This book is a practical guide to ultra-low-latency Java engineering for high-frequency trading, RFQ systems, and market-making desks in banks and hedge funds. It focuses on real techniques used across the industry - from JVM tuning and off-heap design to Unix and network optimization with kernel bypass - showing how modern trading firms are moving from C++ to high-performance Java. Rather than theory, it delivers hands-on patterns, architectures, and system-level practices for building deterministic, production-grade latency-critical systems.