I made a Minecraft clone in processing java. It has basic functions like placing/breaking blocks, sprinting, flying, ambient music, sound effects, and random world generation. The whole thing is about 1000 lines long and took about 10 hours. It’s somewhat optimized (backface/occlusion culling) so it can render 25k blocks at 30 fps.
I don’t understand what you mean by ‘sky sphere’, I just set the background color, and anything not covered by blocks will be the sky. For steering, wasd for movement and mouse is for looking around. I used java.awt.Robot to lock the mouse to the center of the screen.