https://www.developer-tech.com/news/2018/jun/01/10-essential-resources-intermediate-java-programmers/

Java remains one of the most popular programming languages to learn, thanks to its long history and reputation for being easy to pick up yet difficult to master. After you get through the initial tutorials and become more comfortable in the language, finding out how to progress further can be difficult. This article is intended to help with that, offering ten useful resources that will help you further improve your skills and work your way to becoming an expert programmer. Let's start.

The Effective series of programming resources are some of the most commonly recommended, thanks to their compartmentalised format that focuses on delivering well-explained lessons, complete with full source code.