If you have Java 21 installed and registered as a supported runtime, you can turn Java 21 preview features on in the Java build tab of your Eclipse project. By enabling Java 21 preview features, you ...

I've created a new project in Eclipse, put the jar files in that project, and added that to the build path.

Eclipse is an open-source Integrated Development Environment (IDE) supported by IBM. Eclipse is a popular script creator for Java application development (Java SE and Java EE) and Android apps.