Java Exceptions — hierarchy, try/catch/finally, custom exceptions, and best practices from beginner through advanced.

Quick-reference summary of Java exception handling — hierarchy, try/catch/finally, custom exceptions, and best practices — for rapid revision.

A creational pattern that defines an interface for creating an object but lets subclasses decide which class to instantiate.

Hands-on examples of the Factory Method pattern — from classic inheritance-based variants to static factories to Spring @Bean factories.

What functional interfaces are, the built-in types (Function, Predicate, Consumer, Supplier), and how to compose them.

Hands-on examples for Function, Predicate, Consumer, Supplier, and composition patterns.

Consolidated interview Q&A for Java Functional Programming — lambdas, functional interfaces, method references, streams, collectors, parallel streams, and Optional.

Quick-reference summary of Java 8+ functional programming — lambdas, functional interfaces, method references, Streams, Collectors, parallel streams, and Optional — for rapid revision.

How the JVM automatically reclaims heap memory — GC roots, reachability analysis, generational collection, and the major collectors (Serial, Parallel, G1, ZGC) with tuning flags.

How Java generics provide compile-time type safety through parameterised types — covering generic classes, generic methods, bounded type parameters, and the key constraint that generics only work with reference types.

Hands-on code examples for generic classes, generic methods, bounded type parameters, and the diamond operator.

How Git hooks let you automate quality checks at every stage of the commit and push lifecycle — pre-commit linting, commit-msg validation, pre-push tests, and managing hooks across a team with tools like pre-commit.

The Hibernate internals every Spring Boot developer must understand — SessionFactory vs Session thread safety, entity states, dirty checking, first-level cache, second-level cache, and LazyInitializationException.

A deep dive into the HTTP protocol — methods, status codes, headers, content negotiation, and HTTP/2 basics — as used in Java backend development.

Hands-on Spring Boot examples demonstrating HTTP methods, status codes, headers, ETags, and content negotiation.

How to create truly immutable List, Set, and Map instances in Java using the Java 9+ factory methods and how they differ from the older Collections.unmodifiable wrappers.

Hands-on examples comparing List.of, Collections.unmodifiableList, and Arrays.asList — and showing defensive copying at API boundaries.

How database indexes work, when to add them, how to read EXPLAIN plans, and common indexing pitfalls that hurt performance.

Hands-on examples covering index creation, composite indexes, covering indexes, and reading EXPLAIN plans in PostgreSQL.

Extend classes with `extends`, override behavior with `@Override`, use `super` for parent delegation, and learn when inheritance causes more harm than good.

Hands-on code examples and step-by-step walkthroughs for Inheritance in Java.

How to use @SpringBootTest with TestRestTemplate and WebTestClient to write full-context Spring Boot integration tests.

Hands-on walkthrough of @SpringBootTest with TestRestTemplate, @MockBean for external dependencies, and Testcontainers for real database integration.

How Spring's Inversion of Control container manages bean definitions, wires dependencies, and controls object lifecycle using ApplicationContext and BeanFactory.

Consolidated interview Q&A for GoF design patterns in Java — creational, structural, and behavioral — covering beginner through advanced topics.

Quick-reference summary of all 15 GoF design patterns in Java — participants, intent, Java/Spring examples, and top interview questions.

Quick-reference summary of Java's type system — primitives, autoboxing, generics, wildcards, type erasure, and type inference — for rapid revision before interviews.

The high-level concurrency toolkit — ExecutorService, Future, CompletableFuture, CountDownLatch, CyclicBarrier, and Semaphore — that replaces manual thread management in production Java.

Hands-on walkthroughs of ExecutorService, CompletableFuture pipelines, CountDownLatch, and Semaphore.

How to map Java classes to database tables using @Entity, @Id, @GeneratedValue, and relationship annotations (@OneToMany, @ManyToOne), including fetch type defaults and their implications.

Hands-on code examples for JPA entity mapping, relationships, fetch types, and Spring Data auditing in a simple e-commerce domain.

