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You have written Java for 1 to 2 years. You know how to use ArrayList and HashMap . But you don't fully understand generics variance ( ? super T vs. ? extends T ) or the memory implications of inner classes. This book fills those gaps. It serves as the definitive reference for the 20% of the language you rarely touch.

Your current (beginner, intermediate, or advanced)? The specific Java version your current projects utilize?

This is often the headline "deep feature" discussed in modern editions. Introduced in Java 9, the Module System (Project Jigsaw) represents a fundamental shift in how Java code is organized and deployed.