The Sun JDK is no longer in Ubuntu's main package repository. In order to download it, you need to add the appropriate repository and indicate to the system which JDK should be used.

Java 6: for Gingerbread and newer

$ sudo add-apt-repository "deb http://archive.canonical.com/ lucid partner"
$ sudo apt-get update
$ sudo apt-get install sun-java6-jdk

You will need a 64-bit version of Ubuntu. Ubuntu 12.04 is recommended. Building using an older version of Ubuntu is not supported on master or recent releases.

$ sudo apt-get install git gnupg flex bison gperf build-essential \
  zip curl libc6-dev libncurses5-dev:i386 x11proto-core-dev \
  libx11-dev:i386 libreadline6-dev:i386 g++-multilib mingw32 tofrodos \
  python-markdown libxml2-utils xsltproc zlib1g-dev:i386