The JVM's runtime memory layout — heap regions, stack frames, Metaspace, and per-thread vs. shared areas — and why understanding it matters for diagnosing OutOfMemoryErrors and tuning performance.

Hands-on examples for JWT authentication in Spring Boot — generating tokens, validating with Spring Security's resource server, custom claims, and refresh token pattern.

JSON Web Token structure, signing (symmetric vs. asymmetric), validation, and how to wire stateless JWT authentication into a Spring Boot REST API using Spring Security's OAuth2 Resource Server support.

Lambda expressions in Java — syntax, effectively-final capture, `this` behavior, and how they relate to functional interfaces.

Hands-on code examples and step-by-step walkthroughs for Lambda expressions in Java.

Understand Java's List interface, when to use ArrayList vs LinkedList, and how their internal structures drive performance trade-offs.

Hands-on examples for ArrayList vs LinkedList, pre-sizing, safe removal, and subList behavior.

Explicit locking with ReentrantLock, ReadWriteLock, and StampedLock — when and why to reach for them over synchronized.

Hands-on walkthroughs of ReentrantLock, ReadWriteLock, tryLock with timeout, and Condition-based signaling.

The four kinds of method references in Java — static, bound instance, unbound instance, and constructor — and when to prefer them over lambdas.

Hands-on examples for all four kinds of method references — static, bound, unbound, and constructor.

An architectural style that structures an application as a collection of small, independently deployable services — each owning its data and business logic.

Scenario-based walkthrough of building a two-service Order + Inventory microservices topology with service discovery, Kafka events, and Spring Boot 3.

How to create mock objects, stub method calls, verify interactions, and capture arguments in Java unit tests.

Hands-on code examples for creating mocks, stubbing behavior, verifying interactions, and capturing arguments with Mockito.

How to test Spring MVC and Spring WebFlux controllers at the HTTP level without starting a real server, using MockMvc and WebTestClient.

Hands-on walkthrough of HTTP-level controller testing with MockMvc and WebTestClient — requests, assertions, security, and validation.

Consolidated interview Q&A for Java Multithreading — threads, synchronization, wait/notify, ExecutorService, locks, atomics, thread safety, and virtual threads from beginner through advanced.

Quick-reference summary of Java concurrency — threads, synchronization, locks, atomics, ExecutorService, CompletableFuture, virtual threads — for rapid revision.

Practical guide to MySQL, PostgreSQL, and H2 for Java developers — architecture differences, UUID handling, H2 for development, and migration paths to production databases.

Hands-on examples demonstrating how N+1 queries occur and how to fix them with JOIN FETCH, @EntityGraph, and @BatchSize.

CAP theorem, BASE properties, and when to choose Redis, MongoDB, Cassandra, or Elasticsearch over a relational database.

Hands-on Spring Boot examples for Redis caching, MongoDB document CRUD, and Spring Cache abstraction.

The root of every Java class hierarchy — toString, equals, hashCode, clone, wait, notify, and finalize explained with contracts and pitfalls.

Hands-on code examples and step-by-step walkthroughs for equals, hashCode, toString, and clone.

A behavioral pattern where an object (subject) maintains a list of dependents (observers) and notifies them automatically when its state changes.

Hands-on examples of manual Observer, Spring Application Events, @TransactionalEventListener, and @Async observers.

Quick-reference summary of Java OOP concepts — classes, encapsulation, inheritance, polymorphism, abstraction, records, and sealed classes.

Auto-generating interactive API documentation for Spring Boot REST APIs using springdoc-openapi — setup, @Operation/@Schema annotations, security schemes, and customisation.

Hands-on Springdoc setup, @Operation/@Schema annotations, JWT security scheme, and production-safe configuration for Spring Boot REST APIs.

Hands-on examples for Optional creation, chained transformations, orElseGet laziness, and common anti-patterns.

Java's Optional container type — what it is, when it genuinely clarifies code, and the common anti-patterns that make it worse than a null check.

