Java or any other programming language is not a subject to finish in couple of months or to become an expert. The more Scenarios/Logics/Challenges you face, the more you learn. My 2 cents is that you should try Leet Code, watch some tutorials, browse through some articles, try YouTube as well and most importantly.. Try to do hands on practice. Anything you want to solve should be through the programming language you choose.
And after a while, and couple of headaches followed by coffee.. You will start understanding how you should proceed. Everyone tries a different path. Don't try to learn everything in a day. Just like weight loss or cultivating good habits, learning is also a journey. It will take time. Do not lose Hope. PRACTISE!
Satya recommends the following next steps:
Good to see that you're interested in learning Java! I work with Java on a daily basis in my job as a software developer/architect, mostly for backend services on the server side.
In general, I can recommend almost any kind of online Java tutorial, there are quite a few free resources that guide you through the core principles and concepts of Java with hands-on exercises (e.g. https://www.w3schools.com/java/). I can only echo what others already wrote, doing hands-on coding is one of THE most important things you should be doing, regardless of the actual programming/scripting language you choose.
Make sure to take your time, Java is a comprehensive language with many concepts and constructs. See it like learning French or any other spoken language, you can only get better and start conversing if you actually practice it and not just by learning or memorizing the theory. I myself am constantly learning about new stuff in Java and other programming languages, it keeps you on your toes which actually makes being a software developer a very interesting profession.
Moushumi recommends the following next steps:
Since I see that there are already so many great answers to your query, I'd just like to add upon them and give you a few suggestions on books that may help you if you aspire to become a java developer in future. These books are to be picked once you are done learning fundamental java.
-Enterprise patterns for design
-Concurrency in control for multi-threading
-Designing data intensive apps for high volume
All the best !!