The Sybex OCP Oracle Certified Professional Java SE 21 Developer Study Guide (Exam 1Z0-830), by Jeanne Boyarsky and Scott Selikoff, is the book most past candidates start with: more than 1,000 pages written to the exam objectives, with hundreds of practice questions and electronic flashcards in Sybex's online learning environment.
The gap is the format: a book teaches and tests, but it can't tell you on day one where you stand, re-plan your study as you improve, or predict when you'll be ready across Java SE 21's object-oriented concepts, streams and lambdas, collections, concurrency, and I/O topics.
Edureify's Diagnostic maps your understanding across Java SE 21's object-oriented concepts, streams and lambdas, collections, concurrency, and I/O topics on day one, and the Readiness Score keeps updating as you practice — so instead of a vague sense of completion, you get a specific, evolving answer.