This book is a collection of tutorial notes and sample codes written by the author while he was learning JDK (Java Development Kit) core libraries himself. Topics include Time and Calendar, Internationalization, Unnamed Packages, Collections, Character Set and Encoding, Logging, XML related technologies: DOM, SAX, DTD, XSD, and XSL, Cryptography, Certificates, Key stores, Cipher and Encryption, Socket communication, SSL and HTTPS. Updated in 2024 (Version v6.32) with JDK 20. For latest updates and free sample chapters, visit https://www.herongyang.com/JDK.