If you're on Gentoo and trying to play minecraft, this guide is for you. This is a modified version from here. However for completeness sake, I'll have the entire guide here too.
First you need to install icedtea, a FOSS JVM. To install it, emerge it
sudo emerge icedtea
That might take a while to emerge. Once its done, set is as the default system JVM
sudo eselect java-vm list; sudo eselect java-vm system
Of course substitute the number for the list entry that says icedtea. If you want to only set this for your user, don't use sudo.
Now download the executable file for minecraft. Its on mojangs site, but this should be a link to it as of this writing
Now extract it to wherever you would like minecraft to live on your system
tar xvf Minecraft.tar.gz; cd minecraft-launcher
Now that you've cd'd into the launcher folder, run the minecraft-lancher. You may run into an error along the lines of
MessageBox(Minecraft Launcher): Couldn't load launcher core from /home/USER/.minecraft/launcher/liblauncher.so: LoadErrorNotPresent
If you poke around you'll find that shared object file exists and lives in the right place, it just seems to be linking against non existant libraries, but that's OK. (On a unrelated note, its very interesting that the launcher is an electron app that also forces itself to live in your home directory. Just thought that was interesting)
So you need to install the correct libraries for minecraft to load, and its looking for libsecret. So we can emerge that
sudo emerge app-crypt/libsecret
On my system it pulled in 3 other dependencies, so your mileage on may vary on how much it pulls in. Now all you have to do is pull in.
After this all you have to do is run the launcher, sign in and it should work right out of the box! Hope this was helpful, and have fun!