Installing OpenJDK can be very simple but not Oracle JDK.
While Oracle JDK (earlier Sun JDK) is under the Binary Code License (earlier Sun License), OpenJDK is under GPL with a linking exception. From JDK version 7, Oracle has planned to support OpenJDK and withdraw the Operating System Distributor License for Java. This has resulted in a withdrawal of Oracle JDK from the repositories of Linux distributions. Therefore you can’t just use apt-get to install Oracle JDK.