The first thing you need to install before starting to try something is the
Java JDK. You need to consider that different alternatives of the JDK exist, but in all cases, you need to install version 21:
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OracleJDK :1 This version was free until Java 11; after this version, you can use it for development/test environments, but you need to pay a license to use it on production. This version of the JDK offers you the most recent patches of bugs and new features because Oracle owns the language.
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OpenJDK :2 When ...