Five ways to check a prime number in Java: naive, square root, 6k+-1, Sieve of Eratosthenes and BigInteger.isProbablePrime, with measured timings and a program to print primes in a range.
Wrapper classes in Java explained with real output: the eight wrappers, autoboxing and unboxing, the Integer cache (127 vs 128), NullPointerException on unboxing, equals traps, NaN and negative zero.
Enum in Java explained with a runnable program: generated methods, fields and constructors, constant-specific bodies, switch and pattern matching, EnumMap and EnumSet, and the enum singleton.
Lambda expressions in Java explained with a runnable program: syntax forms, target typing, the four method-reference kinds, variable capture, this, and what javac actually generates.
How java.util.HashMap works inside: hash spreading, bucket index, load factor and resize, collision handling and treeification, with a runnable program that peeks into the table and prints real output.
All 15 JEPs in JDK 21: which became final (virtual threads, record patterns, pattern matching for switch, sequenced collections, Generational ZGC, the KEM API) and which are still preview or incubator, each with a short example and real output from JDK 21, plus interview questions.
Everything that became final between Java 12 and Java 17, with examples and real output from JDK 17: records, sealed classes, text blocks, switch expressions, pattern matching for instanceof, helpful NullPointerException messages, the RandomGenerator API and strong encapsulation of JDK internals, plus interview questions.
What Java 11 added over Java 8, with runnable examples and real output from JDK 8 and JDK 11: var in lambdas, the new String methods, Files.readString, the standard HttpClient, single-file launch, nest-based access, and the Java EE and CORBA modules that were removed.
Every headline Java 8 feature with a short example you can run, the real output from a JDK 8, the compile errors you will meet, and the interview questions that come out of each one: lambdas, streams, Optional, default methods, java.time and functional interfaces.
Access modifiers in Java with a visibility matrix: public, protected, default (package-private) and private, the protected-across-packages trap with real compiler errors, which modifiers apply to classes vs members, and how JPMS modules add a layer above public. Verified on JDK 25.