I'm running Ubuntu 12.04 x64 and a Minecraft server on it, the tutorial I followed https://www.digitalocean.com/community/articles/how-to-set-up-a-minecraft-server-on-linux tells me to download the JDK (Java runtime), for obvious reasons. But when I run "java -d64" it says "This Java instance does not support a 64-bit JVM. Please install the desired version." How do I install the desired version correctly?