There are several free resources available for learning game development in Java. Here are a few options you might consider:
Oracle's Java Tutorials: Oracle's Java tutorial website has a section on game programming that covers the basics of creating a simple game in Java. The tutorial uses the JavaFX framework for building the game interface.
LibGDX: LibGDX is a free and open-source game development framework for Java that allows you to build games for desktop, mobile, and web platforms. The LibGDX website has a series of tutorials that walk you through the process of building a game using the framework.
Java Game Development: A Beginner's Guide: This book is a free online resource that covers the basics of game development in Java, using the JavaFX framework. The book includes examples and exercises to help you learn the concepts.
Coursera: Coursera is an online learning platform that offers a variety of courses on game development, including courses on Java game development. Many of these courses are free to audit, although you may have to pay to access certain features or to earn a certificate.
There are also many other resources available online, including forums, blogs, and YouTube channels, that provide tips, tricks, and tutorials for game development in Java.