OpenJDK works best for me. It's simple and I have never faced any problem with it. Just follow these simple steps:

1. From Terminal install open jdk

   sudo apt-get install openjdk-7-jdk
   

2. Write a java program and save the file as filename.java

3. Now to compile use this command from the terminal

   javac filename.java
   

   If everything works well then a new "filename.class" file should be created.

4. To run your program that you've just compiled type the command below in terminal:

   java filename
   

NOTE

You can use any text editor (like gedit) ,

replace the filename with watever name you want

you need to be on same directory as the "present working directory" (got by running pwd) while running the command from terminal.

If you prefer to install Oracle JDK, a step by step instruction on installing Oracle JDK 8 is explained in this article : Install Latest Oracle JDK in Ubuntu

Step 1: Download the latest JDK(jdk-Xuxx-linux-xXX.tar.gz) from this official link.

Step 2: Open the terminal (Ctrl + Alt + T) and enter the following command.

sudo mkdir /usr/lib/jvm

Step 3: Enter the following command to change the directory.

cd /usr/lib/jvm

Step 4: Extract the jdk-Xuxx-linux-xXX.tar.gz file in that directory using this command.

sudo tar -xvzf ~/Downloads/jdk-8u45-linux-x64.tar.gz

Step 5: Enter the following command to open the environment variables file.

sudo gedit /etc/environment

Step 6: In the opened file, add the following bin folders to the existing PATH variable.

/usr/lib/jvm/jdk1.8.0_45/bin
/usr/lib/jvm/jdk1.8.0_45/db/bin
/usr/lib/jvm/jdk1.8.0_45/jre/bin

The PATH variables have to be separated by semicolon. Notice that the installed JDK version is 1.8 update 45. Depending on your JDK version, the paths can be different. Add the following environment variables at the end of the file.

J2SDKDIR="/usr/lib/jvm/jdk1.8.0_45"
J2REDIR="/usr/lib/jvm/jdk1.8.0_45/jre"
JAVA_HOME="/usr/lib/jvm/jdk1.8.0_45"
DERBY_HOME="/usr/lib/jvm/jdk1.8.0_45/db"

The environment file before the modification:

PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games"

The environment file after the modification:

PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/usr/lib/jvm/jdk1.8.0_45/bin:/usr/lib/jvm/jdk1.8.0_45/db/bin:/usr/lib/jvm/jdk1.8.0_45/jre/bin"
J2SDKDIR="/usr/lib/jvm/jdk1.8.0_45"
J2REDIR="/usr/lib/jvm/jdk1.8.0_45/jre"
JAVA_HOME="/usr/lib/jvm/jdk1.8.0_45"
DERBY_HOME="/usr/lib/jvm/jdk1.8.0_45/db"

So two things you'll need to do:

First create a small shell script to start your java program from a terminal. As you have packaged as a jar have a look at this, specifically the JAR Files as Applications section.

This may be sufficient: (although you'll want to use the full path to Java)

#!/bin/bash
java -jar path_to_jar_file

You should be able to run your script and successfully start your program.

Once you've got it starting from a script you can use standard linux tools to start the script. Either putting it in /etc/rc.local, or as you're using Ubuntu, use update-rc.d to start it on boot. See here for a very simple example of using update-rc.d

Hope this helps,

Will

Guys let's understand the syntax of it.

1. If class file is present in the Current Dir.

   java -cp . fileName

2. If class file is present within the Dir. Go to the Parent Dir and enter below cmd.

   java -cp . dir1.dir2.dir3.fileName

3. If there is a dependency on external jars then,

   java -cp .:./jarName1:./jarName2 fileName

   Hope this helps.