How to use Optional correctly — creation, retrieval patterns, chaining, and the anti-patterns that make Optional worse than null.

Hands-on examples for Optional creation, safe retrieval, chaining, and the anti-patterns to avoid.

How LoanApplication is mapped to H2 using JPA @Embeddable value objects, @ElementCollection for rejection reasons, and @PrePersist for UUID generation.

Compile-time (overloading) vs. runtime (overriding) dispatch — how Java decides which method to call and why this is the foundation of flexible design.

Hands-on code examples and step-by-step walkthroughs for Polymorphism in Java.

How Java's eight primitive types differ from object references — stack vs. heap, null safety, autoboxing, unboxing, and the performance tradeoffs involved.

Hands-on code examples and step-by-step walkthroughs for Java primitives, autoboxing, unboxing, the Integer cache, and null NPE traps.

A creational pattern that creates new objects by copying (cloning) an existing instance rather than instantiating from scratch.

A structural pattern that provides a surrogate object which controls access to another object, adding security, caching, logging, or lazy initialization transparently.

Hands-on examples showing manual proxies, JDK dynamic proxies, and how Spring AOP uses proxies for @Transactional and @Cacheable.

Understand Java's Queue and Deque interfaces, how ArrayDeque and PriorityQueue work internally, and when to use BlockingQueue for producer-consumer patterns.

Hands-on examples for ArrayDeque as stack/queue, PriorityQueue for task scheduling, and BlockingQueue for producer-consumer.

The difference between git merge and git rebase — when each produces a better history, how interactive rebase cleans up commits, and the golden rule you must never break.

Hands-on code examples and step-by-step walkthroughs for Records (Java 16+) in Java.

Java's concise immutable data carrier — understand what records generate automatically, compact constructors, and when records replace traditional value classes.

Circuit Breaker, Retry, Bulkhead, Timeout, and Rate Limiter — the five resilience patterns that prevent cascading failures in distributed Spring Boot services, implemented with Resilience4j.

Scenario-based walkthrough of implementing Circuit Breaker, Retry, Bulkhead, and Timeout with Resilience4j in a Spring Boot 3 microservice.

How to design clean, coherent REST APIs — resource naming, HTTP verb mapping, versioning strategies, HATEOAS, and idempotency patterns used in production Spring Boot services.

Hands-on examples of REST resource naming, versioning, idempotency keys, HATEOAS links, and pagination in Spring Boot.

Step-by-step Flyway and Liquibase migration examples in Spring Boot — from first migration to rollback-safe production patterns.

How to manage database schema evolution safely in Spring Boot using versioned migrations with Flyway and Liquibase.

Hands-on code examples and step-by-step walkthroughs for Spring Security's filter chain configuration.

How the LoanApplicationService evaluates a loan — risk classification, interest rate calculation, EMI formula, and the two-tier eligibility check — step by step.

Understand Java's Set interface, how each implementation handles uniqueness and ordering, and when to choose HashSet, LinkedHashSet, or TreeSet.

Hands-on examples for HashSet, LinkedHashSet, and TreeSet — uniqueness enforcement, ordering differences, and set operations.

A creational pattern that ensures a class has exactly one instance and provides a global access point to it.

Hands-on step-by-step examples of all four Java Singleton idioms with thread-safety analysis.

Five design principles that make object-oriented code easier to maintain, extend, and test — the foundation of clean Java design.

Step-by-step scenario showing a notification system refactored through all five SOLID principles in a Spring Boot context.

Understand the difference between Comparable (natural ordering defined on the class) and Comparator (external ordering strategy), and how to use both with collections, streams, and sorted data structures.

Hands-on examples for Comparable, Comparator factory methods, multi-field sorting, and null-safe ordering.

How Spring's Aspect-Oriented Programming model applies cross-cutting concerns — logging, transactions, security, caching — using proxies, pointcuts, and advice without touching business logic.

Spring Boot Actuator exposes built-in HTTP/JMX endpoints for health checks, metrics, environment inspection, and live bean graphs — essential for production monitoring and operations.

