A complete guide to String in Java — immutability and the String pool, basic operations, StringBuilder and StringBuffer, String formatting, parsing and conversion, regular expressions, Text Blocks, and efficient usage patterns for production.
A complete guide to I/O in Java — comparing java.io and java.nio, file operations with Files and Path, reading and writing text and binary files, stream I/O, directories and recursion, file watching, object serialization, and safe resource handling patterns.
A complete guide to mathematics in Java — the Math class and all its functions, safe integer arithmetic with overflow detection, BigDecimal for financial calculations, BigInteger for arbitrarily large numbers, random numbers with Random and SecureRandom, and accurate conversion and rounding.