final vs finally vs finalize in Java: a comparison table, final variables, methods and classes, the finally edge cases (return in finally, System.exit), and what finalize() does on JDK 25 and what to use instead. Verified with real output.
Constructors in Java explained with real output: default, parameterised, copy and private constructors, this() vs super() chaining, the shallow-copy trap, why constructors are not inherited, and the five compile errors they cause. Verified on JDK 25.
The static keyword in Java explained with real output: static variables, methods, blocks and nested classes, the exact initialisation order, why main is static (and Java 25 instance main), and the static bugs that reach production. Verified on JDK 25.
Method overloading vs overriding in Java: compile-time vs run-time polymorphism, the three-phase overload resolution order with real output, overriding rules for access, exceptions and covariant returns, @Override, and static and field hiding. Verified on JDK 25.
Abstract class vs interface in Java in a 12-row comparison table, with default, static and private interface methods, sealed interfaces, the diamond problem with the real compiler error, when to choose which, and the standard interview answer. Verified on JDK 25.
JDK vs JRE vs JVM explained with a comparison table, nested diagram, .java to bytecode to JIT flow, real javap output, class loaders, runtime memory areas and why the JRE no longer ships separately. All verified on JDK 25.
OOPs concepts in Java explained with one bank-account example: encapsulation, inheritance, polymorphism and abstraction, plus class vs object, IS-A vs HAS-A, diagrams, real compiler errors and 15 interview questions. Verified on JDK 25.
Keycloak 26.8 with Spring Boot 4.1 and Spring Security 7.1, tested against a real Keycloak: why roles are silently ignored, how a missing audience or a different host turns a valid token into a 401, method security, and managing users with the Keycloak admin client.
File upload and download in Spring Boot 4.1: why MockMvc accepts files Tomcat rejects, the filename you must not trust, Range downloads, a 200 MB transfer under a 64 MB heap, and what an S3 presigned URL does and does not protect.
30 Java String interview programs, each with a brute-force version, an optimal version, its time complexity and a stream variant where it helps. All code was compiled and run on JDK 25, and the output shown is the real output.