Hands-on examples for enabling and securing Actuator endpoints, writing custom health indicators, and exporting metrics with Micrometer/Prometheus.

How Spring Boot uses @EnableAutoConfiguration and @ConditionalOn* annotations to wire beans automatically based on what is on the classpath.

Quick-reference summary of Spring Boot auto-configuration, properties, starters, Actuator, and testing — key concepts, annotations, and interview questions at a glance.

How to use @WebMvcTest, @DataJpaTest, and @JsonTest to write fast, focused Spring Boot tests that load only the layers you need.

Hands-on walkthrough of @WebMvcTest, @DataJpaTest, and @JsonTest with realistic Order API examples.

How to test Spring Boot applications using @SpringBootTest, test slices (@WebMvcTest, @DataJpaTest), @MockBean, and Testcontainers for integration tests with real infrastructure.

Hands-on examples for @SpringBootTest, @WebMvcTest, @DataJpaTest, @MockBean, and Testcontainers in a realistic order service project.

How Spring's cache abstraction works with @Cacheable, @CacheEvict, @CachePut, and @Caching, plus integrating Caffeine and Redis as backing stores.

Hands-on examples for @Cacheable, @CacheEvict, @CachePut, Caffeine, and Redis cache configuration in a Spring Boot application.

Quick-reference summary of Spring Data JPA — entity mapping, repository methods, transactions, N+1, projections, and caching.

How to use interface projections, DTO projections, and dynamic projections in Spring Data JPA to select only the columns you need in a single optimized query.

Hands-on examples for closed interface projections, nested projections, DTO projections with records, and dynamic projections.

How Spring Data JPA's repository interfaces (CrudRepository, JpaRepository) eliminate DAO boilerplate through query method derivation and @Query for JPQL and native SQL.

Hands-on examples for Spring Data JPA query methods, @Query JPQL/native SQL, @Modifying, pagination, and the Specification API.

How Spring's ApplicationEventPublisher and @EventListener provide a decoupled, in-process observer pattern for reacting to application state changes — including async and transactional event variants.

Consolidated interview Q&A for Spring Framework covering IoC container, dependency injection, bean lifecycle, bean scopes, AOP, and Spring Events — beginner through advanced.

Quick-reference summary of Spring Framework concepts — IoC container, dependency injection, bean lifecycle, scopes, AOP, and events — for rapid revision before interviews.

How Spring MVC processes HTTP requests — DispatcherServlet, @RestController, request mapping, parameter binding, and ResponseEntity patterns used in everyday Spring Boot APIs.

Hands-on Spring MVC examples covering DispatcherServlet internals, parameter binding, validation, content negotiation, and interceptors.

How Spring Security authenticates users — UserDetailsService, PasswordEncoder, AuthenticationManager, and the authentication flow from credentials to SecurityContext.

How Spring Security enforces access control — URL-based rules with requestMatchers, method-level security with @PreAuthorize and @Secured, and the role vs. authority distinction.

How Spring Security's ordered chain of servlet filters intercepts every HTTP request and applies authentication and authorization before the request reaches your controller.

Consolidated interview Q&A for Spring Security covering filter chain, authentication, authorization, JWT, OAuth2, CSRF, and CORS — beginner through advanced.

Quick-reference summary of Spring Security's filter chain, authentication, authorization, JWT, OAuth2, CSRF, and CORS — key concepts, annotations, and interview questions at a glance.

Core SQL syntax and concepts — SELECT, JOIN types, GROUP BY, window functions, and subqueries — with Spring Boot JDBC context.

Hands-on SQL examples covering JOINs, GROUP BY, window functions, and subqueries in a Spring Boot JDBC context.

A behavioral pattern that allows an object to change its behavior when its internal state changes, as if the object changed its class.

A behavioral pattern that defines a family of algorithms, encapsulates each one, and makes them interchangeable at runtime without changing the client.

