To start preparing for the Java certification, it is important to check their website and see the exam schedules. This will help you determine whether you have ample time for preparation. On average, 1 to 3 months for preparation would suffice.
Having known that, it is critical to read the Oracle Java 8 documentation. Yes, read it like a book. You can also check out textbooks that explain clearly the concepts you need to understand to pass the certification exam. If you prefer videos, you can search for an online Java programming course that provides adequate preparation for the exams.
Finally, you should take mock tests prior to the exam. This way, you have a feel of what to expect on the D-day, dispelling any kind of fear.

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