If you can, where do you use it?

If you have mastered the programming language of Java, then you should be able to use it somehow in minecraft. How is that meant? Let's say I can use the Java programming language. What does that bring me in minecraft. It is supposed to be used in minecraft somehow. So minecraft Java edition. What can I do with it in minecraft? Or how do I use it to program in minecraft (where)?

You can use it to program plugins with the Spigot API, for example. Mods are also possible with it.

Minecraft plugins and Minecraft mods and Minecraft clients

For example, you could program a client.

This is not possible in the game itself. Minecraft was programmed with Java.