Hands-on examples of Strategy using classic classes, lambdas, Spring DI, and the strategy registry for runtime selection.

How Java streams work — pipeline anatomy, lazy evaluation, intermediate vs. terminal operations, and common pitfalls.

Hands-on walkthroughs of stream pipelines — filtering, mapping, flatMap, reduce, and lazy evaluation.

Java's text APIs — String immutability and the string pool, mutable alternatives with StringBuilder, and declarative list-joining with StringJoiner.

Hands-on examples for String immutability, StringBuilder loop-building, and StringJoiner delimited assembly.

How Java's synchronized keyword, intrinsic locks, volatile, and the happens-before relationship prevent race conditions in multithreaded programs.

Hands-on examples showing race conditions, synchronized fixes, volatile usage, and the happens-before relationship.

A behavioral pattern that defines the skeleton of an algorithm in a base class, deferring specific steps to subclasses without changing the overall structure.

How to use Testcontainers to run real PostgreSQL, MySQL, Redis, and Kafka instances inside Docker containers during Java tests.

Hands-on examples for running PostgreSQL, Redis, and Kafka in Docker containers during tests, with Spring Boot 3.1+ @ServiceConnection.

Quick-reference summary of Java testing concepts — JUnit 5, Mockito, Spring Boot test slices, integration tests, Testcontainers, MockMvc, and WebTestClient.

How LoanApplicationService is unit-tested with JUnit 5 and Mockito — covering risk classification, EMI calculation, and all eligibility rules using @Nested test classes.

What causes N+1 queries in JPA/Hibernate, how to diagnose them, and how to fix them with JOIN FETCH, @EntityGraph, DTO projections, and batch fetching.

Three design-level strategies for writing correct concurrent code — immutability, ThreadLocal confinement, and explicit object confinement — without reaching for locks.

Hands-on walkthroughs of immutable objects, defensive copying, ThreadLocal usage (and cleanup), and safe publication.

Hands-on examples for @Transactional propagation, readOnly optimization, rollbackFor, the self-invocation trap, and programmatic transactions.

Database transactions, the four ACID properties, isolation levels, and how deadlocks occur — with Spring @Transactional context.

Hands-on examples of Spring @Transactional — propagation, isolation levels, deadlock handling, and common pitfalls.

How Spring's @Transactional works under the hood with AOP proxies, propagation levels, isolation levels, readOnly optimization, rollback rules, and the self-invocation trap.

How the Java compiler removes all generic type information before producing bytecode, why this design choice was made for backward compatibility, and what limitations it imposes at runtime.

Hands-on code examples demonstrating what you can and cannot do because of Java's type erasure — instanceof, new T(), overloading, and the super type token pattern.

How the Java compiler deduces types automatically — covering the diamond operator, generic method type inference, `var` (Java 10+), and lambda target typing — and where inference has limits.

Hands-on code examples for the diamond operator, generic method inference, var for local variables, and lambda target typing.

Quick-reference summary of Git basics, object model internals, branching strategies, rebase vs. merge, remotes, conflict resolution, and hooks for Java backend engineers.

How Object.wait(), notify(), and notifyAll() coordinate threads using the intrinsic monitor — including the spurious wakeup rule and when to use each method.

Hands-on walkthroughs of Object.wait(), notify(), notifyAll() with producer-consumer patterns and timed waits.

Quick-reference summary of HTTP fundamentals, REST design, Spring MVC, exception handling, WebFlux, and OpenAPI for Java backend engineers.

How Java's wildcard type arguments — `?`, `? extends T`, and `? super T` — express variance in generic APIs, and how the PECS rule guides correct usage.

Hands-on code examples for unbounded wildcards, upper-bounded (? extends T), lower-bounded (? super T), and the PECS rule.

Java's eight primitive wrapper types — Integer, Long, Double, and friends — covering autoboxing, caching traps, parsing, and when to use primitives vs objects.

Hands-on examples for autoboxing, the Integer cache trap, parse methods, and unboxing NullPointerException.