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Before becoming a “professional” Minecraft developer, I spent considerable time when I was 15-17 building Bukkit plugins from public plugins.
A lot of them can be found here: https://dev.bukkit.org/members/_forgeuser7002297/projects
The largest one being Snowballer.
Besides that, I worked on a few C++ 2D games and know a thing or two about OpenGL (but not that much!)
For example this Sokoban clone I developed using SFML as a teen:
